Schistosomiasis(*)

Article excerpt

1. Brief description of the condition/disease

Schistosomiasis is a tropical disease caused by several species of parasitic worms of the genus Schistosoma that live within human blood vessels. The female worm produces eggs that are fertilized by male worms and deposited in the vessel walls. The eggs either leave the body in the faeces or urine, or remain in the tissues where they cause inflammation and scarring. Symptoms are related to the location and number of eggs. S. mansoni and S. japonicum primarily cause disease of the intestines and liver, including diarrhoea, abdominal pain, fibrosis of the liver and collateral circulation; S. haematobium primarily affects the bladder and urogenital system, causing bloody urine and problems with micturition and fertility. If the eggs reach fresh water, they hatch, and the embryo (miracidium) swims in search of a susceptible intermediate host (snail species). Within the snail, they develop into larvae (cercariae) which are shed into the water and then become infective for humans.

Infection occurs when human skin contacts these fresh-water cercariae, which penetrate the skin, develop into adult worms, and migrate to the veins of the abdominal cavity, where the adult females release millions of eggs. The eggs escape to the lumen of the urinary bladder or intestine and are passed out during micturition or defecation, reach fresh water, and hatch into miracidia which infect the host snails that produce cercariae, and begin new infections.

2. Current global burden and rating within the overall burden of disease

An estimated 200 million persons in tropical areas of the world are infected with the parasites. Of these, 120 million persons are symptomatic and 20 million have severe disease, with manifestations that include hepatosplenomegaly and portal hypertension for intestinal schistosomiasis. For urinary schistosomiasis the disease manifestations range from haematuria to squamous cell cancer of the bladder. The loss in productivity resulting from schistosomiasis is 4-44 person-days per year.

3. Feasibility (biological) of elimination/eradication

Humans are the principal reservoir of S. mansoni and S. haematobium. Although other mammals (e.g. baboons) can be permissive hosts and can be infected, they do not contribute significantly to human transmission in most endemic areas. Domestic and other animals can also be infected with S. japonicum and figure significantly in transmission where they share living areas or their faeces are used as fertilizer.

Comprehensive control programmes, including mass treatment with antischistosomal drugs, health education, application of moluscicides, and other measures have reduced the prevalence dramatically in many areas. However, although they often reduce morbidity and mortality, control programmes have not usually led to elimination or eradication of transmission. …