People contract snail fever, also known as bilharzia or schistosomiasis, when worm larvae that have been released into freshwater ponds and rivers by infected snails burrow through their skin. It affects more than 100 million people in sub-Saharan Africa and is particularly common in children.
The disease can affect growth, development and cognition and can damage internal organs. People can build up resistance to the infection over time but until now, scientists were unclear what eventually triggers an effective immune response and why (unlike in many other diseases), this only happens after several years of repeated infection.
A team of researchers from the University of Edinburgh used sophisticated computer models to analyse data collected from Zimbabwe over 20 years and check this against different theories relating to the body’s immune response. Millions of computer simulations were run to find out which theory was correct.
Reporting in the ‘Proceedings of the National Academy of Sciences’, they show that the body’s immune response is only triggered once the parasitic worms, which can live in human blood for several years, start to die. The dying or dead worms release molecules known as antigens that are recognised by the body’s immune system, which in turn creates antibodies to fight against the infection. These antibodies prevent the remaining worms from laying eggs, thereby stopping the cycle of infection.
The findings are useful for helping scientists to design effective vaccines, a process that has been slowed down by the uncertainty over the body’s immune reaction. They also have implications for maximising the effectiveness of drug treatment programmes to ensure that the disease is treated without hindering a person’s ability to build up their own immunity.
Kate Mitchell, who carried out the research as part of her PhD in the University of Edinburgh’s School of Biological Sciences, said: “Very young children can be infected because they do not have access to clean water. Although they may build up immunity over time, by the time they become resistant to snail fever, long-term damage has often been done.
“This study will help further research to inform the optimum duration and intensity of drug programmes to target the disease, while ensuring an individual’s ability to build up immunity to it is not hampered, as well as help research to find vaccines for snail fever.”
The study was funded by the Wellcome Trust and the Medical Research Council.
Image: The parasite Schistosoma haematobium, the cause of schistosomiasis. Credit: Lanarkshire Infectious Unit, Wellcome Images.
Mitchell KM et al. Protective immunity to Schistosoma haematobium infection is primarily an anti-fecundity response stimulated by the death of adult worms. Proc Natl Acad Sci 2012 (epub ahead of print).