Question

Intermediate host is absent in the infection of:
A. Plasmodium
B. Trypanosoma
C. Entamoeba
D. Filarial worm

Answer 1

Hint: It is an anaerobic parasite which is a major cause of diarrhoea in developing countries. It is responsible for the transmission of amoebiasis in countries that have poor sanitary conditions. This parasite is known to thrive in such conditions and cause the disease.

Complete answer: Entamoeba histolytica is an anaerobic parasite that causes amoebiasis in humans and other primates. It is the cause of tissue destruction which leads to amoebiasis. It acts in three stages which are as follows; direct host cell death, inflammation and parasite invasion. The life cycle of the parasite can be categorised in two forms; the cyst and the trophozoite stage. The cyst form of the parasite is infective and non-motile, however, the trophozoite form is motile. The infective form of the parasite is a spherical mature four-nucleate cyst which can enter into the alimentary canal from fecally contaminated food and water. The refractile wall after cyst passes through the stomach and enters the caecum of the alimentary canal where excystation takes place. Afterwards, the four nuclei undergo binary fission to form 8 trophozoites which then travel to the large intestine and enter the submucosal tissues where trophozoites multiply by binary fission and cause amoebiasis. A proportion of trophozoites enter the lumen of the bowel and transform into cysts which are then passed in faeces. Since the cysts are themselves able to infect another healthy host, therefore, they do not require any intermediate host. The parasite infects around 35-50 million people worldwide. It kills about around 55,000 people each year.

Note: The cyst form is excreted in the faeces where it can survive for weeks. Even though it can be prevented by maintaining proper sanitary conditions, it becomes difficult in developing and under-developed countries due to the dense population and extremely unhygienic conditions.