Infection of laboratory bred Biomphalaria alexandrina from Giza and Alexandria governorates with Schistosoma mansoni from Giza in relation to snail size and number of penetrated miracidia.

Snail size and the number of Schistosoma mansoni miracidia penetrated are two important factors among others that might influence Biomphalaria alexandrina infection with S. mansoni. Groups of 16 snails of fixed age but of different size class (2-4 mm), (4-6 mm), (6-8 mm), (8-10 mm) and (10-12 mm) from three successive generations of positive and negative laboratory bred B. alexandrina from Giza and Alexandria governorates were each exposed to five S. mansoni miracidia (TBRI from Giza) under light and in water at 25 degrees C. After 20 days, the snails were examined for cercarial shedding to determine their prepatent periods and to estimate the number of cercariae shed from each snail under light for one hour. Other groups of snails (4-6 mm) from three successive generations of positive and negative laboratory bred B. alexandrina from these two Governorates were each exposed to 3, 5, 10, 15, 20 and 25 miracidia. Snails were checked starting from the 20th day of infection to assess whether or not they had acquired infection. The infection rate of B. alexandrina from Giza and Alexandria governorates and their generations decreased significantly as the snail size increased while it increased significantly as the number of miracidia increased. However, there has been a decline in the susceptibility of snails from one generation to the next. The prepatent period for the shedding of cercariae was long in large sized snails and from those descending from negative snails. It can be concluded that the infection rate of B. alexandriina snails is affected by snail size and the number of miracidia penetrated.