Parasitology: Key Concepts, Organisms, and Diagnostic Methods

Agglutination test

Used to diagnose leishmaniasis and Chagas disease.

EIA

Used to identify antigens or antibodies for organisms such as Giardia duodenalis, Cryptosporidium, and Entamoeba histolytica.

DNA probes and polymerase chain reactions

Used to diagnose selected parasite infections.

Intestinal Protozoa

Includes Amoebae, Flagellates, Ciliates, and Sporozoa.

Infective stage

Cysts.

Reproductive stage

Trophozoites.

Entamoeba histolytica

The only true pathogen in the intestinal amoebae group.

Entamoeba coli

Nonpathogenic amoeba.

Iodamoeba butschlii

Cyst has a large, iodine staining vacuole.

Giardia duodenalis

A principal pathogen among intestinal flagellates.

Traveler's diarrhea

Diarrhea caused by drinking contaminated water in streams and ponds while hiking or camping.

EIA methods

Preferred methods for identification over visual methods.

Trophozoites (Giardia duodenalis)

Kite shaped with central axoneme, 2 nuclei, flagella not always visible.

Cysts (Giardia duodenalis)

4 nuclei along central axoneme.

Dientamoeba fragilis

Worldwide distribution causing diarrhea and anal pruritis.

Trophozoites (Dientamoeba fragilis)

Round and binucleate.

Cysts (Dientamoeba fragilis)

None known.

Intestinal Ciliates

Use cilia for motion.

Balantidium coli

Only known pathogen among intestinal ciliates, causes self-limiting diarrhea.

Cyst (Balantidium coli)

Double walled (cilia in between), very large, kidney-shaped macronucleus.

Trophozoite (Balantidium coli)

Large macronucleus, cilia often visible.

Intestinal Sporozoa

No locomotive structures.

Cryptosporidium parvum

Major cause of watery diarrhea and severe dehydration in patients with AIDS.

Identification (Cryptosporidium parvum)

Acid-fast staining oocytes in stool.

Cystoisospora belli

Acid-fast oocysts in stool with large, ellipsoid shape and 1-2 visible cysts.

Naegleria fowleri

Amoeba found widely in the environment, causes primary amebic meningoencephalitis (PAM).

Identification (Naegleria fowleri)

Trophozoites found in CSF or brain tissue, single nucleus with large, dense karyosome.

Acanthamoeba spp.

Cause amoebic keratitis and encephalitis.

Identification (Acanthamoeba spp.)

Trophozoites or cysts visible in corneal scrapings or brain tissue.

Trichomonas vaginalis

Sexually transmitted flagellate that mostly causes vaginitis.

Identification (Trichomonas vaginalis)

Trophozoites found in urine or vaginal wet prep, characteristic 'jerky', non-directional motility.

Hemoflagellates

Flagellates that inhabit the blood and tissue of humans.

Leishmania spp.

Causes leishmaniasis, a cutaneous or disseminated infection contracted from the bite of a sandfly.

Identification (Leishmania spp.)

Flagellated form or nonmotile stage found in blood or tissue.

Trypanosoma cruzi

Causes Chagas disease, contracted from the feces and bite of the reduviid (kissing) bug.

Identification (Trypanosoma cruzi)

Flagellate found in peripheral blood smears, C-shape, large, dark posterior kinetoplast.

Trypanosoma brucei

Causes African sleeping sickness, contracted from tsetse flies.

Identification (Trypanosoma brucei)

Flagellate found in peripheral blood smears, delicate curve with smaller kinetoplast.

Plasmodium spp.

Sporozoa that cause malaria, a life-threatening illness with 250 million new cases and over 600,000 deaths each year.

Identification (Plasmodium spp.)

Diagnosis made primarily through thick and thin blood smears to view developmental stages of the life cycle.

Plasmodium falciparum

Large, banana-shaped gametocyte with multiple rings per RBC and a 'double dot' or 'headphones' ring form.

Plasmodium malariae

Single ring form, no Schuffner's dots, with a band form trophozoite.

Plasmodium ovale and vivax

Ring and trophozoite forms with Schuffner's dots.

Babesia microti

Sporozoa that cause babesiosis, a bloodborne, usually self-limiting infection spread through tick bites.

Babesia microti Identification

Ring forms similar to Plasmodium and trophozoites in blood smears, usually multiple ring forms per cell, with 'plus-sign' or 'maltese cross' morphology.

Toxoplasma gondii

Causes toxoplasmosis, associated with cat feces and ingestion of undercooked meat, especially in pregnant people.

Toxoplasma gondii Identification

Mostly serological testing; organisms can be found, but not easily; large, curved structures found in CSF, blood, or occasionally tissue samples.

Trematodes

Flukes that are flat, hermaphroditic, with at least two suckers.

Trematodes Life cycles

Eggs usually passed through feces into water where they hatch into free-living organisms.

Schistosoma spp.

Blood flukes; eggs found in feces or urine are diagnostic.

Schistosoma mansoni

Eggs have large lateral spine.

Schistosoma japonicum

Eggs have small lateral spine.

Schistosoma haematobium

Eggs have a terminal spine.

Paragonimus westermani

Lung fluke; identification through eggs in feces or (occasionally) sputum, with eggs having a shouldered operculum.

Clonorchis sinensis

Intestinal fluke; identification through eggs in feces with dome shaped, shouldered operculum opposite a small knob.

Fasciola hepatica

Liver fluke; identification through eggs in feces which are rounded, with a non-shouldered operculum.

Cestodes

Tapeworms; adult worms live in humans who shed eggs, which infect an intermediate host.

Taenia saginata

Beef tapeworm; diagnosis through eggs indistinguishable from T. solium and proglottid segments that are wide with more lateral segments.

Taenia solium

Pork tapeworm; diagnosis through eggs indistinguishable from T. saginata and proglottid segments that are more narrow with fewer lateral segments.

Diphyllobothrium latum

Fish tapeworm; identification through eggs that are oblong and smooth with a smooth operculum.

Hymenolepis nana

Dwarf tapeworm caused by accidental ingestion of infected arthropods; identification through eggs that have 2 layers and 3 hooklets inside.

Nematodes

Roundworms that cause a wide variety of infections, with very complex life-cycles.

Enterobius vermicularis

Pinworms, a common infection in school-age children, causing pruritus and itchiness around the anus.

Enterobius vermicularis Diagnosis

Gravid females deposit eggs in perianal folds.

Trichuris trichiura

Whipworm

Diagnosis of Trichuris trichiura

Eggs in stool; Barrel shaped eggs with transparent plugs at each end

Ascaris lumbricoides

Giant intestinal tapeworm

Diagnosis of Ascaris lumbricoides

Larger worms are so big they can cause intestinal blockages, and can be visualized coming out of the rectum/anus, or in the stool; Eggs in stool have a thick shell with a bumpy outer layer

Strongyloides stercoralis

Threadworm

Diagnosis of Strongyloides stercoralis

Eggs rarely found in stool; Feces contain rhabditiform larvae; Larvae very similar to hookworms, but have a short buccal (mouth) cavity and a prominent genital primordium

Necator americanus

Hookworms

Diagnosis of Necator americanus

Eggs found in stool with translucent wall and clusters of spherical embryos; Rhabditiform larvae also found in stool have long buccal cavity

Trichinella spiralis

Infection of skeletal muscle causes edema and swelling in muscle tissue

Contracted from Trichinella spiralis

Ingestion of undercooked pork

Diagnosis of Trichinella spiralis

Encysted larvae and worms seen in skeletal muscle

Dracunculus medinensis

Guinea worm infections

Diagnosis of Dracunculus medinensis

Adult worms found in ulcerations; Treated by removing worms around a stick, 1 inch per day

Habitat of Filariae

Mainly inhabit the circulatory and lymphatic systems, but can also invade the sinus cavities and skeletal muscles.

Wuchereria bancrofti

Causes elephantiasis

Contracted from Wuchereria bancrofti

Through the bite of mosquitoes

Diagnosis of Wuchereria bancrofti

Adults worms have no tail nuclei

Brugia malayi

Causes elephantiasis

Contracted from Brugia malayi

Through the bite of mosquitoes

Diagnosis of Brugia malayi

Adults worms have 2 distinct tail nuclei

Loa loa

Migrate to the eyes

Diagnosis of Loa loa

Adult worms have nuclei that go all the way to the tip of the tail