Question

Trematodes (flukes)- What is the difference between the definitive and the intermediate hosts, what organism is needed in the lifecycle of all of these flukes, and where are these organisms found?

Answer 1

The name trematodes is derived due to the presence of the conspicuous suckers. They are also called flukes, which causes a variety of infection to human and are belong to digenetic trematodes.

The trematodes complete their life cycle in two different host such as definitive host and intermediate host.

DEFINITIVE HOST: Domestic animals and humans are included, habour adult parasite.

INTERMEDIATE HOST: The larval development occurs first intermediate host are freshwater snails or mollusc, the second intermediate host fish an crab, where the larvae encyst.

LIFE CYCLE :

1. Development in humans and Sexual maturation

The infection acquire by the human and other intermediate host by the ingestion of the aquatic plants ,crab or fish harbours the encysted cercariae or the free living cercariae skin penetration. (e.g., schistosomes). Inside the definitive host , migration of the developing young flukes to the site of localisation, here they develop into the adult flukes, sexually mature and begin to lay eggs. The adult flukes produce eggs which are excreated out through the feces, urine, or sputum and reaches the water. They show different development in water, which depend upon their embryonation, as following

* The egg are embryonated when laid, which have a fresh wimming ciliated embryo ( miracidium), hacth out of the egg and enter into the host. (Example: Schistosoma)

* The egg, when laied not embryonated are first matured on the water and the hatch out the miracidium and penetrate into the snail host. (Example: Paragonimus, Fasciolopsis, Fasciola)

* The embryonated egg , which hacth only on ingestion by a suitable host. (Example: Clonorchis and Metagonimus)

2. Asexual ( Larval) development in snail

The larvae from the embryonated egg penetrate into the intermediate host tissue. A sac like structure is formed in the liver or lymph spaces by the trasformation of miracidium transformation known as sporocyst. The sexual multiplication occurs by resulting in the production of the daughter sporocyst (second generation sporocyst). The rediae developed from the sporocyst, having an oral sucker,a birth pore , a muscular pharynx and a sac-like intestine. The radiae either transform into cercariae or multiply to produce daughter radiae. At the stage of radiae the asexual multiplication of the flukes occurs. The radiae form is absent in the schistosoma.

The final stage of development of the miracidia are cercariae. about 1,00,000-2,50,000 of cercariae are from a single miracidum. The free living aquatic form are cercariae, not able to feed themselves so they have to find new host or they die. They possess a tail. When they comes in contact with definitive host discard their tail and enter into the host by penetration or by the encyst to metacercariae in aquatic plants or fresh water crabs and fishes.