"schistosomiasis" Schistosomiasis was probably the most widespread serious worm infestation at the end of the 20th century. Its two main variants affect over 200 million people in Africa, and South and East Asia. Infestation causes chronic ill-health, chiefly due to anemia, and a variety of specific, often serious, complications.
The agent is a blood fluke, a flatworm that is 6–25 mm long. The life cycle involves stages in snails and mammals and there are two distinct intermediate stages in which the parasite swims as a free-living creature. The adult worm lives in and feeds on blood in veins around the bladder or intestine.
The Egyptian (African) variety usually inhabits veins around the bladder, the Oriental variety is harbored mainly in veins around the large intestine. In the Nile delta, and in the new irrigation regions developed since the Aswan High Dam was completed, and elsewhere in Africa where livelihoods depend on irrigation of crops by people whose bare skin is exposed to contaminated water, almost everyone has schistosomiasis, also called bilharziasis in honor of Theodor Bilharz, who identified the parasite.
The eggs of bladder flukes are excreted in the urine, and those of the intestinal variety are excreted in the feces. The eggs develop into free-swimming larvae, which invade and pass the first developmental stage of their life cycle in freshwater snails.
The larvae reproduce asexually in the snails to give rise to large numbers of free-swimming tiny worms called cercaria that are released in periodic bursts through the snail's skin. The cercaria swim free until they encounter bare human skin, which they penetrate (causing itching and a rash); once in the human host the cercaria develop further into adult worms that migrate to the veins around the bladder or large bowel. Adult schistosome worms may live many years, a male and female together in permanent copulation, laying millions of eggs.
Schistosomiasis is difficult to control. Attention has been aimed mainly at the snails: if these can be eliminated, the life cycle is broken. Education to prevent people defecating and urinating in the water helps, but water buffalo and other mammals may also be hosts to schistosomes, so education alone is not enough.
In China, where manual labor is cheap and plentiful, successful control was achieved in many places by turning over sods of wet mud to allow it to dry in the sun, thus killing many snails. Another strategy was to cut vertical walls in the banks of irrigation canals and ponds; the snails breed at the water's edge on sloping surfaces and cannot breed if the land-water junction is vertical.
Alternative irrigation methods such as sprays rather than canals, and lining canals with cement, have worked well in other parts of the world such as Japan. Poisons that kill snails are less effective for oriental schistosomiasis than for the African variety. Treatment of schistosomiasis is mainly directed at people from outside endemic regions, though if its costs could be covered in places where virtually everyone is infected, it might help to break the transmission cycle.
John M. Last
See also worms
How to cite this entry:
John M. Last "schistosomiasis" The Oxford Companion to Medicine. Stephen Lock, John M. Last, and George Dunea. Oxford University Press 2001. Oxford Reference Online. Oxford University Press. 27 August 2011
