In Uganda, despite many years of control efforts, schistosomiasis (bilharzia) is still a big problem, affecting millions of people in 73 of its districts. People here are generally infected with the intestinal form of the disease, which causes a lot of pain and discomfort in the stomach, diarrhoea and in some cases even liver problems. Schistosomiasis is transmitted in water, with the parasite needing to first infect an intermediate freshwater snail host before it can infect humans. This means that transmission can only occur in areas where the water that people use for cooking, bathing, fishing etc is also suitable habitat for these snails.
Current control programmes, which distribute the treatment praziquantel to high-risk populations such as school-aged children, target areas where they believe transmission is most likely. As conducting surveys to identify at-risk areas takes valuable time and money, control programmes make use of information known about the relationship between the disease and the natural environment to help identify these high transmission risk areas. In our study, published in Infectious Diseases of Poverty this week, we revisited the assumptions made about the relationship between schistosomiasis and altitude, and discussed the impact this may have on the national control programme in Uganda.
Prior to our study, it was thought that transmission at altitudes above 1400m in Uganda were unsuitable for schistosomiasis transmission, primarily because water temperature at higher altitudes was too cold for the snails to survive. To test this assumption, we conducted snail surveys within the Mount Elgon and Crater Lakes regions of the country (below) to establish whether the species needed to transmit schistosomiasis (Biompalaria spp.) could be found. To our surprise, snails were found at altitudes as high as 2000m, where water temperatures were well within the range of that required by the snail (15-31 °C) as can be seen in the left-hand scatter plot below.
Of course, the presence of snails does not necessarily equate to the presence of disease in humans. However, school-based prevalence surveys conducted alongside the snail surveys indicate that local transmission is occurring, with the most sensitive diagnostic test used (SEA-ELISA) detecting a prevalence of infection of 27% (40/150) in the the three most elevated schools (1856-2072m) included in the survey (right-hand scatter plot above). Previous studies that have conducted school-based surveys at high altitude generally used less sensitive diagnostic methods, which may be why it was thought that the disease was not found in these areas. In our study for example, when using the least sensitive diagnostic test (Kato-Katz) only 2 of the 150 children (1.3%) surveyed in the three most elevated schools tested positive for schistosomiasis.
In Uganda, schools at high altitude (> 1400m) are currently excluded from the control programme due to the assumption that risk of transmission is low. The results of our study however suggest that these assumptions need to be revisited, with transmission being possible up as high as 2000m. With 15% of the population of Uganda (6 million people) living within the 1400m-2000m altitude range, we now need to learn more about whether other high altitude locations also show signs of being able to sustain transmission, and encourage the expansion of the control programme accordingly. We’re now planning some field work in Uganda this summer/summer, the results of which will hopefully allow us to explore this further. I’ll let you know how we get on!
To read more about my work, please visit my website www.focal-ntds.org!
M. C. Stanton, M. Adriko, M. Arinaitwe, A. Howell, J. Davies, G. Allison, J. E. LaCourse, E. Muheki, N. B. Kabatereine, and R. J. Stothard (2017) Intestinal schistosomiasis in Uganda at high altitude (>1400 m): malacological and epidemiological surveys on Mount Elgon and in Fort Portal crater lakes reveal extra preventive chemotherapy needs, Infectious diseases of poverty, 6:34