Jack Grimes, Imperial College London, 07/07/2017
Water, sanitation, and potentially hygiene are increasingly seen as key components of a sustainable schistosomiasis control strategy. Schistosomiasis control relies primarily upon distribution of praziquantel, the anthelmintic drug, which is highly effective at killing schistosomes. Praziquantel kills the adult worms, which live in the bloodstream. It thus prevents worm eggs subsequently being put into the blood, and causing inflammation, organ damage and other problems.
However, praziquantel does not prevent re-infection after treatment, and it is not effective against juvenile worms that might have infected a person in the weeks leading up to treatment. There is therefore increasing interest in complementing the praziquantel treatment with interventions to prevent transmission of the parasite. People become infected during dermal contact with water containing schistosome larvae (cercariae), and these cercariae are released by snails that were previously infected with other larvae (miracidia) that hatch from eggs in an infected person’s urine or faeces. There is therefore reason to believe that reducing contact with environmental water bodies (river, streams, lakes and so on, that might be infested with cercariae), and proper containment of excreta (urine and faeces) should reduce transmission of the disease. Furthermore, soap is toxic to cercariae, so its use during bathing and the washing of clothes in the river is thought to protect people from infection. However, there is surprisingly little evidence of the relationships between water, sanitation, and hygiene (WASH), and schistosomiasis. Starting to generate this evidence has been the focus of my PhD and postdoctoral work. This work has consisted of three main projects on schistosomiasis and WASH: a systematic review and meta-analysis, a descriptive review, and a national mapping project in Ethiopia.
If WASH is thought to reduce schistosome transmission, then an obvious place to start is to look at whether people with better access to WASH are less likely to be infected. In collaboration with colleagues at the Swiss TPH and Emory University, we carried out a systematic review and meta-analysis (open access paper) to bring together the data in the published literature, to answer that question. We found no studies on soap use and infection (perhaps because soap use, being an activity, is transient and more difficult to assess than access to water and sanitation infrastructure). However, we found that people with better access to safe water did have significantly lower odds of infection, as did those with better access to sanitation. This applied for all species of schistosome. Interestingly, the effect sizes (how much less likely infection was in people with better WASH) varied substantially from study to study – perhaps suggesting that there are many social and environmental factors that mediate an impact of WASH on infection. It’s well known that correlation does not imply causation, and these studies were all observational: their results (and those of the meta-analysis) must therefore be interpreted with caution. People with better WASH are less likely to be infected, but is this because of the WASH, or is it because they are of higher socio-economic status? Higher socioeconomic status likely confers better access to treatment, better knowledge of schistosome transmission and the seriousness of the disease, and perhaps less occupational exposure (people of higher socio-economic status might be teachers rather than fishermen, for example).
With this in mind, the question of the impact of WASH on schistosomiasis can be approached from a different angle: what role does the biology of the parasites and their snail intermediate hosts suggest for WASH? With the same colleagues, we explored this question in a second, descriptive review (open access paper). This review highlighted some surprising aspects of the parasites’ biology. For example, while complete containment of eggs would cause a complete cessation of transmission, the result of containing 50%, 90%, or 99% of the eggs is unclear. It seems unlikely that these would result in 50%, 90% and 99% reductions in worms in people. Instead, it seems that very small numbers of eggs need to get into the water to maintain intense transmission: a miracidium infecting a snail can, over time, develop into tens of thousands of cercariae, each of which has the potential to develop into an adult worm in a person. Even if people have and always use adequate sanitation, infected animals, and eggs washing off the body or clothes might sustain transmission. While water contact has a more direct impact on infection levels, it is similarly difficult to predict the impact of various levels of reduction in water contact, on level of infection. A person coming into contact with infested water at twice the rate will not necessarily have twice the number of worms: immunology and physiology are important regulators of infection.
The third main project was the national mapping of schistosomes and soil-transmitted helminths in Ethiopia (open access paper), carried out in collaboration with the Ethiopian Public Health Institute, the Ethiopian Ministry of Health, the Schistosomiasis Control Initiative and the Partnership for Child Development. The primary aim of this project was to understand the burdens of these parasites across the country, in order to design a control programme. Happily, as children were tested for the parasites across the country, the school WASH was also assessed – allowing for a comparison between schools’ WASH and their parasitic burdens. Schools with more frequent water collection did indeed have higher Schistosoma burdens, but no significant difference was seen when comparing sanitation with schistosomiasis – lending support to the findings of the descriptive review. Once more, this work was observational: it generated evidence for the relationship between WASH and schistosomiasis, but again, it’s possible that correlation does not imply causation. This is the motivation for a hands-on testing of different water supply solutions, as will be seen in the WISER project.