Species diversity and distribution of schistosome intermediate snail hosts in The Gambia

There is a need for recent information on intermediate snail hosts of schistosomes in The Gambia as the latest studies were conducted over three decades ago. This study assessed the incidence, species diversity, distribution and infection status of schistosome intermediate snail hosts in the country. Malacological surveys were conducted in all 5 regions of The Gambia: Central River Region (CRR), Upper River Region (URR), Western Region (WR), Lower River Region (LRR) and North Bank Region (NBR). Sampling of snails was undertaken at 114 sites that included permanent water bodies such as streams (bolongs), rice fields, irrigation canals and swamps; and temporal (seasonal) laterite pools. Ecological and physicochemical factors of sites were recorded. Snails were identified morphologically and screened for schistosome infections using molecular techniques. Freshwater snails were found at more than 50% (60/114) of sites sampled. While three species of Bulinus were collected, no Biomphalaria snails were found in any of the sites sampled. Of the total 2877 Bulinus snails collected, 75.9% were identified as Bulinus senegalensis, 20.9% as Bulinus forskalii and 3.2% as Bulinus truncatus. Seasonal pools produced the largest number of snails, and CRR was the region with the largest number of snails. Bulinus senegalensis was found more in seasonal pools as opposed to permanent sites, where B. forskalii and B. truncatus were observed to thrive. Bulinus snails were more common in seasonal sites where aquatic vegetation was present. In permanent sites, the abundance of snails increased with increase in water temperature and decrease in water pH. Bulinus senegalensis was found infected with both S. haematobium and S. bovis, while B. forskalii and B. truncatus had only S. bovis infection. While the human parasite S. haematobium was restricted to just four sites, the livestock parasite S. bovis had a much more widespread geographical distribution across both CRR and URR. This new information on the distribution of intermediate snail hosts of schistosomes in The Gambia will be vital for the national schistosomiasis control initiative. Author summary Several studies have been conducted on intermediate snail hosts of schistosomes in The Gambia between the 1950s and 1980s, but few reports of similar studies in recent times are available. After The Gambia initiated its national control programme for schistosomiasis in 2014, the need arose for more and current information on schistosomiasis and the intermediate snail hosts involved indirectly in its transmission. We undertook a malacological survey of freshwater sites in all five regions (CRR, URR, WR, LRR and NBR) of The Gambia. While no Biomplalaria snails were found in any of the 114 sites sampled, three species of Bulinus snails were found. Of the total 2877 Bulinus snails collected, 75.9% were identified as Bulinus senegalensis, 20.9% as Bulinus forskalii and 3.2% as Bulinus truncatus. CRR had the highest number ofsnails amongst the regions and seasonal pools yielded the most snails amongst the habitat types sampled. Habitat type, water temperature and pH of sites were observed to have an influence on snail abundance. All three Bulinus snails were found infected with schistosome parasites. The human parasite (Schistosoma haematobium) had a much more restricted distribution as opposed to the livestock parasite (Schistosoma bovis) which had a wider geographical range. The study provides the first malacological report of intermediate snail hosts of schistosomes in The Gambia in over 3 decades and will be useful to the national schistosomiasis control programme.

Automated summary

This study assessed the incidence, species diversity, distribution, and infection status of schistosome intermediate snail hosts in The Gambia, finding that Bulinus snails were more common in seasonal pools, influenced by water temperature and pH. The study also highlights the importance of the distribution information of intermediate snail hosts of schistosomes in The Gambia for the national schistosomiasis control initiative.