Claire Standley, Department of Zoology, Natural History Museum, London
The study of molluscs enables scientists to engage in a wide variety of different pursuits; for example, to elucidate the phylogenetic relationships between taxa, to assess levels and changes in local or regional biodiversity, or to evaluate conservation actions within a protected area, to name but a few. Here, I would like to introduce another facet to malacological research, and that is its biomedical role. Some snails act as the intermediate host for parasites of medical or veterinary importance, and thus understanding their biogeography, evolutionary history and population structure can assist in predicting and controlling the spread of these parasites in humans and livestock. For the most part, these aims require laboratory work for molecular and morphological analyses, but also extensive fieldwork to collect specimens for analyses.
The snails that I study are of the Biomphalaria genus; these are small, freshwater pulmonates confined mostly to Africa and South America. Specifically, they are hosts to Schistosoma mansoni, a trematode parasite that causes intestinal schistosomiasis in humans, also known as bilharzia. This disease is transmitted to people after penetration of the skin by larval stages of the parasite, which are found in the freshwater habitats used by the snails.
Millions of people ain the tropics live at risk at contracting schistosomiasis, and the World Health Organisation estimates some 200,000 people die each year from the disease. However, distribution of the disease is not always homogeneous, even in areas in which the disease is heavily endemic. One suggestion is that it is variations in the host snail distribution, and perhaps even compatibility to the particular genotype of parasite, that contributes to these uneven patterns of disease prevalence and intensity.
In East Africa, the abundance of small and large water bodies and irrigation systems contribute to making this region particularly favourable for the transmission of schistosomiasis, as well as to the evolution of a diverse and often unique malacological fauna. Nowhere is this most evident in the large freshwater lakes that dot the region, and in particular, the largest of them all, Lake Victoria, which is the study site for my research (see fig. 1).
Fig.1 Africa showing the location of Lake Victoria
Thousands of human settlements and communities depend on the lake’s resources for employment and nourishment, but these everyday activities such as fishing, bathing, collecting water and washing clothes bring people into contact with water that may contain schistosome parasites. It is no surprise that the disease is ubiquitous in these communities that are on the immediate lakeshore, but even in this high transmission zone the disease is not evenly spread, as a recent study by Kabatereine et al. (2004) has shown. That study also showed a trend of increasing prevalence towards the east of Lake Victoria’s northern shore. The question my research seeks to answer is whether the types of snails found at these locations plays any role in the dynamics of prevalence, for the entire perimeter of Lake Victoria.
In order to satisfactorily sample the region, extensive fieldwork surveys were required, and I hope to give here some indication of what life is like on these expeditions, as well as present some of the methods we use and the results we have obtained thus far, based on a field trip I undertook in February and March of this year. Contrary to what some people may think, going out on fieldwork certainly isn’t a holiday! Although Tanzania appears to border over half the lakeshore, Uganda’s many islands and inlets give it the longest coastline on Lake Victoria, around 3300km long, which required long days of driving to cover. In just three weeks, our research team surveyed over 50 sites, from the Tanzanian border in the west all the way to the Kenyan border in the east. We were working under the remit of the EU-CONTRAST grant, which has brought together fourteen collaborating institutions from Africa and Europe to tackle the problem of schistosomiasis transmission and to optimize control strategies in affected regions. My supervisor Dr Russell Stothard and I, both from the Natural History Museum in London, were hosted by the Ministry of Health’s Vector Control Division, and accompanied by two of their field technicians, Moses Adriko and Moses Aguemente. One of the malacologists at the VCD, Dr Francis Kazibwe, also accompanied us, as did a colleague from the Danish Bilharzia Laboratory in Copenhagen, Dr Aslak Jorgensen. In each location, we also established contact with local health officers. This collaborative process has been crucial to the collection and collation of data throughout the study regions. Our group thus contained different forms of expertise, with slightly tangential aims: whereas my research focuses primarily on Biomphalaria and its role in transmission of schistosomiasis, our Danish colleague had a broader interest in malacological biodiversity, and the technicians and local health officers were focused on delivering efficient and effective treatment to affected populations.
These varied perspectives are crucial for understanding the whole picture of schistosomiasis’ influence on the region. However, I will focus on my particular research into snail distributions, habitat type and their infection status. It is reasonably rare to find snails infected with schistosomiasis in the wild, but we found individuals from four separate populations that were shedding cercaria larvae. This rarity is not fully understood, but it may be due to low numbers of eggs reaching the water from faeces, compensated by a high asexual reproductive rate of the proto-cercariae while within the snail. We also recorded the abundance of snails at each site, to if this pattern could be explained by habitat type or water chemistry, and to see if there might be a correlation with the pattern of human prevalence of schistosomiasis. Fig. 2 shows the abundance of Biomphalaria at each site.
After plotting the data, we were excited to note that there appears to be a trend of increasing snail abundance towards the east of the region, which matched the increasing prevalence of human infection in previous studies, such as that of Kabatereine et al. above, as well as our own data from the region.
Some of the snail data was interesting in its own right; for example, whereas the genus is characterised as being discoidal, we found one population of Biomphalaria that did not coil fully in one plane (Fig. 3). This may be simply a case of ecophenotypic variation, or perhaps indicative of a more profound difference between this population and neighbouring ones
Fig. 3. The unusual specimen is on the left; note the leftwards deviation of the final whorl. The right hand sample was collected on the other side of a narrow staright, approximately 10km away.
People always ask how we manage to collect snails without contracting schistosomiasis ourselves! The answer is that we take lots of simple precautions against coming into contact with any water which might contain larval schistosomes. This means waders and long scoops to collect snails from the water, and gloves and forceps whenever the snails themselves are handled. Fig. 4 on the right shows one of the technicians, Moses Adriko, collecting from a reed bed in a site in western Uganda. We also went into almost 30 schools and collected stool samples from children to test whether they had the disease. Processing dozens of such slides is certainly not a glamorous way to spend an evening, but it provided very important data, particularly in terms of monitoring the effectiveness of existing national treatment programmes in Uganda
We also collected water samples from each site, and also more generally from the lake, and were astonished to find how widely the pH values varied; Biomphalaria snails were found in sites with a pH ranging from 6 to more than 10. It has been suggested that the high values could be caused by human contaminants such as cleaning products, but we also found high values in sites far from human habitation, and in the middle of the lake. More likely, they were caused by the acquisition of H+ ions during photosynthesis, which leads to daily variation in the pH level; it could be that the pH drops again to lower values at night when photosynthetic algae respire. Lake Victoria is known for its algal blooms, so this may be a hypothesis we should test on a later field trip, perhaps by sampling water at night as well as during the day.
In all, we worked hard, drove a lot, camped on some very remote islands and rode in a variety of boats, but also managed to collect a large amount of data which we hope will prove relevant to developing a more detailed picture of schistosomiasis in East Africa. And, of course, one of the benefits of the early starts was the opportunity to experience some beautiful African sunrises!
I would like to thank the following for making this research possible:
· The Natural History Museum, London, for funding my PhD studentship
· My supervisors, Dr Russell Stothard (NHM) and Dr Chris Wade (University of Nottingham)
· The Vector Control Division, Ministry of Health, Kampala, Uganda