AIIMS 2009 Zoology Life Cycles MCQ Question

In the context of parasitology, the term "intermediate host" refers to an organism that hosts a parasite during some developmental stage of the parasite's life cycle. Understanding which parasites require intermediate hosts is crucial for understanding their life cycles and transmission.

Explanation of the Correct Answer: C) Entamoeba

Entamoeba is a genus of protozoan parasites, notably Entamoeba histolytica, which causes amoebic dysentery. The life cycle of Entamoeba does not require an intermediate host. Instead, it has a direct life cycle that involves:

1. Ingestion of Cysts: The infectious form of the parasite (cysts) is ingested through contaminated food or water.
2. Trophozoite Stage: Once in the intestine, the cysts transform into trophozoites, which can then invade the intestinal wall and cause disease.

Since Entamoeba can complete its life cycle directly within the definitive host (humans), it does not require an intermediate host for its development.

Why Other Options Are Incorrect

A) Plasmodium: This genus includes the pathogens responsible for malaria. Plasmodium requires two hosts to complete its life cycle: the female Anopheles mosquito (the intermediate host) and humans (the definitive host). In the mosquito, the parasite undergoes sexual reproduction, while in humans, it develops asexual reproduction.

B) Trypanosoma: This genus includes parasites that cause diseases such as sleeping sickness and Chagas disease. Trypanosoma species typically require an insect host (such as the tsetse fly for Trypanosoma brucei) for part of their life cycle, making it necessary to have an intermediate host.

D) Filarial Worm: Filarial worms, such as those causing lymphatic filariasis, also require an intermediate host, usually a mosquito, to complete their life cycle. The larvae develop in the mosquito before being transmitted to humans.

Summary

In summary, Entamoeba is unique among the options listed because it does not require an intermediate host to complete its life cycle, making Option C the correct answer. The other options all involve complex life cycles that necessitate the presence of intermediate hosts to support their development and transmission. Understanding these life cycles is vital for devising effective prevention and treatment strategies for the diseases caused by these parasites.