8.11 - Parasitology

The incorrect match between organism and the appropriate diagnostic procedure is:

A. Onchocerca volvulus - examination of skin snips

B. Cryptosporidium spp - modified acid-fast stain

C. Enterobius vermicularis - routine ova and parasite examination

D. Schistosoma haematobium - examination of urine sediment

C. Enterobius vermicularis - routine ova and parasite examination

parasite examination

• Enterobius vermicularis (pinworm) lays eggs on perianal folds, not in stool.

• Correct diagnostic method: cellophane (Scotch) tape preparation from perianal area collected in the early morning before bathing or defecation.

• Routine O&P exams are often negative, because eggs and adult worms rarely appear in stool.

Other options (correct matches):

• Onchocerca volvulus: diagnosed by skin snips showing microfilariae.

• Cryptosporidium spp: detected via modified acid-fast stain (oocysts = bright red on blue background).

• Schistosoma haematobium: eggs with terminal spine found in urine sediment.

Bold summary:
Routine stool O&P does not reliably detect Enterobius vermicularis — diagnosis requires perianal adhesive tape prep, not fecal exam.

In a patient with diarrhea, occasionally Entamoeba histolytica/E. dispar (four nucleated cysts, no chromatoidal bars) are identified as being present; however, these cells, which are misdiagnosed as Protozoa, are really:

A. Macrophages

B. Polymorphonuclear leukocytes (PMNs)

C. Epithelial cells

D. Eosinophils

B. Polymorphonuclear leukocytes (PMNs)

• In diarrheal specimens, PMNs (neutrophils) may appear rounded, with multi-lobed or fragmented nuclei and granular cytoplasm, sometimes giving the false impression of multi-nucleated cysts.

• When stool is stained (e.g., trichrome or iodine), PMNs can lose their lobation, showing 2–4 dark nuclei and no visible cytoplasmic inclusions — mimicking Entamoeba cysts.

• Epithelial cells, while also present, are typically larger, flatter, and have a single nucleus with abundant cytoplasm — less likely to be confused for Entamoeba cysts in routine stool prep.

• E. histolytica/E. dispar cysts have a defined wall and central karyosomes — features absent in PMNs.

Bold summary:
In diarrheal stool, degenerating PMNs can mimic 4-nucleate Entamoeba cysts, leading to false protozoan ID.

Charcot-Leyden crystals in stool may be associated with an immune response and are thought to be formed from the breakdown products of:

A. Neutrophils

B. Eosinophils

C. Monocytes

D. Lymphocytes

B. Eosinophils

Charcot–Leyden Crystals

• Bipyramidal, colorless, refractile structures seen in stool, sputum, or tissue exudates.

• Formed from lysophospholipase (galectin-10) released by disintegrating eosinophils during allergic or parasitic inflammation.

• Common in helminthic infections (Strongyloides, Ascaris, Trichuris) or eosinophilic colitis.

• Absence doesn’t exclude parasitic disease.

Bold summary:
Charcot–Leyden crystals reflect eosinophil breakdown products, indicating eosinophil-driven immune activity in parasitic infection.

Parasitic organisms that may be transmitted sexually do not include:

A. Entamoeba gingivalis

B. Dientamoeba fragilis

C. Trichomonas vaginalis

D. Diphyllobothrium latum

D. Diphyllobothrium latum

• D. latum (fish tapeworm) is acquired by eating undercooked freshwater fish, not through sexual contact.

• It causes vitamin B₁₂ deficiency and megaloblastic anemia, with proglottids or eggs seen in stool.

Other options (sexually transmissible or contact-associated):

• Entamoeba gingivalis: transmitted via oral contact (kissing, saliva exchange).

• Dientamoeba fragilis: occasionally transmitted fecal-orally or via direct contact, and sexual spread has been documented.

• Trichomonas vaginalis: a true sexually transmitted flagellate, causing vaginitis/urethritis.

Bold summary:
Diphyllobothrium latum spreads through ingestion of infected fish, not sexual contact — making D the only non-sexually transmissible parasite listed.

The incorrect match between the organism and one method of acquiring the infection is:

A. Trypanosoma brucei rhodesiense - bite of sand fleas

B. Giardia lamblia (G. intestinalis, G. duodenalis) - ingestion of water contained with cysts

C. Hookworm - skin penetration of larvae from soil

D. Toxoplasma gondii - ingestion of infected raw or rare meats

A. Trypanosoma brucei rhodesiense - bite of sand fleas

Transmission of Trypanosoma brucei rhodesiense

• Caused by African sleeping sickness (East African form).

• Transmitted by the tsetse fly (Glossina spp.), not sand fleas.

• The parasite enters via the fly bite, multiplies in blood and lymph, and eventually invades the CNS.

Other correct matches:

• Giardia lamblia: infection via ingestion of cysts in contaminated water.

• Hookworm (Necator, Ancylostoma): infection via skin penetration by filariform larvae from contaminated soil.

• Toxoplasma gondii: infection via ingestion of tissue cysts in undercooked meat or exposure to oocysts in cat feces.

Bold summary:
T. brucei rhodesiense is transmitted by the tsetse fly, not sand fleas — making A the incorrect pairing.

Upon examination of stool material for Cystoisospora belli, one would expect to see:

A. Cysts containing sporozoites

B. Precysts containing chromatoidal bars

C. Oocysts that are modified acid-fast positive

D. Sporozoites that are hematoxylin positive

C. Oocysts that are modified acid-fast positive

Cystoisospora belli (formerly Isospora belli)

• Appears in stool as ellipsoidal oocysts measuring ~25–30 µm.

• Modified acid-fast stain positive, showing pink-red oocysts against a blue background.

• Immature oocysts contain one sporoblast; mature forms have two sporocysts, each with four sporozoites.

• Infection acquired by ingestion of sporulated oocysts; causes prolonged watery diarrhea, especially in immunocompromised hosts.

Bold summary:
Cystoisospora belli is identified in stool by acid-fast–positive oocysts, distinguishing it from non–acid-fast protozoa.

Which specimen is the least likely to provide recovery of Trichomonas vaginalis?

A. Urine

B. Urethral discharge

C. Vaginal discharge

D. Feces

D. Feces

Trichomonas vaginalis Recovery

• T. vaginalis is a urogenital flagellate, not an intestinal parasite.

• Found in vaginal secretions, urethral discharge, and sometimes urine sediments.

• Requires fresh, warm specimens for motility observation; trophozoites die quickly outside the body.

• Fecal samples do not contain T. vaginalis — intestinal conditions are incompatible with its survival.

Bold summary:
T. vaginalis inhabits the urogenital tract, so feces are the least likely specimen to yield the organism.

Which of the following is the best technique to identify Dientamoeba fragilis in stool?

A. Formalin concentrate

B. Trichrome-stained smear

C. Modified acid fast-stained smear

D. Giemsa stain

B. Trichrome-stained smear

Dientamoeba fragilis Identification

• A flagellate without external flagella, often confused with an amoeba.

• Exists only in a trophozoite form — no cyst stage, so concentration methods (e.g., formalin-ethyl acetate) are ineffective.

• Best visualized on permanent stained smears, especially trichrome, where nuclei and cytoplasmic inclusions are clearly defined.

• D. fragilis shows 1–2 nuclei with fragmented chromatin and a granular cytoplasm.

Bold summary:
D. fragilis lacks a cyst stage, so trichrome-stained smears are the best method for identification in stool.

One of the following protozoan organisms has been implicated in waterborne and foodborne outbreaks within the United States. The suspect organism is:

A. Pentatrichomonas hominis

B. Dientamoeba fragilis

C. Giardia lamblia (G. intestinalis, G. duodenalis)

D. Balantidium coli

C. Giardia lamblia (G. intestinalis, G. duodenalis)

Giardia lamblia Outbreaks

• Leading cause of waterborne and foodborne protozoal outbreaks in the U.S.

• Infection via ingestion of cysts in contaminated water, food, or by fecal-oral transmission.

• Infective dose is very low (as few as 10 cysts).

• Cysts: oval, 8–12 µm, with 4 nuclei when mature.

• Trophozoites: pear-shaped, bilaterally symmetrical, with two nuclei and ventral sucking disk.

• Causes “beaver fever” — foul-smelling, greasy diarrhea, bloating, and malabsorption.

Bold summary:
Giardia lamblia is the most common protozoan linked to U.S. waterborne and foodborne outbreaks, transmitted by ingestion of infectious cysts.

Gram staining of a sample from a gum lesion showed what appeared to be amoebae. A trichrome smear showed amoebae with a single nucleus and partially digested PMNs. The correct identification is:

A. Trichomonas tenax

B. Entamoeba histolytica / Entamoeba dispar

C. Entamoeba gingivalis

D. Entamoeba polecki

C. Entamoeba gingivalis

Entamoeba gingivalis Characteristics

• The only Entamoeba species found in the oral cavity (gingival crevices, tonsillar crypts).

• No cyst stage — transmission occurs via direct oral contact (e.g., kissing, sharing utensils).

• Trophozoites contain ingested PMNs or epithelial cells, but not RBCs (distinguishes from E. histolytica).

• Single nucleus with a central karyosome and fine peripheral chromatin.

• Common in poor oral hygiene, periodontal disease, or gum lesions.

Bold summary:
Amoebae with ingested leukocytes from oral lesions identify Entamoeba gingivalis, the oral commensal with no cyst form.

An E. histolytica (true pathogen) trophozoite has the following characteristics:

A. Compact karyosome in the nucleus, ingested RBCs, and clear pseudopodia

B. Ingested RBCs, clear pseudopodia, and uneven chromatin on the nuclear membrane

C. Ingested RBCs, clear pseudopodia, and large glycogen vacuoles in cytoplasm

D. Large, blotlike karyosome, ingested white blood cells (WBCs), and granular pseudopods

A. Compact karyosome in the nucleus, ingested RBCs, and clear pseudopodia

Entamoeba histolytica (true pathogen)

• Trophozoite: 10–60 µm, actively motile with a single clear pseudopod.

• Nucleus: Compact (central) karyosome and evenly distributed peripheral chromatin — distinguishes it from commensal E. coli, which has coarse, irregular chromatin.

• Cytoplasm: finely granular, often containing ingested RBCs (a key pathogenic feature).

• Diagnostic significance: RBC ingestion confirms E. histolytica as a true pathogen rather than nonpathogenic E. dispar.

Bold summary:
Trophozoites of E. histolytica show ingested RBCs, a compact central karyosome, and clear, active pseudopodia — defining features of this pathogenic amoeba.

A 12-year-old girl is brought to the emergency room with meningitis and a history of swimming in a warm-water spring. Motile amoebae that measure 10 um in size are seen in CSF and are most likely:

A. Iodamoeba bütschlii trophozoites

B. Endolimax nana trophozoites

C. Dientamoeba fragilis trophozoites

D. Naegleria fowleri trophozoites

D. Naegleria fowleri trophozoites

Naegleria fowleri (Primary Amoebic Meningoencephalitis – PAM)

• Free-living amoeba found in warm freshwater, hot springs, and poorly chlorinated pools.

• Infects via nasal mucosa → olfactory nerve → brain, causing rapidly fatal meningoencephalitis.

• Trophozoites: ~10–20 µm, motile with eruptive pseudopodia (observed in fresh CSF).

• May convert to a flagellate form in water; cyst form occurs only in environment, not tissue.

• CSF findings: motile trophs, ↑WBCs (neutrophils), ↓glucose, ↑protein.

Bold summary:
Amoebic trophozoites in CSF after warm-water exposure indicate Naegleria fowleri, the cause of primary amoebic meningoencephalitis.

Characteristics of the rhabditiform (noninfective) larvae of Strongyloides stercoralis include a:

A. Short buccal capsule and large genital primordium

B. Long buccal capsule and pointed tail

C. Short buccal capsule and small genital primordium

D. Small genital primordium and notch in tail

A. Short buccal capsule and large genital primordium

Strongyloides stercoralis – Rhabditiform Larva (noninfective stage)

• Found in fresh stool (diagnostic stage).

• Measures ~200–250 µm in length.

• Has a short buccal capsule (mouth cavity) — key differentiator from hookworm larvae.

• Features a prominent genital primordium, visible as a clear oval structure mid-body.

• Filariform larva (infective stage): long esophagus and notched tail.

Bold summary:
Rhabditiform larvae of Strongyloides stercoralis have a short buccal capsule and large genital primordium, distinguishing them from hookworm larvae.

Visceral larva migrans is associated with which of the following organisms and diagnostic method?

A. Toxocara - serology

B. Onchocerca - skin snips

C. Dracunculus - skin biopsy

D. Angiostrongylus - CSF examination

A. Toxocara - serology

Visceral Larva Migrans (VLM)

• Caused by migration of Toxocara canis or T. cati (dog or cat roundworms) larvae in human tissue.

• Humans are accidental hosts — infection via ingestion of embryonated eggs from contaminated soil or pet environments.

• Larvae migrate through liver, lungs, CNS, and eyes but do not mature in humans.

• Diagnosis: by serologic testing (ELISA) — larvae rarely seen in tissue; eosinophilia and elevated IgE common.

Bold summary:
Visceral larva migrans results from tissue migration of Toxocara larvae, and serology is the primary diagnostic method.

The following organisms are linked with specific, relevant information. The incorrect combination is:

A. Stronglyoides stercoralis - internal autoinfection

B. Echinococcus granulosus - hydatid examination

C. Toxoplasma gondii - serology

D. Balantidium coli - common in the United States

D. Balantidium coli - common in the United States

Balantidium coli

• Incorrect pairing — it is rare in the U.S., found mostly in tropical areas or among people with pig exposure.

• Causes balantidiasis, with ulcerative colitis–like diarrhea.

• Large ciliated protozoan with trophozoite and cyst stages.

Other correct pairings:

• Strongyloides stercoralis: causes internal/external autoinfection.

• Echinococcus granulosus: produces hydatid cysts; diagnosed by cyst exam/imaging.

• Toxoplasma gondii: identified by serologic testing (IgM/IgG).

Bold summary:
Balantidium coli infections are rare in the U.S., making D the incorrect association.

Examination of 24-hour unpreserved urine specimen is sometimes helpful in the recovery of:

A. Trichomonas vaginalis trophozoites

B. Schistosoma haematobium eggs

C. Enterobius vermicularis eggs

D. Stronglyoides stercoralis larvae

B. Schistosoma haematobium eggs

Schistosoma haematobium (urinary schistosomiasis)

• Adult flukes inhabit the venous plexus of the bladder.

• Eggs (with a terminal spine) are excreted in urine, especially collected around midday when egg output peaks.

• 24-hour unpreserved urine can increase recovery yield by concentrating eggs.

• Infection may cause hematuria, bladder granulomas, and increased risk of squamous cell carcinoma.

Bold summary:
Schistosoma haematobium produces terminal-spined eggs found in urine, making it the correct answer for 24-hour urine examination.

The examination of sputum may be necessary to diagnose infection with:

A. Paragonimus westermani

B. Trichinella spiralis

C. Wucheria bancrofti

D. Fasciola hepatica

A. Paragonimus westermani

Paragonimus westermani (lung fluke)

• Adult flukes reside in lung tissue, producing eggs expectorated in sputum or swallowed and passed in stool.

• Eggs: golden-brown, operculated, ~80–120 µm long.

• Infection acquired by eating undercooked freshwater crustaceans (crabs or crayfish) containing metacercariae.

• Symptoms mimic tuberculosis — chronic cough, blood-tinged sputum, chest pain, and eosinophilia.

Bold summary:
Paragonimus westermani infection is diagnosed by finding operculated eggs in sputum, confirming lung fluke infection.

Two helminth eggs that may resemble one another are:

A. Diphyllobothrium latum and Paragonimus westermani

B. Opisthorchis sinensis and Fasciolopsis buski

C. Taenia saginata and Hymenolepis nana

D. Ascaris lumbricoides and Trichostrongylus

A. Diphyllobothrium latum and Paragonimus westermani

Egg Similarity

• Both have operculated, oval eggs that are yellow-brown and similar in size (~70–90 µm).

• Diphyllobothrium latum (fish tapeworm): inconspicuous operculum, small abopercular knob at opposite end; found in stool.

• Paragonimus westermani (lung fluke): prominent operculum with shoulders, slightly asymmetrical; found in sputum or stool.

• Differentiation depends on source of specimen and egg morphology detail.

Bold summary:
D. latum and P. westermani eggs both appear operculated and oval, but differ by specimen type and subtle morphological features.

Eating poorly cooked infected pork can lead to an infection with:

A. Taenia solium and Trichinella spiralis

B. Taenia saginata and Hymenolepis nana

C. Trichuris trichiura and Hymenolepis diminuta

D. Diphyllobothrium latum and Ascaris lumbricoides

A. Taenia solium and Trichinella spiralis

Pork-Borne Helminths

• Taenia solium (pork tapeworm): infection occurs by eating undercooked pork containing cysticerci; may also cause cysticercosis if eggs are ingested.

• Trichinella spiralis: acquired by eating raw or undercooked pork containing encysted larvae in muscle tissue; larvae migrate to striated muscle, causing myalgia, fever, and eosinophilia.

• Both are zoonotic parasites linked to poorly cooked pork.

Bold summary:
Consumption of undercooked pork can transmit both Taenia solium (tapeworm) and Trichinella spiralis (tissue roundworm) infections.

An operculated cestode egg that can be recovered from human feces is:

A. Clonorchis siensis

B. Diphyllobothrium latum

C. Paragonimus westermani

D. Dipylidium caninum

B. Diphyllobothrium latum

Diphyllobothrium latum (fish tapeworm)

• The only operculated cestode egg found in human stool.

• Eggs: oval, yellow-brown, operculated, 55–75 µm long, with a small abopercular knob opposite the operculum.

• Acquired by eating undercooked freshwater fish containing plerocercoid larvae.

• May cause vitamin B₁₂ deficiency and megaloblastic anemia.

Bold summary:
Diphyllobothrium latum produces operculated eggs in stool, unique among cestodes infecting humans.

The adult tapeworm of Echinococcus granulosus is found in the intestine of:

A. Dogs

B. Sheep

C. Humans

D. Cattle

A. Dogs

Echinococcus granulosus (Hydatid tapeworm)

• Definitive host: Dogs and other canids, where the adult tapeworm resides in the small intestine.

• Intermediate hosts: Sheep, cattle, and humans, which develop hydatid cysts in organs (especially liver and lungs).

• Humans acquire infection by ingesting eggs shed in dog feces.

• Adult worm is small (3–6 mm), with only three proglottids — immature, mature, and gravid.

Bold summary:
The adult Echinococcus granulosus lives in the intestine of dogs, while humans serve as accidental intermediate hosts developing hydatid cysts.

In infections with Taenia solium, humans can serve as the:

A. Definitive host

B. Intermediate host

C. Either the definitive or the intermediate host

D. None of the above

C. Either the definitive or the intermediate host

Taenia solium (Pork tapeworm)

• Definitive host: Humans harbor the adult tapeworm in the intestine after eating undercooked pork containing cysticerci.

• Intermediate host: Humans can also become accidental intermediate hosts by ingesting eggs, leading to cysticercosis (larval cysts in tissues such as brain, eye, and muscle).

• Normal intermediate host: pigs.

Bold summary:
Humans can act as either the definitive host (intestinal infection) or the intermediate host (cysticercosis) in Taenia solium infection.

Humans acquire infections with Diphyllobothrium latum adult worms by:

A. Ingestion of freshwater crabs

B. Skin penetration of cercariae

C. Ingestion of water chestnuts

D. Ingestion of raw freshwater fish

D. Ingestion of raw freshwater fish

Diphyllobothrium latum (fish tapeworm)

• Humans are infected by eating raw or undercooked freshwater fish containing the plerocercoid (larval) stage.

• Fish become infected after consuming infected copepods (first intermediate host).

• Adult worms develop in the human small intestine, sometimes reaching up to 10 meters long.

• Can cause vitamin B₁₂ deficiency and megaloblastic anemia.

Bold summary:
Diphyllobothrium latum infection occurs after eating raw freshwater fish harboring the plerocercoid larval stage.

Humans can serve as both the intermediate and definitive host in infections caused by:

A. Enterobius vermicularis

B. Hymenolepis nana

C. Schistosoma japonicum

D. Ascaris lumbricoides

B. Hymenolepis nana

Hymenolepis nana (dwarf tapeworm)

• Unique among cestodes because it can complete its life cycle entirely in humans.

• Humans act as both intermediate and definitive host through internal autoinfection.

• Infection occurs by ingesting eggs containing oncospheres, which hatch in the intestine → develop into cysticercoid larvae in intestinal villi → mature into adult worms in the same host.

• Common in children and associated with crowded or unsanitary conditions.

Bold summary:
Hymenolepis nana is the only human tapeworm capable of autoinfection, allowing humans to serve as both intermediate and definitive host.

Babesia is an organism that has been implicated in disease from both splenectomized and nonsplenectomized patients. Morphologically, the parasites resemble:

A. Plasmodium falciparum rings

B. Leishmania donovani amastigotes

C. Trypanosoma cruzi trypomastigotes

D. Microsporidian spores

A. Plasmodium falciparum rings

Babesia spp. (babesiosis)

• Intraerythrocytic protozoa transmitted by the Ixodes tick (same vector as Lyme disease).

• Morphology: small ring forms within RBCs that closely resemble P. falciparum trophozoites but lack hemozoin pigment and do not produce schizonts or gametocytes.

• Diagnostic clue: may show Maltese cross formation (tetrads of merozoites).

• Causes hemolytic anemia and fever, more severe in asplenic or immunocompromised patients.

Bold summary:
Babesia parasites mimic Plasmodium falciparum ring forms in RBCs but are distinguished by absence of pigment and possible Maltese cross formations.

Organisms (and infections) that under normal conditions cannot be transmitted in the laboratory are:

A. Cryptosporidium - cryptosporidiosis

B. Taenia solium - cysticercosis

C. Ascaris lumbricoides - ascariasis

D. Enterobius vermicularis - pinworm infections

C. Ascaris lumbricoides - ascariasis

Ascaris lumbricoides (large intestinal roundworm)

• Eggs passed in stool are not immediately infective — they must embryonate in soil for 2–3 weeks before becoming infective.

• Because of this environmental maturation requirement, Ascaris cannot be transmitted directly in a laboratory setting.

• Infection occurs only by ingesting embryonated eggs from contaminated soil or food, not fresh specimens.

Other options:

• Cryptosporidium: oocysts are immediately infective; accidental ingestion is a real lab hazard.

• Taenia solium: eggs are infective if ingested, posing risk of cysticercosis.

• Enterobius vermicularis: eggs are airborne and infective within hours, easily spread.

Bold summary:
Ascaris lumbricoides eggs require time to embryonate, so fresh specimens aren’t infective — making it non-transmissible in the lab.

Toxoplasma gondii is characterized by:

A. Possible congenital infection and ingestion of oocysts

B. Cosmopolitan distribution and possible difficulties with interpretation of serological results

C. None of the above

D. Both A and B

D. Both A and B

Toxoplasma gondii Characteristics

• Transmission: by ingestion of oocysts from cat feces or tissue cysts in undercooked meat.

• Congenital infection: occurs when a nonimmune pregnant woman acquires primary infection — can cause hydrocephalus, chorioretinitis, and intracranial calcifications in the fetus.

• Distribution: worldwide (cosmopolitan); infection is common but often subclinical in immunocompetent hosts.

• Diagnosis: primarily by serology (IgM/IgG), though results can be difficult to interpret due to persistence of IgG antibodies.

Bold summary:
Toxoplasma gondii shows global distribution, is transmitted by ingestion of oocysts or cysts, and may cause congenital infection — hence both A and B are correct.

Oocysts of Cryptosporidium spp can be detected in stool specimens by using:

A. Modified acid-fast stain

B. Gram stain

C. Methenamine silver stain

D. Trichrome stain

A. Modified acid-fast stain

Cryptosporidium spp. Detection

• Oocysts are acid-fast positive, appearing bright red or pink against a blue background with the modified Ziehl–Neelsen or Kinyoun stain.

• Size: 4–6 µm, smaller than Cyclospora (8–10 µm).

• Common cause of self-limited watery diarrhea in immunocompetent hosts and severe, chronic diarrhea in immunocompromised patients (e.g., AIDS).

• Other detection methods: immunofluorescence assays and EIA antigen tests.

Bold summary:
Cryptosporidium oocysts are identified in stool using the modified acid-fast stain, which highlights small red oocysts on a blue background.

Which microfilariae are usually not found circulating in peripheral blood?

A. Brugia malayi

B. Wuchereria bancrofti

C. Onchocerca volvulus

D. Loa loa

C. Onchocerca volvulus

Onchocerca volvulus (river blindness)

• Microfilariae occur in skin and subcutaneous tissue, not in blood.

• Transmitted by the blackfly (Simulium spp.) near fast-flowing rivers.

• Adults form subcutaneous nodules; larvae cause itching, dermatitis, and blindness.

• Diagnosed by skin snips.

Other species:

• Brugia malayi & Wuchereria bancrofti: microfilariae in blood (lymphatic filariasis).

• Loa loa: diurnal blood microfilariae, causes Calabar swellings.

Bold summary:
Onchocerca volvulus microfilariae stay in skin, not blood—hence the correct answer.

Massive hemolysis, blackwater fever, and central nervous system involvement are most common with:

A. Plasmodium vivax

B. Plasmodium falciparum

C. Plasmodium ovale

D. Plasmodium malariae

B. Plasmodium falciparum

Plasmodium falciparum (malignant tertian malaria)

• Most severe and fatal form of malaria.

• Causes massive intravascular hemolysis, leading to blackwater fever (hemoglobinuria).

• CNS involvement (cerebral malaria) due to parasitized RBCs adhering to endothelium, causing microvascular obstruction.

• High parasitemia possible because P. falciparum infects RBCs of all ages.

• Characteristic multiple ring forms per RBC and banana-shaped gametocytes.

Bold summary:
Plasmodium falciparum causes severe hemolysis, blackwater fever, and cerebral malaria, making it the most dangerous Plasmodium species.

Organisms that should be considered in a nursery school outbreaks of diarrhea include:

A. Endolimax nana, Giardia lamblia (G. intestinalis, G. duodenalis), and Entamoeba coli

B. Giardia lamblia (G. intestinalis, G. duodenalis), Dientamoeba fragilis, and Cryptosporidium parvuvm

C. Cryptosporidium spp, Trichomonas vaginalis, and Entamoeba coli

D. Trichomonas hominis, Dientamoeba fragilis, and Endolimax nana

B. Giardia lamblia (G. intestinalis, G. duodenalis), Dientamoeba fragilis, and Cryptosporidium parvuvm

Nursery School Diarrhea Outbreaks

• Common pathogens: Giardia lamblia, Dientamoeba fragilis, and Cryptosporidium parvum.

• All spread fecal-orally through contaminated hands, toys, food, or water.

• Giardia: greasy, foul diarrhea and bloating.

• Dientamoeba: mild, chronic diarrhea without cyst stage.

• Cryptosporidium: watery diarrhea, especially in children and immunocompromised hosts.

Bold summary:
Daycare and nursery outbreaks of diarrhea typically involve Giardia, Dientamoeba, and Cryptosporidium, all transmitted fecal-orally.

The incorrect match between disease and symptoms is:

A. Paragonimiasis - hemoptysis

B. Cryptosporidiosis - watery diarrhea

C. Toxoplasmosis in compromised host - central nervous system symptoms

D. Enterobiasis - dysentery

D. Enterobiasis - dysentery

Enterobiasis (pinworm infection)

• Caused by Enterobius vermicularis.

• Incorrect pairing — it causes perianal itching, not dysentery.

• Eggs are laid on perianal folds at night → intense pruritus → secondary bacterial infection possible.

Other correct pairings:

• Paragonimiasis: hemoptysis from lung fluke infection.

• Cryptosporidiosis: watery diarrhea, especially in immunocompromised hosts.

• Toxoplasmosis: in compromised hosts, causes CNS involvement (encephalitis, seizures).

Bold summary:
Enterobius vermicularis causes perianal itching, not dysentery — making D the incorrect match.

The formalin-ether (ethyl acetate) concentration procedure for feces is used to demonstrate:

A. Motility of helminth larvae

B. Protozoan cysts and helminth eggs

C. Formation of amoebic pseudopods

D. Trophozoites

B. Protozoan cysts and helminth eggs

Formalin–ether (ethyl acetate) concentration method

• A stool concentration technique that separates parasites from fecal debris.

• Preserves morphology of protozoan cysts, helminth eggs, and larvae for easier detection.

• Uses formalin to fix organisms and ether or ethyl acetate to extract fat and debris, forming a sediment for microscopic exam.

• Not suitable for demonstrating trophozoite motility or pseudopod formation (live activity lost during fixation).

Bold summary:
The formalin–ether concentration method concentrates protozoan cysts and helminth eggs, not motile trophozoites.

Cysts of Iodamoeba bütschlii typically have:

A. Chromatoidal bars with rounded ends

B. A heavily vacuolated cytoplasm

C. A large glycogen vacuole

D. Many ingested bacteria and yeast cells

C. A large glycogen vacuole

Iodamoeba bütschlii (nonpathogenic intestinal amoeba)

• Cysts: 8–20 µm, single nucleus with large, eccentric karyosome.

• Characterized by a prominent glycogen vacuole that stains brown with iodine — its most distinctive feature.

• Cytoplasm is coarsely granular, without chromatoidal bars.

• Trophozoites may contain bacteria or debris but are nonpathogenic.

Bold summary:
Cysts of Iodamoeba bütschlii show a single nucleus and a large glycogen vacuole, the hallmark diagnostic feature.

The miracidial hatching test helps to demonstrate the viability of eggs of:

A. Taenia species

B. Schistosoma species

C. Hookworm species

D. Opisthorchis species

B. Schistosoma species

Miracidial Hatching Test

• Used to determine viability of Schistosoma eggs in feces or urine.

• Eggs are placed in freshwater under light, which stimulates hatching of miracidia (the ciliated larval stage).

• Presence of swimming miracidia confirms live, viable eggs — important for assessing infection activity or treatment success.

• Schistosoma eggs hatch directly in water; other helminths (e.g., Taenia, hookworm) require intermediate hosts or specific environmental conditions.

Bold summary:
The miracidial hatching test confirms live Schistosoma eggs by detecting the emergence of ciliated miracidia in water.

Organisms that should be considered in a waterborne outbreak of diarrheal disease include:

A. Giardia lamblia (G. intestinalis, G. duodenalis) and Cryptosporidium spp

B. Endolimax nana and Entamoeba histolytica

C. Blastocystis spp and Trichomonas vaginalis

D. Toxoplasmosis gondii and Schistosoma mansoni

A. Giardia lamblia (G. intestinalis, G. duodenalis) and Cryptosporidium spp

Waterborne Diarrheal Pathogens

• Both Giardia and Cryptosporidium cause major waterborne outbreaks via ingestion of resistant cysts or oocystsin contaminated water.

• Giardia: produces greasy, foul-smelling diarrhea, bloating, and malabsorption.

• Cryptosporidium: causes watery diarrhea, especially severe in immunocompromised hosts.

• Both are chlorine resistant, allowing transmission even in treated water supplies.

Bold summary:
Giardia and Cryptosporidium are chlorine-resistant protozoa responsible for waterborne diarrheal outbreaksworldwide.

Fecal immunoassays have become more commonly used to diagnosis infections with:

A. Endolimax nana and Blastocystis spp

B. Giardia lamblia (G. intestinalis, G. duodenalis) and Cryptosporidium spp

C. Ascaris lumbricoides and Trichuris trichiura

D. Strongyloides stercoralis and Trichomonas vaginalis

B. Giardia lamblia (G. intestinalis, G. duodenalis) and Cryptosporidium spp

Fecal Immunoassays

• Detect parasite antigens directly in stool using EIA (enzyme immunoassay) or rapid immunochromatographic tests.

• Most widely used for Giardia and Cryptosporidium, which may be missed on routine O&P exams due to intermittent shedding or small size.

• Provide higher sensitivity and specificity than microscopy.

• Results available in hours, useful for outbreak and clinical screening.

Bold summary:
Modern fecal immunoassays are primarily used to detect Giardia and Cryptosporidium antigens in stool with greater accuracy than microscopy.

The most common infections with the microsporidia often originate in:

A. The lung

B. The nervous system

C. The gastrointestinal tract

D. Mucocutaneous lesions

C. The gastrointestinal tract

Microsporidia Infections

• Obligate intracellular spore-forming parasites (now classified as fungi-related).

• Most common human infections originate in the GI tract, causing chronic watery diarrhea, malabsorption, and weight loss—especially in immunocompromised patients (e.g., AIDS).

• Species such as Enterocytozoon bieneusi and Encephalitozoon intestinalis infect intestinal epithelial cells.

• Infection occurs via ingestion of spores in contaminated food or water.

• Other less common sites: ocular, respiratory, and disseminated infections.

Bold summary:
Microsporidia most often infect the gastrointestinal tract, producing chronic diarrhea in immunocompromised hosts.

Eye infections with Acanthamoeba spp have most commonly been traced to:

A. Use of soft contact lenses

B. Use of hard contact lenses

C. Use of contaminated lens care solutions

D. Failure to remove lenses which swimming

C. Use of contaminated lens care solutions

Acanthamoeba Keratitis

• Acanthamoeba are free-living amoebae found in tap water, soil, and lens-cleaning solutions.

• Most eye infections result from contaminated contact lens solutions or cases, allowing amoebae to colonize the cornea.

• Risk factors: poor lens hygiene, homemade saline, or rinsing lenses with tap water.

• Causes severe keratitis with pain, photophobia, and possible vision loss.

• Diagnosed via corneal scrapings showing trophozoites and cysts.

Bold summary:
Acanthamoeba eye infections most often stem from contaminated contact lens solutions, not the lenses themselves.

Select the most sensitive recovery method for Acanthamoeba spp from lens care solutions or corneal biopsies.

A. The trichrome staining method

B. The use of monoclonal reagents for the detection of antibody

C. The use of non-nutrient agar cultures seeded with Escherichia coli

D. The Giemsa stain method

C. The use of non-nutrient agar cultures seeded with Escherichia coli

Acanthamoeba Recovery and Diagnosis

• Most sensitive method: inoculating non-nutrient agar plates seeded with E. coli.

• Acanthamoeba trophozoites feed on the bacterial lawn, producing visible tracks or clear zones within 24–48 hours.

• Useful for culturing from corneal scrapings, biopsies, or contaminated lens solutions.

• Microscopic stains (trichrome, Giemsa) can show trophozoites and cysts but are less sensitive and mainly confirmatory.

• Serologic and antibody-based methods have limited diagnostic value for keratitis.

Bold summary:
Acanthamoeba is best recovered by non-nutrient agar culture seeded with E. coli, the most sensitive diagnostic method for lens-related and corneal infections.

The microsporidia are organisms (now classified with the fungi) that have been implicated in human disease primarily in:

A. Immunocompromised patients

B. Pediatric patients under age 5 years

C. Adult patients with congenital immunodeficiencies

D. Patients who have been traveling in the tropics

A. Immunocompromised patients

Microsporidia (fungus-related obligate intracellular parasites)

• Cause opportunistic infections, mainly in immunocompromised individuals, especially those with AIDS or undergoing immunosuppressive therapy.

• Most commonly infect the gastrointestinal tract, producing chronic watery diarrhea, but may also involve ocular, respiratory, or disseminated disease.

• Transmission occurs via ingestion or inhalation of spores from contaminated water or surfaces.

• Rare in immunocompetent hosts, where infections are usually mild and self-limited.

Bold summary:
Microsporidia primarily cause opportunistic infections in immunocompromised patients, especially those with advanced HIV/AIDS.

When staining Cystoisospora belli oocysts with modified acid-fast stains, the important difference between these methods and the acid-fast stains used for acid-fast bacilli (AFB) is:

A. The staining time is much longer with regular AFB acid-fast stains

B. The decolorizer is much weaker than acid alcohol used for AFB decolorizing

C. A counterstain must be used for the modified methods

D. The staining is more concentrated when staining for AFB

B. The decolorizer is much weaker than acid alcohol used for AFB decolorizing

Modified Acid-Fast Stain (for intestinal coccidia)

• Used to detect oocysts of Cystoisospora belli, Cryptosporidium, and Cyclospora in stool.

• The key difference from traditional AFB staining is the use of a weaker decolorizer (e.g., 1% sulfuric acid) instead of acid alcohol.

• This mild decolorization allows the delicate oocyst wall to retain carbol fuchsin stain, appearing red or pink on a blue background.

• AFB methods for mycobacteria use strong acid-alcohol, which would over-decolorize these protozoan oocysts.

Bold summary:
Modified acid-fast stains use a weaker decolorizer than AFB stains, allowing intestinal oocysts to retain the red stain.

The incorrect match between symptoms and disease is:

A. Dysentery and amebiasis

B. Malabsorption syndrome and giardiasis

C. Cardiac involvement and chronic Chagas disease

D. Myalgias and trichuriasis

D. Myalgias and trichuriasis

Trichuriasis (whipworm infection)

• Caused by Trichuris trichiura and involves the large intestine, not muscle tissue.

• Symptoms: abdominal pain, tenesmus, bloody diarrhea, and in heavy infections, rectal prolapse — not myalgia.

Other correct matches:

• Amebiasis (Entamoeba histolytica): causes dysentery with bloody, mucus-filled stools.

• Giardiasis: causes malabsorption syndrome, steatorrhea, and bloating.

• Chronic Chagas disease (Trypanosoma cruzi): involves the heart, leading to cardiomyopathy and arrhythmias.

summary:
Trichuriasis affects the intestine, not muscle, so myalgia is incorrectly paired — making D the wrong match.

The incorrect match between organisms and characteristic is:

A. Chilomastix mesnili and Shepherd's crook and lemon shape

B. Plasmodium malariae and "band trophozoite"

C. Hymenolepis nana and striated shell

D. Wuchereria bancrofti and sheathed microfilariae

C. Hymenolepis nana and striated shell

Hymenolepis nana (dwarf tapeworm)

• Incorrect pairing — H. nana eggs have a thin, smooth shell, not striated.

• Characteristic features: hexacanth embryo (oncosphere) and two polar thickenings with filaments extending inward.

Other correct matches:

• Chilomastix mesnili: trophozoite has a “shepherd’s crook” cytostome, and cyst is lemon-shaped with a single nucleus.

• Plasmodium malariae: trophozoite often forms a band across the RBC.

• Wuchereria bancrofti: produces sheathed microfilariae in night blood.

Bold summary:
Hymenolepis nana eggs have a smooth, thin shell, not a striated one — making C the incorrect match.

The incorrect match between method and method objective is:

A. Direct wet examination and detection of organism motility

B. Knott concentration and the recovery of operculated helminth eggs

C. Baermann concentration and the recovery of Strongyloides

D. Permanent stained fecal smear and confirmation of protozoa

B. Knott concentration and the recovery of operculated helminth eggs

Knott Concentration Method

• Incorrect pairing — the Knott technique is designed for concentration of microfilariae from blood, notoperculated helminth eggs.

• Used in diagnosis of filariasis caused by Wuchereria bancrofti, Brugia malayi, and Loa loa.

Other correct matches:

• Direct wet exam: detects motility of trophozoites and larvae.

• Baermann concentration: used for recovery of Strongyloides stercoralis larvae from stool.

• Permanent stained smear (e.g., trichrome): confirms protozoan morphology and nuclear detail.

Bold summary:
The Knott concentration method targets microfilariae, not operculated helminth eggs, making B the incorrect match.

The incorrect match between organism and characteristic/diagnostic method is:

A. Dientamoeba fragilis and tetrad karyosome in the nucleus

B. Toxoplasma gondii and diagnostic serology

C. Echinococcus granulosus and daughter cysts

D. Schistosoma mansoni and egg with terminal spine

D. Schistosoma mansoni and egg with terminal spine

Schistosoma mansoni

• Incorrect pairing — S. mansoni eggs have a large lateral spine, not a terminal one.

• Found in feces, measuring 114–180 µm, and are non-operculated.

• Causes intestinal schistosomiasis with portal hypertension and hepatosplenomegaly.

Other correct matches:

• Dientamoeba fragilis: trophozoites with 1–2 nuclei, fragmented karyosomes, seen on trichrome stains.

• Toxoplasma gondii: diagnosed by serology (IgM/IgG titers).

• Echinococcus granulosus: forms hydatid cysts with daughter cysts in intermediate hosts.

Bold summary:
Schistosoma mansoni eggs have a lateral, not terminal, spine — making D the incorrect match.

There are few procedures considered STAT in parasitology. The most obvious situation would be:

A. Ova and parasite examination for giardiasis

B. Baermann concentration for strongyloidiasis

C. Blood films for malaria

D. Culture of amebic keratitis

C. Blood films for malaria

STAT Testing in Parasitology

• Malaria is a medical emergency, requiring immediate diagnosis and treatment to prevent severe disease or death.

• Thick and thin blood films are examined as soon as possible (STAT) to detect Plasmodium species and estimate parasitemia.

• Delay in diagnosis can lead to rapid clinical deterioration, especially with P. falciparum.

Other options:

• Giardiasis, strongyloidiasis, and amebic keratitis are important but not emergent; routine processing is acceptable.

Bold summary:
Blood films for malaria are among the few true STAT procedures in parasitology due to the potential for rapid disease progression.

An immunosuppressed man has several episodes of pneumonia, intestinal pain, sepsis with gram-negative rods, and a history of military service in Southeast Asia 20 years earlier. The most likely cause is infection with:

A. Trypanosoma cruzi

B. Strongyloides stercoralis

C. Naegleria fowleri

D. Paragonimus westermani

B. Strongyloides stercoralis

Strongyloides Hyperinfection

• Immunosuppression (e.g., steroids) → risk of hyperinfection/dissemination with autoinfection.

• Larvae penetrate gut → carry enteric gram-negative rods into bloodstream → sepsis/pneumonia.

• Epidemiology: endemic in Southeast Asia; remote exposure (e.g., military service) can persist for decades.

• Diagnosis: multiple stool exams (larvae, not eggs), sputum larvae, or serology.

Bold summary:
Past exposure plus immunosuppression and gram-negative sepsis points to Strongyloides stercoralis hyperinfection.

In a pediatric patient, the recommended clinical specimens for recovery of Enterobius vermicularis are the:

A. Stool specimens

B. Sigmoidoscopy scrapings

C. Duodenal aspirates

D. Series of cellophane tape preparations

D. Series of cellophane tape preparations

Enterobius vermicularis (pinworm)

• Best diagnostic method: cellophane (Scotch) tape preparation applied to the perianal region in the early morning before bathing or defecation.

• Adult females migrate to perianal folds at night to lay eggs; eggs are rarely found in stool.

• Recommended to collect 3 consecutive morning samples to maximize recovery.

• Microscopic exam reveals oval, flattened eggs with a colorless shell and developing larva inside.

Bold summary:
Diagnosis of Enterobius vermicularis relies on cellophane tape preps, not stool exams, since eggs are deposited perianally.

Eosinophils meningoencephalitis is a form of larva migrans, causing fever, headache, stiff neck, and increased cells in the spinal fluid. It is caused by:

A. Necator americanus

B. Angiostrongylus cantonensis

C. Ancylostoma braziliense

D. Strongyloides stercoralis

B. Angiostrongylus cantonensis

Angiostrongylus cantonensis (rat lungworm)

• Causes eosinophilic meningoencephalitis, a neural form of larva migrans.

• Humans = accidental hosts, infected by eating raw or undercooked snails, slugs, or contaminated producecontaining larvae.

• Larvae migrate to the CNS, causing fever, headache, neck stiffness, and marked eosinophilia in CSF.

• Endemic in Southeast Asia, Pacific Islands, and the Caribbean.

Bold summary:
Angiostrongylus cantonensis infection leads to eosinophilic meningoencephalitis after ingestion of infected snails or contaminated produce.

"Cultures of parasites are different from bacterial cultures; no quality control is needed." This statement is:

A. True, if two tubes of media are set up for each patient

B. True, if the media are checked every 24 hours

C. False, unless two different types of media are used

D. False, and organism and media controls need to be set up

D. False, and organism and media controls need to be set up

Parasite Culture Quality Control

• This statement is false — parasitology cultures require the same QC standards as bacteriology and mycology.

• Media must be quality-controlled for sterility and ability to support growth before use.

• Known positive control organisms should be inoculated periodically to verify media performance.

• Regular checks ensure media integrity, temperature control, and absence of contamination.

Bold summary:
Parasitic cultures require full QC, including media and organism controls, just like bacterial cultures—making D the correct answer.

Protozoan cysts were seen in a concentration sediment and tentatively identified as Entamoeba coli. However, the organisms were barely visible on the permanent stained smear because:

A. The organisms were actually not present in the concentration sediment

B. There were too few cysts to allow identification on the stained smear

C. Entamoeba coli cysts were present but poorly fixed due to the thick cysts wall

D. The concentrate and permanent stained smear were not from the same patient

C. Entamoeba coli cysts were present but poorly fixed due to the thick cysts wall

• Entamoeba coli cysts have a thick, refractile wall that can interfere with proper fixation and staining during permanent smear preparation.

• As a result, cysts may appear faint or barely visible on trichrome or iron hematoxylin stains, even though they were seen clearly in the concentrated wet mount.

• The thick wall resists penetration by fixatives and stains, reducing contrast and detail.

• This problem is specific to certain cystic protozoa (especially E. coli and Endolimax nana) and not due to low numbers or poor sampling.

Bold summary:
Entamoeba coli cysts may stain poorly because their thick cyst walls resist fixation, causing them to appear barely visible on permanent stained smears.

When humans have hydatid disease, the causative agent and host classification are:

A. Echinococcus granulosus - accidental intermediate host

B. Echinococcus granulosus - definitive host

C. Taenia solium - accidental intermediate host

D. Taenia solium - definitive host

A. Echinococcus granulosus - accidental intermediate host

Hydatid Disease (Echinococcosis)

• Caused by the larval stage of Echinococcus granulosus (dog tapeworm).

• Normal cycle:

  • Definitive host: dogs and other canids (harbor the adult tapeworm in the intestine).

  • Intermediate host: sheep, cattle, etc. (develop hydatid cysts in organs).

• Humans become accidental intermediate hosts by ingesting eggs from dog feces.

• The larvae migrate to organs (usually liver or lungs), forming hydatid cysts containing daughter cysts and protoscolices.

Bold summary:
In hydatid disease, humans accidentally serve as intermediate hosts for Echinococcus granulosus after ingesting infective eggs.

A 45-year-old hunter developed fever, myalgia, and periorbital edema. He has a history of bear meat consumption. The most likely causative agent is:

A. Toxoplasma gondii

B. Taenia solium

C. Hymenolepis nana

D. Trichinella spiralis

D. Trichinella spiralis

Trichinellosis (Trichinosis)

• Acquired by eating undercooked or raw meat (especially bear, pork, or wild game) containing encysted larvaeof Trichinella spiralis.

• After ingestion, larvae mature in the intestine and migrate via the bloodstream to striated muscle, where they encyst.

• Classic triad of symptoms:

  • Fever

  • Myalgia (muscle pain)

  • Periorbital edema

• Lab findings: eosinophilia, elevated muscle enzymes (CK, LDH).

Bold summary:
Consumption of bear meat followed by fever, myalgia, and periorbital edema is diagnostic of Trichinella spiralisinfection (trichinellosis).

In a condition resulting from the accidental ingestion of eggs, the human becomes the intermediate host, rather than the definitive host. The correct answer is:

A. Trichinosis

B. Cysticercosis

C. Ascariasis

D. Strongyloidiasis

B. Cysticercosis

Cysticercosis (larval Taenia solium infection)

• Occurs when humans accidentally ingest eggs of the pork tapeworm (Taenia solium), rather than cysticerci in meat.

• The oncospheres hatch in the intestine, penetrate the intestinal wall, and migrate to tissues (brain, eye, muscle) where they form cysticerci (larval cysts).

• In this scenario, humans act as the intermediate host instead of the definitive host.

• Can lead to neurocysticercosis, the most common parasitic cause of seizures worldwide.

Bold summary:
In cysticercosis, humans ingest Taenia solium eggs and become the intermediate host, with larvae encysting in tissues.

A transplant recipient on immunosuppressive drugs developed increasing diarrhea. The most likely combination of disease and diagnostic procedure is:

A. Trichinosis and trichrome staining

B. Microsporidiosis and modified trichrome staining

C. Toxoplasmosis and Gram staining

D. Paragonimiasis and wet preparation

B. Microsporidiosis and modified trichrome staining

Microsporidiosis (Microsporidia infection)

• Opportunistic infection in immunocompromised or transplant patients, causing chronic watery diarrhea and malabsorption.

• Caused by obligate intracellular spore-forming parasites such as Enterocytozoon bieneusi and Encephalitozoon intestinalis.

• Diagnosis:

  • Modified trichrome stain (chromotrope 2R) highlights tiny oval spores (1–2 µm) that stain bright red with a clear halo.

  • Can also use electron microscopy or PCR for confirmation.

Bold summary:
In an immunosuppressed transplant patient with diarrhea, Microsporidia infection is suspected and confirmed by modified trichrome staining of stool or tissue.

After returning from a 2-year stay in India, a patient has eosinophilia, an enlarged left spermatic cord, and bilateral inguinal lymphadenopathy. The most likely clinical specimen and organism match is:

A. Thin blood films and Leishmania

B. Urine and concentration for Trichomonas vaginalis

C. Thin blood films and Babesia

D. Thick blood films and microfilariae

D. Thick blood films - microfilariae

Lymphatic Filariasis (likely Wuchereria bancrofti)

• Clinical clues:

  • Eosinophilia, lymphangitis, enlarged spermatic cord, and inguinal lymphadenopathy are classic for lymphatic filariasis.

• Causative agent: Wuchereria bancrofti (most common), transmitted by mosquito bites in tropical regions such as India.

• Diagnostic specimen: Thick blood film, collected at night (nocturnal periodicity) to detect sheathed microfilariae.

• Complications: chronic infection can lead to elephantiasis of the legs or genitals.

Bold summary:
Post-India eosinophilia with lymphatic swelling indicates Wuchereria bancrofti infection, diagnosed by finding microfilariae on a nocturnal thick blood film.

Patients with severe diarrhea should use "enteric precautions" to prevent nosocomial infections with:

A. Giardia lamblia (G. intestinalis, G. duodenalis)

B. Ascaris lumbricoides

C. Cryptosporidium spp

D. Cystoisospora belli

C. Cryptosporidium spp

Cryptosporidium and Enteric Precautions

• Cryptosporidium oocysts are immediately infective upon excretion and are highly resistant to chlorine, enabling easy person-to-person and waterborne transmission.

• Causes profuse watery diarrhea, particularly severe in immunocompromised patients.

• Enteric (contact) precautions are essential in healthcare settings to prevent nosocomial spread via contaminated hands, surfaces, or shared equipment.

• Handwashing with soap and water (not alcohol-based sanitizer) is required because alcohol does not kill oocysts.

Bold summary:
Patients with Cryptosporidium infection require enteric precautions since the oocysts are immediately infective and highly resistant to disinfectants.

A 60-year-old Brazilian patient with cardiac irregularities and congestive heart failure suddenly dies. Examination of the myocardium revealed numerous amastigotes, an indication that the cause of death was most likely:

A. Leishmaniasis with Leishmania donovani

B. Leishmaniasis with Leishmania braziliense

C. Trypanosomiasis with Trypanosoma gambiense

D. Trypanosomiasis with Trypanosoma cruzi

D. Trypanosomiasis with Trypanosoma cruzi

Chagas Disease (American Trypanosomiasis)

• Caused by Trypanosoma cruzi, transmitted by reduviid (kissing) bugs, endemic in Central and South America, including Brazil.

• Amastigotes are the intracellular stage found in cardiac muscle, smooth muscle, and ganglia.

• Chronic infection leads to myocarditis, cardiomegaly, arrhythmias, and congestive heart failure — often resulting in sudden cardiac death.

• Diagnosis in chronic cases may involve serology or histologic ID of amastigotes in tissue; acute cases show trypomastigotes in blood.

Bold summary:
Numerous amastigotes in heart tissue of a Brazilian patient indicate Trypanosoma cruzi infection (Chagas disease), the cause of fatal myocarditis and heart failure.

When analysis of blood smears for malaria are requested, what patient information should be obtained?

A. Diet, age, gender

B. Age, antimalarial medication, gender

C. Travel history, antimalarial medication, date of return to the United States

D. Fever patterns, travel history, diet

C. Travel history, antimalarial medication, date of return to United States

Malaria Workup Essentials

• Accurate malaria diagnosis depends heavily on epidemiologic and exposure information.

• Travel history identifies potential Plasmodium species based on geographic distribution.

• Antimalarial medication history clarifies whether symptoms are due to treatment failure, resistance, or relapse.

• Date of return to the U.S. helps determine incubation period, which varies by species (P. falciparum typically 7–10 days; P. vivax/ovale may relapse months later).

• Fever pattern alone isn’t diagnostic without exposure context.

Bold summary:
When malaria smears are ordered, always document travel history, antimalarial drug use, and return date to guide species identification and clinical interpretation.

In an outbreak of diarrheal disease traced to a municipal water supply, the most likely causative agent is:

A. Cryptosporidium spp

B. Cystoisospora belli

C. Entamoeba histolytica

D. Dientamoeba fragilis

A. Cryptosporidium spp

Cryptosporidium and Waterborne Outbreaks

• Cryptosporidium is the most common cause of municipal waterborne diarrheal outbreaks in the U.S. and worldwide.

• Oocysts are immediately infective, highly resistant to chlorine, and small enough to pass through some filtration systems.

• Causes profuse watery diarrhea, abdominal cramps, and nausea; severe or chronic in immunocompromised patients.

• Diagnosis: modified acid-fast stain or stool antigen testing (EIA).

Bold summary:
Cryptosporidium is the leading cause of chlorine-resistant, waterborne diarrheal outbreaks linked to municipal water supplies.

Within the United States, sporadic mini-outbreaks of diarrheal disease have been associated with the ingestion of strawberries, raspberries, fresh basil, mesclun (baby lettuce leaves), and snow peas. The most likely causative agent is:

A. Dientamoeba fragilis

B. Cyclospora cayetanensis

C. Schistosoma mansoni

D. Cystoisospora belli

B. Cyclospora cayetanensis

Cyclospora cayetanensis (Cyclosporiasis)

• Causes prolonged watery diarrhea often linked to fresh produce (raspberries, basil, lettuce, snow peas, etc.).

• Transmission: ingestion of sporulated oocysts from contaminated food or water.

• Key point: oocysts passed in stool are not immediately infective — they must sporulate in the environment(unlike Cryptosporidium).

• Diagnosis: detection of oocysts (8–10 µm) that are variably acid-fast positive or autofluorescent under UV light.

• Treatment: trimethoprim-sulfamethoxazole (TMP-SMX).

Bold summary:
Cyclospora cayetanensis causes foodborne diarrheal outbreaks from fresh produce like berries and herbs due to ingestion of sporulated oocysts.

Which of the following statements is true regarding onchocerciasis?

A. The adult worm is present in blood

B. The microfilariae are in blood during the late evening hours

C. The diagnostic test of choice is the skin snip

D. The parasite resides in the deep lymphatics

C. The diagnostic test of choice is the skin snip

Onchocerciasis (Onchocerca volvulus, “river blindness”)

• Transmission: via blackfly (Simulium spp.) breeding in fast-flowing rivers.

• Adults: reside in subcutaneous nodules (onchocercomas), not blood or lymphatics.

• Microfilariae: migrate through skin and eyes, causing pruritus, dermatitis, and blindness; absent from blood.

• Diagnosis: skin snip test — a shallow dermal incision (often from the iliac crest or scapula) is placed in saline and examined microscopically for emerging motile microfilariae.

Bold summary:
Onchocerca volvulus is diagnosed by a skin snip, detecting microfilariae emerging from skin tissue rather than circulating in blood.

The most prevalent helminth to infect humans is:

A. Enterobius vermicularis, the pinworm

B. Ascaris lumbricoides, the large intestinal roundworm

C. Taenia saginata, the beef tapeworm

D. Schistosoma mansoni, one of the blood flukes

A. Enterobius vermicularis, the pinworm

In the United States (and other industrialized regions):

• Enterobius vermicularis is by far the most common helminth infection, particularly among children.

• Transmitted by fecal–oral spread of eggs, often via contaminated hands, bedding, or clothing.

• Causes perianal itching, especially at night, due to nocturnal egg deposition.

• Diagnosis: early morning cellophane tape prep from perianal folds.

A helminth egg is described as having terminal polar plugs. The most likely helminth is:

A. Hookworm

B. Trichuris trichiura

C. Fasciola hepatica

D. Diphyllobothrium latum

B. Trichuris trichiura

Trichuris trichiura (whipworm)

• Egg morphology: barrel-shaped with a thick brown shell and distinctive bipolar (terminal) plugs at each end.

• Eggs are unembryonated when passed in stool and become infective after embryonation in soil.

• Infection occurs via ingestion of embryonated eggs from contaminated soil or produce.

• Adults inhabit the cecum and ascending colon, where they embed in mucosa and can cause bloody diarrhea, tenesmus, and rectal prolapse in heavy infections.

Bold summary:
Trichuris trichiura eggs are barrel-shaped with terminal polar plugs, a classic diagnostic feature of whipworm infection.

Ingestion of which of the following eggs will result in infection?

A. Strongyloides stercoralis

B. Schistosoma japonicum

C. Toxocara canis

D. Opisthorchis sinensis

C. Toxocara canis

Toxocara canis (Visceral/Ocular Larva Migrans)

• Humans become infected by ingesting embryonated eggs from dog feces–contaminated soil or hands.

• The eggs hatch in the intestine → larvae migrate through liver, lungs, CNS, or eyes, but do not mature in humans (accidental host).

• Causes visceral larva migrans (fever, eosinophilia, hepatomegaly) or ocular larva migrans (retinal granuloma).

• Diagnosis by serology (ELISA) since larvae are rarely recovered.

Bold summary:
Ingestion of embryonated Toxocara canis eggs from contaminated soil causes larval migration through tissues—humans act as accidental hosts.

Plasmodium vivax and Plasmodium ovale are similar because they:

A. Exhibit Schüffner dots and have a true relapse in the life cycle

B. Have no malarial pigment and multiple rings

C. Commonly have appliqué forms in the red cells

D. Have true stippling, do not have a relapse stage, and infect old red cells

A. Exhibit Schüffner dots and have a true relapse in the life cycle

Plasmodium vivax & Plasmodium ovale – Key Similarities

• Both species infect young (reticulated) RBCs.

• Infected RBCs show Schüffner dots (fine red granules visible on stained smears).

• Both produce enlarged, pale RBCs with amoeboid trophozoites.

• Most importantly, both have hypnozoite stages (dormant liver forms) that can reactivate weeks to months later, causing true relapse.

Bold summary:
P. vivax and P. ovale both show Schüffner dots in RBCs and can cause relapsing malaria due to dormant hypnozoites in the liver.

The term internal autoinfection can be associated with the following parasites:

A. Cryptosporidium spp and Giardia lamblia (G. intestinalis, G. duodenalis)

B. Cystoisospora belli and Strongyloides stercoralis

C. Cryptosporidium spp and Strongyloides stercoralis

D. Giardia lamblia (G. intestinalis, G. duodenalis) and Cystoisospora belli

C. Cryptosporidium spp. and Strongyloides stercoralis

Internal Autoinfection

• Means reinfection within the same host without leaving the body.

• Strongyloides stercoralis: intestinal rhabditiform larvae develop into infective filariform larvae that penetrate mucosa or perianal skin, causing chronic or disseminated infection.

• Cryptosporidium spp: forms thin-walled oocysts that excyst in the intestine, reinfecting epithelial cells and prolonging diarrhea.

Bold summary:
Strongyloides and Cryptosporidium both cause internal autoinfection, enabling persistent infection without external reinfection.

Microsporidia have been identified as causing severe diarrhea, disseminated disease in other body sites, and ocular infections. Routes of infection have been identified as:

A. Ingestion

B. Inhalation

C. Direct contamination from the environment

D. Ingestion, inhalation, and direct contamination from the environment

D. Ingestion, inhalation, and direct contamination from the environment

Microsporidia Transmission

• Microsporidia are spore-forming, obligate intracellular parasites (fungal-related).

• Ingestion: most common route via contaminated food or water, causing intestinal infection and diarrhea.

• Inhalation: spores can enter through respiratory droplets or aerosols, leading to pulmonary or disseminated disease.

• Direct contamination: spores can enter through broken skin or the eye, causing keratoconjunctivitis.

Bold summary:
Microsporidia infect humans through ingestion, inhalation, or direct environmental contamination, producing intestinal, disseminated, or ocular disease.

An immunocompromised pt continues to have diarrhea after repeated ova and parasites (O&P) exams (sedimentation concentration, trichrome permanent stained smear) were reported as neg; organisms that might be responsible for the diarrhea include:

A. Cryptosporidium spp, Giardia lamblia (G. intestinalis, G. duodenalis), & Cystoisospora belli

B. Giardia lamblia (G. intestinalis, G. duodenalis), microsporidia, & Endolimax nana

C. Taenia solium & Endolimax nana

D. Cryptosporidium spp & microsporidia

D. Cryptosporidium spp & microsporidia

• Both Cryptosporidium and Microsporidia are common causes of chronic diarrhea in immunocompromised patients, especially those with AIDS or post-transplant immunosuppression.

• These organisms are not detected on routine O&P exams using sedimentation or trichrome stains.

• Require special stains or tests:

  • Cryptosporidium: modified acid-fast or antigen detection (EIA/IFA).

  • Microsporidia: modified trichrome (chromotrope 2R) or fluorescent stains (calcofluor white).

Bold summary:
Persistent diarrhea with negative O&P results in an immunocompromised patient suggests Cryptosporidium or Microsporidia — both require special stains for detection.

Confirmation of an infection with microsporidia can be achieved by seeing:

A. The oocyst wall

B. Sporozoites within the oocyst

C. Evidence of the polar tubule

D. Organisms stained with modified acid-fast stains

C. Evidence of the polar tubule

Microsporidia Identification

• Microsporidia are obligate intracellular, spore-forming parasites related to fungi.

• Diagnostic confirmation is based on identifying the polar tubule (polar filament) — a unique structure used by the spore to inject infective material into host cells.

• Visualization methods:

  • Electron microscopy (definitive) clearly shows the coiled polar tubule.

  • Modified trichrome (chromotrope 2R) or calcofluor white stain may highlight the spores (1–2 µm), but not the tubule itself.

• Modified acid-fast stain is not reliable for Microsporidia.

Bold summary:
Confirmation of Microsporidia infection relies on demonstrating the polar tubule, the hallmark invasive apparatus of the organism.

Early ring forms of the fifth human malaria, Plasmodium knowlesi, resemble those of:

A. Plasmodium vivax

B. Plasmodium ovale

C. Plasmodium malariae

D. Plasmodium falciparum

D. Plasmodium falciparum

Plasmodium knowlesi Characteristics

• Originally a simian malaria parasite found in macaques, now recognized as the fifth human malaria species.

• Early ring forms closely resemble those of P. falciparum — small, delicate rings that may be multiple per RBCand sometimes appliqué (accolé) forms.

• Later stages, however, mimic P. malariae with band forms and 72-hour schizogony cycle (daily fever pattern).

• Diagnosis: confirmed by PCR, since P. knowlesi is easily misidentified microscopically.

Bold summary:
Early P. knowlesi rings look like P. falciparum, but its later stages and 24-hour replication cycle resemble P. malariae.

Parasite stages that are immediately infective for humans on passage from the gastrointestinal tract include:

A. Schistosoma spp eggs

B. Toxoplasma gondii bradyzoites

C. Ascaris lumbricoides eggs

D. Cryptosporidium spp oocysts

D. Cryptosporidium spp. oocysts

Immediately Infective Stage

• Cryptosporidium oocysts are fully sporulated at excretion, making them immediately infective to others or the same host.

• Ingestion of even a few oocysts causes watery diarrhea.

• They are chlorine-resistant, promoting rapid waterborne spread.

Other options:

• Schistosoma eggs: infect snails, not humans.

• Toxoplasma bradyzoites: from tissue cysts in meat.

• Ascaris eggs: require embryonation in soil to become infective.

Bold summary:
Cryptosporidium oocysts are immediately infective upon passage, allowing direct and waterborne transmission.

Older developing stages (trophozoites, schizonts) of the fifth human malaria, Plasmodium knowlesi, resemble those of:

A. Plasmodium vivax

B. Plasmodium ovale

C. Plasmodium malariae

D. Plasmodium falciparum

C. Plasmodium malariae

Plasmodium knowlesi – Morphologic Overlap

• P. knowlesi exhibits dual resemblance:

  • Early ring forms look like P. falciparum (delicate, multiple rings per RBC).

  • Later trophozoite and schizont stages closely resemble P. malariae — showing band forms across RBCs and rosette-shaped schizonts with up to 10 merozoites.

• Unlike P. malariae, P. knowlesi has a 24-hour replication cycle, leading to daily fevers and potentially rapidly progressive infection.

• Diagnosis: requires PCR to differentiate from P. malariae.

Bold summary:
The trophozoite and schizont stages of P. knowlesi mimic P. malariae, while its rings resemble P. falciparum.

Autofluoresence requires no stain and is recommended for the presumptive identification of:

A. Entamoeba histolytica cysts

B. Toxoplasma gondii tachyzoites

C. Dientamoeba fragilis trophozoites

D. Cyclospora cayetanensis oocysts

D. Cyclospora cayetanensis oocysts

Cyclospora cayetanensis – Autofluorescence

• Cyclospora oocysts exhibit natural (autofluorescent) blue-green fluorescence under UV or epifluorescence microscopy, requiring no stain.

• This property helps distinguish Cyclospora from Cryptosporidium and Cystoisospora.

• Oocysts measure 8–10 µm, are spherical, and may show variable acid-fast staining.

• Commonly linked to foodborne outbreaks involving fresh produce like berries or basil.

Bold summary:
Cyclospora cayetanensis oocysts show blue-green autofluorescence under UV light, allowing presumptive identification without staining.

Key characteristics of infection with Plasmodium knowlesi include:

A. Rapid erythrocyte cycle (24 hours), will infect all ages of RBCs, and can cause serious disease

B. Erythrocytic cycle limited to young RBCs and causes a relatively benign disease

C. The possibility of a true relapse from the liver, infection in older RBCs, and causes serious disease

D. Extended life cycle (72 hours), will infect all ages of RBCs, and disease is similar to that caused by Plasmodium ovale

A. Rapid erythrocyte cycle (24 hours), will infect all ages of RBCs, and can cause serious disease

Plasmodium knowlesi – Key Features

• Has the fastest erythrocytic cycle of all human malarias — 24 hours — leading to quotidian (daily) fevers.

• Infects RBCs of all ages, similar to P. falciparum, resulting in high parasitemia and potentially severe, life-threatening disease (e.g., ARDS, renal failure).

• Does not form hypnozoites, so no relapse stage.

• Morphologically, early stages resemble P. falciparum, while later stages mimic P. malariae.

Bold summary:
P. knowlesi causes daily fevers and severe malaria due to its 24-hour cycle and ability to infect RBCs of all ages.

Microsporidial infections can be confirmed by using:

A. Light microscopy and modified trichrome staining

B. Phase contrast microscopy and routine trichrome staining

C. Electron microscopy and modified acid-fast staining

D. Fluorescence microscopy and hematoxylin staining

A. Light microscopy and modified trichrome staining

Microsporidia Diagnosis

• Confirmed by light microscopy of stool, urine, or tissue using modified trichrome stain (chromotrope 2R).

• Spores appear oval, 1–2 µm, staining bright pink to red with a distinct clear halo.

• This stain highlights the spore wall and occasionally the polar tubule, aiding identification.

• Fluorescent stains (e.g., calcofluor white) and electron microscopy can further confirm species, but modified trichrome is the routine diagnostic method.

Bold summary:
Microsporidia infection is routinely confirmed by light microscopy with modified trichrome stain, showing tiny red spores with a clear halo.

Although pathogenicicty of Blastocystis spp remains controversial, newer information suggests that:

A. Most organisms are misdiagnosed as artifacts

B. Approximately 10 subtypes/strains are included in the name, some of which are pathogenic and some are nonpathogenic

C. The immune status of the host is solely responsible for symptomatic infections

D. The number of organisms present determines pathogenicity

B. Approximately 10 subtypes/strains are included in the name, some of which are pathogenic and some are nonpathogenic

Blastocystis spp – Updated Understanding

• Once thought to be a harmless commensal, Blastocystis is now recognized as a complex of genetically distinct subtypes (STs 1–10+).

• Pathogenicity varies by subtype, explaining why some infections are asymptomatic while others cause diarrhea, abdominal pain, or IBS-like symptoms.

• Human infections most often involve ST1–ST4, with variable clinical significance.

• Diagnosis is typically made via O&P exam or molecular methods (PCR).

Bold summary:
Blastocystis spp includes multiple genetic subtypes, some pathogenic and others benign, accounting for its inconsistent clinical presentation.

Potential problems using EDTA anticoagulant and holding the blood too long prior to preparation of thick and thin blood films include:

A. Changes in parasite morphology, loss of organisms within several hours, and poor staining

B. Loss of Schüffner dots, poor adherence of the blood too the glass slide, and parasites beginning the vector cycle within the tube of blood

C. None of the above

D. All of the above

D. All of the above

When EDTA-anticoagulated blood is held too long before preparing malaria thick and thin smears, multiple issues occur:

• Parasite morphology changes: Rings and trophozoites distort or degenerate.

• Loss of organisms: Especially delicate forms, which can lyse or disappear within hours.

• Poor staining quality: Stains unevenly, obscuring key diagnostic features.

• Loss of Schüffner dots: Subtle RBC inclusions fade with time.

• Poor slide adherence: Blood held too long tends to clot or form artifacts, causing poor film quality.

• Parasites beginning vector cycle: Although theoretical, prolonged delay allows partial developmental changes mimicking later stages.

• summary:
  Delays in making smears from EDTA blood cause morphologic distortion, organism loss, and staining artifacts—so all listed problems can occur.

Key characteristics of the thick film include:

A. The ability to see the parasite within the RBCs

B. The ability to identify the parasites to the species level

C. The examination of less blood than the thin blood film

D. The necessity to lake the RBCs during or prior to staining

D. The necessity to lake the RBCs during or prior to staining

Thick Blood Film – Key Features

• Used for screening and detecting low levels of parasitemia, not for species identification.

• The film contains a large volume of blood, allowing concentration of parasites for easier detection.

• RBCs are “laked” — meaning they are intentionally lysed (ruptured) during or before staining so that only white blood cells, platelets, and parasites remain visible.

• Because the RBCs are destroyed, the parasites appear free in the background, making species ID difficult without a corresponding thin film.

Bold summary:
In a thick blood film, RBCs are laked (lysed) before staining to release parasites for easier detection of low parasitemia.