Snail borne larval trematodes of the Okavango Delta, Botswana

The Okavango Delta, situated in northwestern Botswana is one of the worlds largest inland delta systems formed by the Okavango River, flowing in a southeasterly direction from Angola. The snail fauna comprises 20 species occurring in the Okavango Delta and surrounding areas. The aim of this project was to determine what types of cercaria were found infecting freshwater snails in the system as well as to determine whether human schistosomiasis was present in the system. Snails are economically important because they serve as the intermediate hosts for a number of parasitic diseases such as schistosomiasis in humans and animals and paramphistomiasis in cattle. During two consecutive field trips to the Okavango Delta in 1999 and 2000 freshwater snails were collected from various localities within the delta. A total of eight different cercariae were shed from six different freshwater snail species. Cercariae were described and placed into their respective families: Pila occidentalis shed a vivax cercaria belonging to the family Cyathocotylidae which are common parasites of birds, reptiles and mammals. Lanistes ovum shed a xiphidio cercariae belonging to the family Plagiorchiidae and are known to parasitize amphibians and birds. Cleopatra elata shed a parapleurolophocercous cercaria belonging to the family Heterophyidae which parasitize fish. Lymnaea natalensis shed two types of cercaria from different localities within the delta, the first type being a strigeid cercaria belonging to the family Diplostomidae and parasitising birds. The second type was a 27-echinostomatid cercaria belonging to the family Echinostomatidae and are known to be parasites of birds. Biomphalaria pfeifferi also shed two types of cercariae, a schistosome cercaria belonging to the family Schistosomatidae known to parasitise mammals and another strigeid cercaria belonging to the family Diplostomidae. Bulinus globosus shed a dark bodied amphistome cercaria belonging to the Family Paramphistomidae and are known to be parasites of livestock. The prevalence of infection was higher in the 2000 survey than in the 1999 survey since the localities that were sampled in 2000 were closer to human settlements. No snails were found to be infected with larval trematodes, which could result in human schistosomiasis. This study of snail borne larval trematodes provided insight into the different kinds of larval trematodes that are present in this unique system and has laid the foundation for further research of the different trematode life cycles occurring in the Okavango Delta.