Answer to Question #64431 in Zoology for Sanjukta Ghosh

Question #64431
Definitive hosts are hosts in which an organism attains sexual maturity usually followed by sexual reproduction(fertilisation). (from Parasitology: A Conceptual Approach By Eric Loker, Bruce Hofkin) Anophles is a definitive host of plasmodium in which the fertilisation is occuring. But I don't understand what is meant by sexual maturity? It would be great if you could explain the determination of definitive host depending on the sexual maturity of a parasite with example.
Expert's answer
Sexual maturity is the ability to perform sexual reproduction. In males it is the ability to produce fertile sperms and in females it is the ability to produce mature eggs. So, definitive host is the host, in which parasite performs sexual reproduction. In case of malaria plasmodium, the macrogametocytes and microgametocytes fuse to perform sexual reproduction in the organism of mosquito, which is a definitive host. It leads to the formation of sporozoits, which can enter the human organism, the intermediate host. Inside the human organism, the parasite reproduces asexually only by means of specific divisions.

The other example is beef tapeworm. It is a flatworm, which resides in the human intestine when sexually mature. Thus, human is a definitive host. The worms in the human intestine reproduce sexually, and the fertilized eggs are released with feces. The eggs preserve in soil for days, and if cattle occasionally engulfs them, the fertilized eggs develop in the intestines through several stages and become larvae. They cross the wall of the intestines, and get into muscle tissue with blood flow. There it resides in the form of cysticerci and does not perform sexual reproduction. Thus, cattle is the intermediate host. If human eats the infected meat, the larva develop into the adult worms inside the human intestine, where sexual reproduction takes place, and the cycle repeats.

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