B Webster2; D Rollinson2; T Pennance2; F Allan2; A Emery2; A Gouvras2; K Poulton2; E Lugli2; S Kinung'hi1; T Angelo1;
1 National Institute for Medical Research, Dar es Salaam, Tanzania; 2 Natural History Museum, UK
DiscussionFrom a global health perspective, the Neglected Tropical Disease (NTD), human schistosomiasis, is the most important water-borne parasitic disease. It is a chronic and debilitating disease caused by infections with trematodes of the genus Schistosoma, which are highly endemic in many subtropical and tropical regions. The parasites, 'schistosomes', have a two-host lifecycle with an asexual stage in particular species of fresh water snail vectors and a sexual stage living in the blood vessels of their human hosts. Whilst Mass Drug Administration (MDA) with praziquantel, behavioural change, education and snail control are having a major impact on schistosomiasis transmission, further research into schistosome transmission biology together with more tailored tests and tools for transmission monitoring and surveillance are required to help achieve these ambitious goals. Additionally, schistosomiasis transmission is highly dynamic with water resource development/management, climate change and the high mobility/migration of people all being important factors that can dramatically and rapidly affect the spread, intensity and introduction/re-introduction of the disease, exacerbating the requirement for accurate ways to monitor transmission.
Molecular vector (mainly insects) xenomonitoring is a powerful tool for monitoring the transmission of several parasitic infections. Detection of infection/transmission of schistosomes in snail intermediate hosts (vectors) has primarily relied on morphological methods based on observed parasite emergence but this method lacks sensitivity (very few snails are observed shedding), specificity (animal and human schistosome cercariae can not be easily morphologically identified) and is a highly labour intensive process. Here we will report on the development of a molecular assay for the xenomonitoring of S. mansoni, which causes human intestinal schistosomiasis, in Biomphalaria spp snails in Mwanza Tanzania and how this has been applied to investigate transmission dynamics in different endemic settings in the Mwanza region. Biomphalaria snails were collected from multiple water contact sites from 3 low transmission areas and 1 high transmission area in Mwanza. Snail were checked for patent infections by cercarial shedding and all negative snails were preserved for molecular xenomonitoring. Up to 10% more snails were found to be infected following the molecular xenomonitoring analysis and this varied by location and site. Our data shows that even in low transmission settings xenomonitoring methods can be used to detect and monitor transmission but specificity and sensitivity of the assays is important due to multiple other trematode infections existing in such settings.