The correct option for the primary and secondary hosts of bilharzia is C. man and snail. Detailed Explanation: What is Bilharzia? Bilharzia, also known as schistosomiasis, is a disease caused by parasitic worms of the genus Schistosoma. These worms are transmitted to humans through contact with freshwater that is contaminated with the larvae released by infected snails. Hosts of Bilharzia: 1. Primary Host (Definitive Host): This is the host in which the adult form of the parasite lives and reproduces. In the case of bilharzia, the primary host is humans (man). The adult Schistosoma worms reside in the blood vessels of the human host, where they can lay eggs that are excreted in urine or feces, depending on the species of Schistosoma.

1. Secondary Host (Intermediate Host): This is the host in which the larval stages of the parasite develop. For bilharzia, the secondary host is snails. Specifically, freshwater snails of the genus Biomphalaria (for Schistosoma mansoni) and Bulinus (for Schistosoma haematobium) serve as the intermediate hosts. The adult worms release eggs into the water, which hatch into larvae (miracidia) that infect the snails. Inside the snails, the larvae undergo asexual reproduction and develop into another larval stage called cercariae, which are then released back into the water, where they can penetrate human skin.

Why Option C is Correct:

• Man as the Primary Host: Humans are the definitive hosts where the adult worms live and reproduce.
• Snail as the Secondary Host: Snails are the intermediate hosts where the larvae develop before they can infect humans.

Why Other Options are Incorrect:

• Option A: Fish and Man
• Fish are not involved in the life cycle of Schistosoma. The primary and secondary hosts are specifically humans and snails, respectively. Therefore, this option is incorrect.
• Option B: Man and Dog
• While dogs can be hosts for some parasites, they are not involved in the life cycle of Schistosoma. The definitive host is humans, and the intermediate host is snails. Thus, this option is also incorrect.
• Option D: Fish and Snail
• Similar to option A, fish do not play a role in the life cycle of Schistosoma. The correct intermediate host is the snail, but the primary host must be a human, making this option incorrect as well.

Common Pitfalls:

• Confusing the roles of different hosts: It’s important to remember that the primary host is where the adult parasite lives and reproduces, while the secondary host is where the larval stages develop.
• Misidentifying the types of organisms involved: Knowing the specific organisms (e.g., snails of the genus Biomphalaria or Bulinus) is crucial for understanding the transmission of bilharzia.

Revision Summary:

• Bilharzia (schistosomiasis) is caused by Schistosoma worms.
• The primary host is man, where adult worms live and reproduce.
• The secondary host is snail, where larvae develop before infecting humans.
• Understanding the life cycle of the parasite is key to preventing and treating the disease.