TransmissionThe schistosome life cycle is just that – a cycle. From human to water to snail to water and back to human. The adult worms live in the blood vessels of the human host, and the eggs that the female worms lay break from the blood vessels into the bladder or intestine of the host. These eggs then leave the human body in the excreta of an infected person, hatch on contact with water and release microscopic larvae called miracidia. In order to survive, the tiny larvae must find and penetrate a specific fresh water snail. Once inside the snail host, the miracidium transforms through an asexual reproduction cycle to produce thousands of new parasite larvae (cercariae), which leave the snail into the water. People are infected with these cercariae during contact with infested water during their normal daily activities such as hygiene, recreation or farming by irrigation. The cercariae are capable of penetrating an individual's unbroken skin within a few seconds, and having done so they continue their biological cycle by migrating through the lungs to reach the liver. It takes about 45 days from penetration before the worms are adult and mature in the liver, and they then pair off, with the male worm wrapped around the female as shown. The male transports his mate to their final site in the human host blood vessel where they can live for several years producing many eggs every day. |
