Pathogenesis+-+Schistosomiasis

Infection occurs when skin comes in contact with contaminated freshwater. Freshwater becomes contaminated by Schistosoma eggs when infected people urinate or defecate in the freshwater. In the water the eggs hatch, and if the appropriate species of snails are present the parasites penetrate the snail tissue and multiply within. The parasite then leaves the snail tissue and enters the water. The parasite is able to live about 48 hours outside of its host. Schistosoma parasites can bore through the skin of people who come in contact with contaminated water, typically while wading, swimming and bathing. Over the course of several weeks, the parasites travel through the hosts tissue and develop into adult worms within the blood vessels of the body. After the worm reaches maturity, it mates and females produce eggs. Some of these eggs travel to the bladder or intestine where they are excreted through urine or stool.

//http://www.cdc.gov/parasites/schistosomiasis/gen_info/faqs.html//

After being excreted through urine or stool the eggs may enter the freshwater directly as a result of poor sewage systems or direct elimination into the water. If the conditions are optimal, the eggs hatch and release miracidia. Miracidia swim and penetrate specific snails to and multiply within. Within the snail the miracidia go through 2 stages of sporocyts and production of cercariae (free-swimming larva). After being released, the cercariae can survive outside the snail host for about 48 hours. Within that time the cercariae can penetrate the skin of a human host, shed their tail and develop into schistosomulae. Schistosomulae travel through multiple tissues and stage to ultimately reside in the blood vessels. Adult worms live in the mesenteric venules, particularly in the intestine and rectum (specifically in the case of //S. Japonicum. S. Japonicum// is often found in the superior mesenteric veins draining the large intestine. Schistosomulae are capable of moving between sites and are not only present in one location. The females, around 7 to 20 mm lay eggs in the small venules of the portal and perivesical systems. The eggs move progressibly towards the opening of the instestine and the bladder. Later the eggs are eliminated with feces and urine to repeat the whole process again.. Pathology includes katayama fever, hepatic perisinusoidal egg granulmas, Symmers' pipe stem periportal fibrosis, portal hypertension and occasional embollic egg granulomas in brain and spinal cord.

Human contact with water is thus necessary for infection by schistosomes. Various animals, such as dogs, cats, rodents, pigs, hourse and goats, serve as reservoirs for //S. japonicum//.

//http://www.cdc.gov/parasites/schistosomiasis/biology.html//