Experimental Infections of Biomphalaria Glabrata, B. Pfeifferi and B. Sudanica with Schistosoma Mansoni
A laboratory study was carried out to determine quantitatively the degree of compatibility between biomohalaria glabrata, B. pfeifferi and B. sudanica and Schistosoma mansoni. The effect of host diet and mode of exposure to miracidia, continuous darkness, origin of host species and parasite strains, miracidial dosage level, age and intensity of primary infection in dual infections were examined by measuring snail survival, growth, fecundity, prepatent period, infection rate and cercarial production. The results of experiments on host diet and exposure technique showed that individual exposure of B. glabrata snails to mircidia resulted in injection rates varying from 31% - 85% compared with a range of 6% - 22% obtained in mass exposed snails. Lettuce plus Tetramin and lettuce plus rabbit pellets were found to be superior diets to lettuce alone. Light-deprivation did not significantly reduce the survivorship, growth, fecundity and cercarial production of B. glabrata. The compatibility of large (1479.1mh), medium (493.5mg) and small 101.5mg) B. glabrata with S. mansoni was tested. Medium-sized snails were found to produce the highest average cercarial yield of 628 cercariae per snail per day compared to 377 cercariae produced by small and large snails respectively. This trend was confirmed in a subsequent modified replicate. An investigation of the compatibility between Sudanese B. pfeifferi, Ethiopian B. sudanica and Brazilain B. glabrata to Kenya and Brazilian strains of S. mansoni strain from Kenya. The highest infection rate (60%) and the highest cercarial yield (2792 cercariae per snail) were produced by this host-parasite combination. Exposure of B. glabrata to varying doses of miracidia ranging from 1 to 50 resulted in decreased cercarial production at miracidial densities above fifteen. Four different types of shell deformations were observed in addition to polyembryony and the production of eggmasses which were completely devoid of eggs and embryos.Superimposition of homologous miracidia on pre-existing S. mansoni infections was easily achieved in B. glabrata. However, the effect of superinfection on snail survival, growth, fecundity and cercarial production depended on the sequence of infection and the interval between infections. A challenge infection of five miracidia within an interval of one week, was found to enhance cercarial production and inhibit ovipository activity to a high degree (P < 0.001). On the contrary, snails which received a challenge infection of fifteen miracidia three weeks after a primary dose of five miracidia showed less evidence of ovipository inhibition and had a significantly reduced mean cercarial production (P < 0.05). The significance of the age of the primary infection is discussed.