Practice 5 structured short- and long-answer medical questions on Weekly Year 2: Parasitology Exam - May 29, 2026 (Sections B & C). Review model answers for...
Answer: The life cycle of Plasmodium falciparum in humans begins with the injection of sporozoites into the bloodstream by an infected Anopheles mosquito. Sporozoites migrate to the liver and undergo asexual reproduction (exo-erythrocytic schizogony), producing merozoites. These merozoites then invade red blood cells (RBCs), initiating the erythrocytic cycle. Within RBCs, merozoites develop into ring forms, then trophozoites, and finally schizonts, each containing multiple merozoites. Rupture of infected RBCs releases merozoites to infect new RBCs, causing the cyclical fever and chills characteristic of malaria. Some merozoites develop into gametocytes, which are taken up by mosquitoes, completing the cycle.
Answer: In adult tapeworms, Taenia solium (pork tapeworm) has a rostellum with hooks and 4-5 primary suckers, while Taenia saginata (beef tapeworm) lacks a rostellum and has only 4 prominent suckers. Regarding gravid proglottids, Taenia solium has a uterus with 7-13 lateral branches on each side, whereas Taenia saginata has a uterus with 17-32 lateral branches. In larval stages, Taenia solium forms cysticerci (bladder worms) in pigs that are typically smaller and more numerous than those of Taenia saginata, which are larger and fewer in number in cattle.
Answer: Praziquantel is a broad-spectrum anthelmintic drug highly effective against schistosomes. Its mechanism of action is not fully elucidated but is believed to involve disrupting the parasite's tegument and causing calcium influx, leading to paralysis and death. This increased membrane permeability also makes the parasite vulnerable to host immune responses. Clinically, praziquantel is the drug of choice for treating all forms of schistosomiasis in humans, administered orally. It is generally well-tolerated with mild side effects.
Answer: The definitive host is the organism in which the parasite reaches sexual maturity and reproduces. The intermediate host is the organism that harbors the larval or asexual stages of the parasite. This distinction is crucial for understanding transmission and developing control strategies. For example, in the life cycle of Echinococcus granulosus, dogs are the definitive hosts where adult tapeworms reside and lay eggs, while sheep and other herbivores are intermediate hosts that ingest the eggs and develop hydatid cysts in their organs.
Answer: 1. **Life cycle of Giardia lamblia:** Giardia lamblia exists in two forms: the trophozoite and the cyst. Trophozoites are motile, pear-shaped organisms found in the small intestine, where they attach to the intestinal lining. Cysts are oval, thick-walled structures resistant to environmental conditions and found in feces. Infection occurs by ingesting Giardia cysts, usually through contaminated food or water. In the acidic environment of the stomach, cysts excyst in the small intestine, releasing infective trophozoites. Trophozoites multiply by binary fission and attach to the duodenal and jejunal mucosa. Some trophozoites encyst before being passed in the stool. 2. **Pathogenesis and clinical manifestations of giardiasis:** Giardia trophozoites attach to the villi of the small intestine, interfering with nutrient absorption, particularly fats and disaccharides. This can lead to villous atrophy and inflammation. The clinical manifestations of giardiasis are highly variable, ranging from asymptomatic infection to severe diarrhea. Common symptoms include sudden onset of watery, foul-smelling diarrhea (often described as 'rotten egg' smell), abdominal cramps, bloating, gas, nausea, and greasy stools (steatorrhea). Weight loss can occur with chronic infection due to malabsorption. Some individuals may experience post-infectious irritable bowel syndrome. 3. **Diagnostic methods:** Diagnosis is typically made by identifying Giardia cysts or trophozoites in stool samples. Multiple stool samples (at least three) collected on different days are often required due to the intermittent shedding of cysts. Microscopic examination of stool smears (saline or iodine wet mounts, or permanent stains like trichrome or acid-fast) is the most common method. Antigen detection assays (ELISA or immunofluorescence) are highly sensitive and specific for Giardia antigens in stool. In cases of persistent symptoms or inconclusive stool findings, duodenal aspiration or biopsy can be performed for direct examination or culture, though this is less common. 4. **Principles of management and prevention:** The primary treatment for symptomatic giardiasis is antiparasitic medication. Common drugs include metronidazole, tinidazole, nitazoxanide, and furazolidone. Treatment duration and dosage depend on the chosen drug and patient factors. Supportive care includes hydration and nutritional support, especially for patients with malabsorption. Prevention strategies focus on reducing transmission. This includes safe drinking water practices (boiling, filtering, or using chemical disinfection), proper sanitation, handwashing with soap and water, especially after using the toilet and before handling food, and thorough cooking of food. Public health education plays a vital role in raising awareness and promoting these preventive measures.