Veterinary Parasitology Flashcards

A dog presents with a pot-bellied appearance, poor coat, and a positive fecal flotation showing oval eggs with thick, mammillated shells. Which parasite is most likely responsible?

Answer: Toxocara canis

Toxocara canis produces large, oval eggs with a thick, pitted (mammillated) outer shell. Ascarid infections are common in puppies and can cause pot-bellied appearance and poor thrift. Ancylostoma produces thin-shelled eggs, Trichuris produces barrel-shaped eggs with bipolar plugs, and Dipylidium proglottids contain egg packets not individual eggs.

Which of the following Plasmodium-like protozoa infects cattle erythrocytes, is transmitted by Ixodes ticks, and can cause severe hemolytic anemia in splenectomized animals?

Answer: Babesia bovis

Babesia bovis is an intraerythrocytic protozoan transmitted by Rhipicephalus (Boophilus) microplus ticks that causes bovine babesiosis (redwater fever). Splenectomized animals are especially vulnerable because the spleen normally removes parasitized red cells. Theileria infects lymphocytes first, Trypanosoma is flagellated and extracellular, and Anaplasma is a rickettsial organism, not a protozoan.

Ivermectin exerts its antiparasitic effect primarily by:

Answer: Potentiating glutamate-gated chloride channels, causing paralysis

Ivermectin binds to glutamate-gated chloride channels (and to a lesser degree GABA-gated channels) in invertebrate nerve and muscle cells, causing hyperpolarization and irreversible paralysis. Benzimidazoles inhibit tubulin polymerization; organophosphates block acetylcholinesterase; and antiprotozoals like atovaquone target mitochondrial transport.

The intermediate host responsible for transmission of Taenia solium cysticercosis to humans is:

Answer: Pigs ingesting tapeworm eggs

Taenia solium uses pigs as the intermediate host. Pigs ingest proglottids or eggs shed in human feces; oncospheres hatch and develop into cysticerci in pig muscle (pork measles). Humans become definitive hosts by eating undercooked pork. Taenia saginata uses cattle. Dipylidium requires fleas. Mosquitoes transmit filarial nematodes, not cestodes.

Giardia duodenalis trophozoites are best characterized by which morphological feature on a fecal smear?

Answer: Two nuclei and a ventral adhesive disc giving a 'falling leaf' motility

Giardia trophozoites are binucleate, pear-shaped flagellates with a concave ventral adhesive disc used to attach to intestinal epithelium. Their characteristic 'falling leaf' tumbling motility is seen in fresh wet mounts. Cysts (not trophozoites) contain 4 nuclei. Tritrichomonas has an undulating membrane and single nucleus. Axostyle prominence is a feature of Trichomonas.

A horse in Florida develops crusting dermatitis with intensely pruritic skin lesions in summer. Skin scraping reveals no mites, but the lesions improve when the horse is stabled at night. The most likely causative agent is:

Answer: Culicoides spp. hypersensitivity

Sweet itch (Queensland itch, summer seasonal recurrent dermatitis) is caused by IgE-mediated hypersensitivity to the saliva of Culicoides biting midges, which are most active at dawn and dusk. Stabling at night reduces exposure and improves signs. Chorioptes and Psoroptes are mange mites identified on scraping. Habronema causes cutaneous habronemiasis (summer sores) with granulomatous lesions, not diffuse pruritic crusting.