Schistosomiasis is an infection of humans caused by a parasitic trematode worm. There are three main species causing the infection. Schistosoma haematobium is endemic to Africa and the Middle East. Schistosoma mansonii occurs in the Caribbean, South America, Africa and the Middle East. Schistosoma japonicum occurs in the Far East. Two other species also infect humans but they are not considered so important. They are Schistosoma mekongi found in South East Asia and Schistosoma intercalatum found in Central West Africa.
When one is looking for environmental methods to control this parasite it is first necessary to know something about its’ lifecycle. During its lifetime, the parasite has two free-swimming forms and two different hosts. The primary host in which the adult resides is the human while the secondary host for the larval form is a water snail. While the snails acting as secondary hosts to Schistosoma mansonii, Schistosoma haematobium and Schistosoma intercalatum live in rivers and lakes those of the Asian varieties, live in the rice producing paddy fields.
Starting with the adult form eggs produced by the parasite pass out of its’ human host in the feces or in the case of Schistosoma haematobium the urine. If the eggs reach a suitable water source, they hatch to release the first of the free-swimming forms, known as a miracidium. The miracidia will swim until they encounter a suitable water snail which they enter via its’ foot. Once in the snail the Schistosoma will develop through two generations of sporocysts until it can release the second of the free-swimming forms the very mobile cercariae. Each infected snail can release thousands of cercariae in a circadian pattern the release depending on light and the water temperature. The cercariae can penetrate human skin and once within the host will travel first to the lungs then after about ten days to the liver. It is in the liver that the parasite finally matures and forms a bonded pair. The larger female schistosome lies within the gynaecophoric channel of its’ smaller mate. Once fully matured the pair migrates to either the mesenteric veins of the gut or the veins of the kidney, ureters or bladder in the case of Schistosoma haematobium. Once at their final destination the pair starts producing eggs to restart the cycle. Normally the adult schistosomes remain alive within their human host for about four years although infections can persist for up to 20 years.
The first point at which to break this cycle is to prevent the eggs reaching the water. A modern sewage system for handling human waste is the ideal solution. This solution is not always feasible in the often poverty-stricken countries, which are home to this parasite. There are some cheaper alternatives such as the composting toilet, which in addition to keeping the eggs away from water until they are no longer viable also provides a safe, and valuables source of soil nutrition.
The next stage at which to break the parasites life cycle is by controlling the secondary host. A number of chemicals have been used in attempts to eradicate the snails in lakes and rivers. These include copper sulfate, niclosamide and acrolein. There is evidence that the Gopo Berry, native to Africa, can be used to control the snails as can the sapindus plant. Another method, which has been proposed, is the introduction or augmentation of predator species such as crayfish. The introduction of a foreign species into an ecosystem should always be handles with caution. The smoothing of the sides of irrigation canals so removing the crevices where snails used to lodge has proved to be an effective measure for control.
The next measure, which can be employed, is preventing humans getting into the water. It has been found that the younger a person is when first infected the higher the parasitic load they carry and the more severe the illness. In many of the countries one of the few play times the children have is when they get to swim in the rivers and lakes, which are in effect making them ill. The provision of safe cheap swimming pools in parts of South America has led to a marked decrease in the number of infected individuals.
While any or all of these provisions can help prevent infections caused by Schistosoma haematobium, Schistosoma mansonii and Schistosoma intercalatum the control of the Asian species present a different problem. Schistosoma japonicum has a number of other species, which can act as hosts to the adult worm. These other hosts include dogs, rodents, cats, goats and horses. Dogs are also hosts to the adult form of Schistosoma mekongi. Keeping these animals out of the rice paddies and so preventing the snails residing there becoming infected is an impossible task. The use of chemicals to control the snails is also to be avoided, as the rice is such a major food source for the population. It is also impossible to keep the humans out of the rice paddies, as the plants need tending. In endemic areas of Asia, the best form of control for the parasite is regular testing and, when required, treatment with a single dose of the drug praziquantel.
