Practice 5 structured short- and long-answer medical questions on Weekly Year 2: Parasitology Exam - May 15, 2026 (Sections B & C) — Essay Questions. Review...
Answer: Human infection begins with the bite of an infected Anopheles mosquito, injecting sporozoites into the bloodstream. Sporozoites travel to the liver, where they develop into merozoites within hepatocytes (exoerythrocytic stage). Merozoites are released into the bloodstream and invade red blood cells (erythrocytic stage). Inside RBCs, merozoites mature through ring, trophozoite, and schizont stages, eventually releasing more merozoites upon lysis of the RBC, causing fever and chills. Some merozoites develop into gametocytes (male and female), which are taken up by another mosquito during a blood meal, initiating the sexual cycle (sporogony) in the mosquito vector.
Answer: The most likely causative organism is *Entamoeba histolytica*. The stool microscopy finding of a large, spherical cyst with four nuclei is characteristic. The mode of transmission is fecal-oral, typically through the ingestion of contaminated water or food containing mature cysts.
Answer: *Schistosoma mansoni* adults reside in the mesenteric veins. Female worms lay eggs that penetrate the intestinal wall and are shed in feces. However, some eggs may become trapped in host tissues, particularly the liver and intestinal wall. These trapped eggs elicit a strong granulomatous inflammatory response. Eosinophils and lymphocytes surround the eggs, leading to fibrosis, granuloma formation, and potentially portal hypertension and hepatosplenomegaly. This immune response to the eggs, rather than the adult worms, is responsible for the severe pathology.
Answer: The infective stage of *Giardia duodenalis* is the cyst. Humans ingest cysts from contaminated water or food. Excystation occurs in the small intestine, releasing trophozoites. The diagnostic stage of *Giardia duodenalis* is the trophozoite, which is found in the small intestine and stool, and the cyst, which is found in the stool. Therefore, both trophozoites and cysts can be used for diagnosis via stool examination.
Answer: ## Life Cycle of *Babesia microti*: * **Vector:** Primarily ticks (e.g., *Ixodes scapularis*). * **Transmission:** Ticks acquire *Babesia* by feeding on an infected host (e.g., rodent, deer). During a subsequent blood meal on a human, infected ticks transmit the parasite as sporozoites. * **Human Infection:** Sporozoites enter the bloodstream and invade erythrocytes (red blood cells). * **Erythrocytic Cycle:** Within RBCs, the parasite undergoes asexual reproduction (similar to malaria), forming ring forms and then mature trophozoites, which can divide to produce merozoites. The parasite can also form characteristic tetrads (Maltese cross formations), though this is not always seen. RBCs rupture, releasing merozoites to infect new RBCs. * **Transmission to Mosquito:** If an infected tick bites an uninfected mammal, the tick ingests infected blood. Gametocytes within the RBCs undergo sexual development in the tick's gut, leading to the formation of the ookinete and oocyst. Sporozoites develop within the oocyst and migrate to the tick's salivary glands, ready to be transmitted to a new host. ## Mode of Transmission: * **Tick bites:** The primary mode of transmission to humans is through the bite of infected ticks. * **Blood transfusion:** Although rare, transmission can occur through contaminated blood transfusions. ## Pathogenesis and Clinical Manifestations: * **Hemolysis:** The invasion and destruction of red blood cells by *Babesia* leads to hemolytic anemia, which is a hallmark of the disease. This can result in fatigue, jaundice, and splenomegaly. * **Immune response:** The body mounts an immune response, but it is often insufficient to clear the parasite completely. * **Clinical Presentation:** Symptoms are often flu-like and can vary widely in severity, from asymptomatic infection to severe, life-threatening disease. Common symptoms include: * Intermittent fever, chills, and drenching sweats * Myalgia and arthralgia * Headache * Fatigue and weakness * Anemia (hemolytic) * Nausea and vomiting * Splenomegaly and hepatomegaly (less common) * **Risk Factors for Severe Disease:** Immunocompromised individuals (e.g., those with HIV/AIDS, post-splenectomy, elderly) are at higher risk for severe and complicated babesiosis, which can lead to acute respiratory distress syndrome (ARDS), renal failure, disseminated intravascular coagulation (DIC), and death. ## Diagnosis: * **Microscopy:** Peripheral blood smears stained with Giemsa or Wright stain are the primary diagnostic tool. *Babesia* parasites are observed within erythrocytes, often appearing as ring forms, tetrads (Maltese cross), or pleomorphic shapes. Distinguishing *Babesia* from *Plasmodium* species (malaria) is crucial. * **Serological Tests:** Indirect fluorescent antibody (IFA) tests can detect antibodies against *Babesia*. However, these are often not available in resource-limited settings and may not be useful in early stages of infection. * **Molecular Methods:** Polymerase chain reaction (PCR) is highly sensitive and specific for detecting *Babesia* DNA, particularly useful in cases where microscopy is equivocal or for identifying specific species. ## Management: * **Asymptomatic Patients:** Typically do not require treatment. * **Symptomatic Patients:** Treatment is indicated for symptomatic individuals. * **Standard Regimen:** A combination of atovaquone and azithromycin is the first-line treatment for most patients. * **Alternative Regimen:** For severe or complicated babesiosis, or in patients who cannot tolerate the standard regimen, clindamycin with quinine may be used. However, this regimen is associated with more significant side effects. * **Duration of Treatment:** Treatment duration typically ranges from 7 to 10 days, but may be extended in immunocompromised individuals or those with prolonged symptoms. * **Supportive Care:** Management of anemia and other complications, such as fluid and electrolyte balance, is important.