Schistosomiaisis is a disease caused by parasitic worms. You become infected when your skin comes in contact with contaminated freshwater in which certain types of snails that carry schistosomes are living.
An estimated 85% of the world’s cases of schistosomiasis are in Africa, where prevalence rates can exceed 50% in local populations.
There are many varieties of schistosomes, but only four which are particularly important in man:
1. S. haematobium occurs throughout Africa and in Arabia, South West Asia, and around the Mediterranean. The urinary tract and the portal system are mainly affected, but the lungs and colon do not escape, and the central nervous system may occasionally be involved.
2. S. mansoni is also prevalent throughout Africa, particularly in the north, in Arabia, and in the north of South America. It mainly affects the colon, the portal system and the lungs, very rarely the central nervous system.
3. S. japonicum is found mostly in Asia; in China and Japan, the Philippines, and other Pacific islands. It primarily affects the colon and small intestine, the portal system and the lungs, rarely the central nervous system.
4. S. intercalatum is much less common and occurs only in equatorial Africa, particularly Zaire, and affects the digestive tract and the portal system.There is one other, S. mekongi, clinically similar to S. japonicum, but found only in the Mekong river basin.
The clinical manifestations of chronic schistosomiasis are the result of host immune responses to schistosome eggs. Eggs secreted by adult worm pairs enter the circulation and lodge in organs and cause granulomatous reactions. Eosinophilia may be present.
S. mansoni and S. japonicum eggs most commonly lodge in the blood vessels of the liver or intestine and can cause diarrhea, constipation, and blood in the stool. Chronic inflammation can lead to bowel wall ulceration, hyperplasia, and polyposis and, with heavy infections, to periportal liver fibrosis
S. haematobium eggs typically lodge in the urinary tract and can cause dysuria and hematuria. Calcifications in the bladder may appear late in the disease. S. haematobium infection has been associated with increased risk of bladder cancer.