**Question:** A 40-year old veterinarian presented with complaints of abdominal pain. On examination he was found to be icteric, the liver was palpable. Ultrasonography revealed the presence of cyst in the right lobe of liver. The definitive hosts of the agent causing this infection are -

A. Dogs
B. Cats
C. Birds
D. Rodents

**Core Concept:** Schistosomiasis, also known as bilharziasis, is a parasitic infection caused by flatworms of the genus Schistosoma. The definitive hosts for these parasites are mammals that serve as intermediate hosts for the parasite larvae.

**Why the Correct Answer is Right:** The definitive hosts of Schistosoma parasites are mammals that help in the maturation and reproduction of the parasite. In this case, the correct answer is D. Rodents, as they are known to harbor Schistosoma mansoni, the primary cause of human schistosomiasis in the tropics. Rodents serve as the definitive host for the parasite, allowing it to mature and reproduce.

**Why Each Wrong Option is Incorrect:**

A. Dogs (Dogs are not the definitive host for Schistosoma mansoni. They can act as intermediate hosts, but humans or rodents are the definitive hosts.)
B. Cats (Cats are not the definitive host for Schistosoma mansoni. They can act as intermediate hosts, but humans or rodents are the definitive hosts.)
C. Birds (Birds are not the definitive host for Schistosoma mansoni. They can act as intermediate hosts, but humans or rodents are the definitive hosts.)

**Clinical Pearl:** Schistosomiasis is a zoonotic infection that affects millions of people globally. It is crucial for healthcare providers to understand the definitive hosts of the parasites causing these infections to effectively diagnose and treat the disease.

**Correct Answer Explanation:**

In the case of schistosomiasis, the definitive hosts are rodents (e.g., rats and mice), which are the primary reservoir for Schistosoma mansoni. Humans serve as accidental intermediate hosts when they come into contact with the parasite-infested freshwater snails. The infection typically occurs when the cercariae (mature parasites) penetrate the skin of humans during contact with contaminated water. The parasites then develop into schistosomula and migrate to the lungs, where they penetrate the blood vessels, reaching the liver, spleen, and finally the mesenteric veins, where they mature into adult worms. These adult worms form reproductive pairs in the mesenteric veins, causing an inflammatory response and leading to the characteristic signs and symptoms of the disease.

**Core Concept Explanation:**

Schistosomiasis is a parasitic infection caused by flatworms called schistosomes. Humans and rodents serve as intermediate and definitive hosts, respectively. Humans become infected by coming into contact with freshwater snails, which are the intermediate hosts of these parasites. Schistosomula then migrate to