Biomphalaria pfeifferi is a keystone intermediate host snail for Schistosoma mansoni. Broadly speaking, wherever this snail species appears transmission foci of intestinal schistosomiasis soon follow. Over the past five years, HUGS has been tracking its expansion along the Lake Malawi shoreline and today, we are regularly detecting infected snails shedding human cercariae in Mangochi District. In February 2023, our parallel investigation into the genetic diversity of this species was published in Tropical Medicine and Infectious Diseases. After DNA typing all collected snails, Bi. pfeifferi exhibited very low genetic diversity and we have confirmed autochthonous transmission of intestinal schistosomiasis locally.
At the same time and now highlighting our ongoing snail surveillance fieldwork, the HUGS Malawi team returned to our snail collecting sites in Mangochi, Chikwawa and Nsanje districts in late February. This was for the first time in 2023, with our surveillance seeking to address the question: “Which environmental-, ecological- or genetic-drivers enhance or diminish the transmission of hybrid schistosomes?”. Our snail surveys take place quarterly throughout the year, this present survey being our fifth, with later activities continuing until summer 2024.
The HUGS Malawi team inspects a total of twelve carefully chosen freshwater habitats ; seven collection sites are in Mangochi District, two collection sites are in Chikwawa District and three collection sites are in Nsanje District. As expected, Bulinus snail species were found at all sites bar one in Mangochi District. Further testing revealed a single snail from Mangochi District shedding human cercariae. In comparison with earlier surveillance data, snail shedding rates are at their lowest ebb this time of year.
The importance of regular forward-looking snail surveillance cannot be overstated. While we continue to find Bi. pfeifferi in Mangochi District, it came as a very big surprise to find this species in Nsanje District. Through Gladys Namacha’s keen eyes this species was spotted at Nsanje Port being the very first time this snail has been found in the Lower Shire River. This unprecedented finding expands its known range in Malawi some 200 km southwards for it has never been known from Chikwawa or Nsanje Districts. Our observation raises a new array of research questions that need to be answered, and quickly, if we are to stop another outbreak of intestinal schistosomiasis in Malawi.
The HUGS Malawi and UK teams are soon to return to Nsanje District to conduct a more thorough search for this snail. Our surveys will be later embellished by advanced geospatial mapping analyses by Clinton Nkolokosa, a Wellcome Trust MSc Research Fellow. Over the next year Clinton, who recently graduated with a distinction in his taught course component, will be assisting both HUGS and SHIRE_Vec projects with his environmental research.