Comprehensive Parasitology: Protozoa, Nematodes, and Cestodes

Vocabulary practice flashcards covering protozoan parasites (coccidia, hemoflagellates, enteric flagellates, amoebae, ciliates), agricultural and medical nematodes (soil-transmitted helminths and filarial worms), and cestodes (tapeworms and hydatid disease), including drug mechanisms of action, diagnostic morphology, and life cycles.

Babesiosis

A tick-borne hemolytic illness caused by the intraerythrocytic protozoan Babesia, which does not produce hemozoin; it is characterized by fever, fatigue, chills, and hemolytic anemia, with life-threatening severity in splenectomized, elderly, or immunocompromised patients.

Ixodes scapularis

The blacklegged tick vector that transmits Babesia microti to humans and mice, and frequently co-transmits Lyme disease, Powassan virus, and Human Granulocytic Anaplasmosis.

Atovaquone and azithromycin

The standard frontline combination chemotherapy used to treat Babesia microti infections in humans (with clindamycin plus quinine reserved for severe cases).

Equine Piroplasmosis

Equine babesiosis in horses caused by Babesia equi and Babesia caballi, managed by quarantine and administration of imidocarb dipropionate.

Cyclospora cayetanensis

A monoxenic coccidian parasite of the human gastrointestinal tract transmitted via the fecal-oral route by ingesting sporulated oocysts on contaminated food or water, causing prolonged watery diarrhea and food-borne traveler's diarrhea.

Sulfa Drugs

Antimicrobial agents (such as sulfamethoxazole in Bactrim) that competitively inhibit dihydropteroate synthetase, blocking microbial folate synthesis and nucleic acid production to exert a static inhibitory effect.

Toxoplasma gondii

An apicomplexan coccidian tissue parasite first identified by Charles Nicolle in 1908; it possesses a broad intermediate host range of warm-blooded vertebrates, forms persistent tissue cysts in immunoprivileged sites such as the brain, and can alter host behavior.

Definitive Host (Toxoplasma gondii)

Members of the feline family (cats), which are the only hosts in which Toxoplasma gondii reaches sexual maturity and undergoes sporogony to produce infectious oocysts shed into the environment via feces.

TORCH Complex

A medical acronym designating vertically transmitted congenital infections: Toxoplasmosis (Toxoplasma gondii), Other (e.g., HIV, Syphilis), Rubella, Cytomegalovirus, and Herpes.

Congenital Toxoplasmosis

A disease resulting from vertical transmission of Toxoplasma gondii when a previously unexposed mother is infected during pregnancy; clinical manifestations include hydrocephalus, chorioretinitis, liver cirrhosis, and splenomegaly.

Pyrimethamine (Daraprim)

An antiparasitic medication targeting microbial folic acid synthesis, routinely combined with sulfadiazine or clindamycin for the treatment of toxoplasmosis.

Kinetoplastids

A group of flagellated protozoans related to Euglena distinguished by the presence of a kinetoplast, an organized DNA-containing network within a single large mitochondrion.

Trypanosoma brucei

An extracellular hemoflagellate protozoan parasite transmitted by the tsetse fly (Glossina) that causes African sleeping sickness and possesses the ability to cross the blood-brain barrier into the central nervous system.

Trypanosoma brucei gambiense (TbG)

The subspecies of Trypanosoma brucei responsible for over $97\%$ of reported human African trypanosomiasis cases, with humans serving as the primary reservoir.

Trypanosoma brucei rhodesiense (TbR)

The subspecies of Trypanosoma brucei causing acute African sleeping sickness, for which cattle and wild ungulates serve as the primary animal reservoirs.

Variant Surface Glycoprotein (VSG)

A dense macromolecular coat covering the surface of bloodstream Trypanosoma brucei that undergoes extensive antigenic variation via genomic expression site cassettes to persistently evade host antibody-mediated humoral immunity.

Pentamidine (Pentamine)

A chemotherapy agent highly effective for both primary stage treatment and prophylaxis of early hemolymphatic African sleeping sickness.

Fexinidazole

An all-oral medication approved for both Stage I and Stage II African sleeping sickness, achieving a $97\%$ cure rate with a 10-day treatment course.

Wigglesworthia

An obligate bacterial endosymbiont housed within the bacteriome organ of the tsetse fly that synthesizes essential B vitamins required for fly growth and reproduction.

Sodalis glossinidius

A bacterial endosymbiont residing in the hemolymph and gastrointestinal tract of tsetse flies that bolsters insect immunity and helps protect the fly from adverse effects of Trypanosoma infection.

Wolbachia (in Diptera)

A bacterial endosymbiont of insects that can act as a reproductive parasite or mutualist, facilitating host iron metabolism under nutritional stress and aiding antiviral defense.

Chagas Disease (American Trypanosomiasis)

A chronic zoonotic infection caused by Trypanosoma cruzi, discovered by Dr. Carlos Chagas, transmitted when a triatomine ('kissing') bug excretes infective metacyclic trypomastigotes in its feces near a bite wound.

Romaña's Sign

A classic clinical marker of acute Chagas disease characterized by unilateral, painless periorbital edema and conjunctivitis resulting from contamination of the eye by infected triatomine feces.

Benznidazole

An antiparasitic prodrug activated by Trypanosoma cruzi type I nitroreductase that generates free radical species to induce extensive parasite DNA damage and cell death.

Diplomonads

A group of anaerobic protists within Excavata characterized by possessing double cells with two identical nuclei, eight flagella, and mitosomes rather than classical mitochondria.

Giardia lamblia

A flagellated diplomonad intestinal parasite observed by Antonie van Leeuwenhoek and described by Dr. Vilem Lambl; it attaches to epithelial cells of the duodenum and upper jejunum, producing steatorrhea, bloating, and abdominal cramps.

Cryptosporidium hominis

An apicomplexan coccidian parasite with small ($4\text{--}6\,\mu\text{m}$) thick-walled oocysts that are highly chlorine-resistant, which establishes intracellular, extracytoplasmic infection in the ileum to cause profuse watery diarrhea.

1993 Milwaukee Cryptosporidiosis Outbreak

The largest documented waterborne disease outbreak in United States history, in which Cryptosporidium bypassed public municipal water filtration, sickening over $400,000$ individuals and causing $104$ fatalities.

Nitazoxanide (NTZ)

An oral prodrug converted into tizoxanide that selectively inhibits the enzyme pyruvate ferredoxin oxidoreductase, disrupting anaerobic energy metabolism in both Giardia lamblia and Cryptosporidium.

Trichomonas vaginalis

A flagellated, microaerophilic parabasalid protozoan parasite that causes the sexually transmitted infection trichomoniasis, colonizing the human urogenital epithelium and increasing the risk of preterm birth and HIV transmission.

Metronidazole (Flagyl)

A nitroimidazole prodrug that enters anaerobic protozoan cells by passive diffusion and undergoes reduction to generate reactive metabolites that fragment DNA and halt protein synthesis; effective against Entamoeba, Giardia, and Trichomonas.

Entamoeba histolytica

A pathogenic, tissue-destroying amoebozoan that invades the large intestinal mucosa via motile trophozoites (often containing phagocytosed red blood cells), causing amoebic dysentery with bloody, mucus-laden stools and liver abscesses.

Diloxanide (Furamide)

A poorly absorbed lumenal amebicide that disrupts microbial protein synthesis to kill Entamoeba histolytica trophozoites and cysts confined to the intestinal lumen.

Dientamoeba fragilis

A fragile, non-flagellated trichomonad protozoan inhabiting the human cecum and colon that decomposes rapidly outside the body and can be transmitted inside the protective shell of Enterobius vermicularis eggs.

Naegleria fowleri

A free-living, thermophilic amoeba found in warm freshwater that penetrates the nasal olfactory neuroepithelium to migrate into the brain, causing acute Primary Amebic Meningoencephalitis (PAM) with a fatal outcome in over $99\%$ of cases.

Acanthamoeba

A genus of opportunistic free-living amoebae that cause sight-threatening amoebic keratitis in contact lens users and granulomatous amebic encephalitis (GAE) in immunocompromised hosts.

Balantidium coli

The only known ciliated protozoan parasite of humans; it is acquired by ingesting cysts shed by reservoir hosts such as pigs and invades the human colon wall to cause balantidiasis.

Geo-helminths

Soil-transmitted parasitic nematodes that possess direct life cycles without intermediate hosts, undergoing embryonic maturation in soil and infecting humans through fecal contamination of ground, food, or water.

Fearful Triad

A common global parasitic co-infection consisting of three soil-transmitted gastrointestinal nematodes: Ascaris lumbricoides, Trichuris trichiura, and human hookworms (Ancylostoma duodenale or Necator americanus).

Type 1 Nematodes

Soil-transmitted nematode roundworms whose route of human infection occurs strictly through the oral ingestion of embryonated eggs (including Ascaris lumbricoides, Trichuris trichiura, and Enterobius vermicularis).

Enterobius vermicularis

The human pinworm, an intestinal nematode dwelling in the cecum; gravid females migrate out to the perianal folds at night to deposit infective embryonated eggs, causing perianal pruritus.

Scotch Tape Test

The diagnostic method for detecting Enterobius vermicularis in which clear cellulose tape or an adhesive paddle is pressed against the perianal skin in the early morning to capture characteristic asymmetrical pinworm eggs for microscopic examination.

Pyrantel pamoate

An over-the-counter antihelminthic drug that acts as a depolarizing neuromuscular blocker, inducing persistent spastic paralysis in intestinal roundworms so they lose their grip on the gut wall and are expelled.

Ascaris lumbricoides

The human giant roundworm; ingested eggs hatch in the small intestine, and larvae undergo a transpulmonary circulatory migration via the liver and alveolar lungs before being swallowed back into the bowel to mature into large adults.

Trichuris trichiura

The human whipworm, a Type 1 geo-helminth whose adults embed their thin anterior ends into the mucosal lining of the cecum; heavy infections produce bloody diarrhea with mucus and can precipitate rectal prolapse.

Mebendazole (Vermox)

A broad-spectrum benzimidazole anthelmintic that selectively and irreversibly blocks glucose and nutrient uptake in susceptible intestinal roundworms, depleting ATP and causing immobilisation and death.

Type 2 Nematodes

Soil-transmitted roundworms whose infectious third-stage ($\text{L3}$) filariform larvae actively enter the human host via direct percutaneous penetration of intact skin (e.g., Ancylostoma duodenale, Necator americanus, and Strongyloides stercoralis).

Ground Itch

An intense, pruriginous, erythematous dermal allergic reaction occurring at the cutaneous site of hookworm filariform larval entry, facilitated by parasite secretion of hyaluronidase and elastase enzymes.

Necator americanus

The New World hookworm historically endemic across the American South, once dubbed the 'germ of laziness' because chronic hookworm-induced blood loss and microcytic anemia produced widespread physical weakness and lethargy.

Strongyloides stercoralis

A soil-transmitted threadworm capable of alternating between a free-living sexual reproductive cycle in soil and a parasitic cycle in humans, wherein only female worms are parasitic and reproduce via parthenogenesis in the intestinal mucosa.

Autoinfection (Strongyloides)

An internal infection cycle in which rhabditiform Strongyloides larvae transform into infective filariform larvae within the host's intestinal lumen, burrowing back through the gut mucosa or perianal skin to sustain lifelong chronic infection.

Strongyloides Hyperinfection Syndrome

A catastrophic acceleration of autoinfection typically triggered by corticosteroid therapy or immunosuppression, resulting in massive systemic dissemination of larvae to pulmonary, central nervous, and gastrointestinal organs accompanied by enteric bacteremia.

Ivermectin

A macrocyclic lactone anthelmintic that activates invertebrate glutamate-gated and GABA-receptor chloride channels to cause hyperpolarization and flaccid paralysis; highly active against nematodes and arthropods, but ineffective against cestodes and trematodes lacking a GABA system.

Trichinella spiralis

A foodborne tissue nematode contracted by eating undercooked meat (especially pork) containing encysted larvae, causing an initial gastrointestinal phase followed by periorbital edema, myalgia, and encystment of new larvae inside skeletal muscle cells.

Wuchereria bancrofti

A mosquito-borne filarial nematode whose adult worms reside in human lymphatic vessels, provoking chronic inflammatory lymphatic obstruction, recurrent bacterial infections, and severe disfiguring elephantiasis.

Loa loa

The African eye worm, a filarial nematode transmitted by deerflies of the genus Chrysops that migrates through human subcutaneous tissue and across the subconjunctiva of the eye, releasing day-periodic microfilariae into peripheral circulation.

Heterodera glycines

The soybean cyst nematode (SCN), an agricultural plant-parasitic roundworm introduced into the US from Asia in 1954 that infects soybean root systems and accounts for over $\$1\text{ billion}$ in annual crop damage, favored by soil pH values of $7.0\text{--}8.0$.

Cestodes (Tapeworms)

Dorsoventrally flattened, segmented parasitic flatworms (phylum Platyhelminthes) that are monoecious (hermaphroditic), lack a coelomic body cavity, and absorb all nutrients directly across their external syncytial tegument due to the total absence of an alimentary canal.

Scolex

The specialized anterior anchoring organ of a tapeworm, which carries muscular suckers and, in armed species such as Taenia solium, a rostellum with circular chitinous hooks to attach to the host intestine.

Proglottid

A single hermaphroditic body segment in a tapeworm strobila that originates at the neck, matures with male and female reproductive organs, and becomes filled with fertilized egg capsules before detaching.

Taeniasis

An intestinal infection caused by the presence of a mature adult tapeworm (Taenia solium or Taenia saginata) in the gut lumen, acquired specifically by ingesting viable larval cysticerci embedded in raw or undercooked meat.

Cysticercosis

A systemic tissue infection that develops when humans accidentally ingest the microscopic embryonated eggs of the pork tapeworm Taenia solium, leading to the development of encysted larval cysticerci in muscles, eyes, and subcutaneous tissue.

Neurocysticercosis (NCC)

A severe form of cysticercosis resulting from Taenia solium cysticerci lodging in the human central nervous system and brain parenchyma, representing a leading cause of acquired adult-onset epilepsy and seizure disorders globally.

Praziquantel

A drug of choice for cestode and trematode infections that increases cell membrane permeability to calcium ions ($\text{Ca}^{2+}$) to produce rapid sucker tetany and paralysis while inhibiting purine uptake, exposing the worm to host digestion.

Niclosamide

An oral anthelmintic that inhibits glucose uptake and uncouples oxidative phosphorylation in cestodes, dismantling the tapeworm's anaerobic energy metabolism within the bowel.

Taenia saginata

The beef tapeworm; an intestinal cestode up to several meters in length that uses cattle as intermediate hosts and lacks hooks on its four-suckered scolex, and which does not cause human cysticercosis upon egg ingestion.

Diphyllobothrium latum

The broad fish tapeworm; a giant cestode reaching lengths greater than $10\text{ meters}$ that uses copepods and freshwater fish as intermediate hosts, attaching to human mucosa via bothria and predisposing infected individuals to severe vitamin $\text{B}_{12}$ deficiency and megaloblastic anemia.

Operculated Egg (Diphyllobothrium)

The diagnostic oval egg passed in the stool of patients infected with Diphyllobothrium latum, identifiable by a distinct apical operculum lid at one end and a small abopercular knob at the opposite end.

Plerocercoid Larva (Sparganum)

The infective metacestode stage of Diphyllobothrium latum that develops in the muscular tissue and flesh of freshwater or anadromous fish, which establishes adult tapeworm infection in humans when raw or undercooked fish is ingested.

Hymenolepis nana

The dwarf tapeworm; the most common human cestode, notable for its ability to complete its entire life cycle within a single human host through internal autoinfection or via ingestion of arthropod intermediate hosts containing cysticercoids.

Hydatid Cyst Disease

A severe, space-occupying larval tapeworm infection of the liver, lungs, and brain caused by accidental ingestion of the microscopic eggs of the canine tapeworms Echinococcus granulosus or Echinococcus multilocularis.