Reducing prevalence of schistosomiasis

Schistosomiasis is a serious parasitic disease affecting approximately 350 million people, with a further 600 million at risk. Four species of Schistosoma are pathogenic for humans. The infection is acquired through contact with water inhabited by aquatic snails, who are the intermediate hosts. The infective larvae penetrate the skin and develop into worms that inhabit the blood vessels, the intestines, or the urinary bladder. Schistosomiasis is a serious chronic disease that seriously impedes child growth and human productivity. In its more severe forms, it can cause paralysis and death. Schistosomiasis is an environmental health risk, and is exacerbated by any human activity that increases the habitat for aquatic snails, such as irrigation and damming projects.

This strategy features in the framework of Agenda 21 as formulated at UNCED (Rio de Janeiro, 1992), now coordinated by the United Nations Commission on Sustainable Development and implemented through national and local authorities.

The aim is to implement control programmes in countries where major human parasitic infections are endemic and achieve an overall reduction in the prevalence of schistosomiasis and of other trematode infections by 40% and 25% respectively by the year 2000, from a 1984 baseline, as well as a marked reduction in incidence, prevalence and intensity of filarial infections.

Although the disease is clearly environmental in nature, the health risk is increased by human activity. Currently, the conventional environmental fix is to spray bodies of water with a chemical molluscicide to kill the intermediate snail host. The major drawbacks of this intervention are that it is difficult to sustain (because of its cost) and that it seriously contaminates the environment. The recommended molluscicide is toxic to many aquatic organisms, and is persistent in the environment, thus amplifying its toxic effects on nontarget organisms. In the early 1960s, a group of researchers in Ethiopia discovered that when tiny amounts of a dried soapberry ( Phytolacca dodecandra) were crushed and mixed with water, the resulting solution killed freshwater snails, the intermediate host for the schistosome parasite. The active ingredient in this solution was Endod, a saponin produced by the soapberry plant. Because the soapberry plant was endemic to the region, scientists thought that this plant could replace the conventional synthetic molluscicide. Soapberry was already used by local populations to wash clothes; thus, its acceptance for schistosomiasis control would be relatively easy. Scientists at the Institute of Pathobiology of Addis Ababa selected some high-yielding cultivars of the plant and studied their life history. However, the use of this natural molluscicide could not be promoted until it was proven safe. A series of international workshops and consultations was initiated, culminating in a full battery of toxicity testing in accordance with OECD Minimal Data Requirements for Pre-market chemicals. It was established that Endod is either nontoxic or only slightly toxic to mammals and is nonmutagenic. Ecotoxicity testing has indicated that Endod is no more toxic than the recommended synthetic molluscicides. Indeed, after application, Endod does not remain or build up in the environment. The use of Endod as a botanical molluscicide would offer many advantages. It is both biodegradable and locally available. It can be grown on marginal land by rural people, with the added advantage of stabilizing soils and preventing erosion. Its acceptance as a method for the control of schistosomiasis may also increase its value as a cash crop. Furthermore, it would put the means of controlling a severe environmental health hazard in the hands of the people most at risk, increasing the sustainability of the solution. A team of researchers at the Blair Research Laboratories in Zimbabwe are now started studying the effectiveness of Endod application for schistosomiasis control, looking at the actual decrease of infection rates, the community acceptance of the scheme and the cost efficiency of the intervention.