The effect of some biological and physical factors on infection of Biomphalaria glabrata with Angiostrongylus cantonensis.

Several biological and physical factors which may influence infection of Biomphalaria glabrata snails with the first stage larvae of Angiostrongylus cantonensis were studied. These factors were: the size of snails, the number of first stage larvae to which snails were exposed, the age of larvae, individual exposure compared with mass exposure of snails, the length of exposure period and the effect of temperature and light. The results showed that young snails, 2 mm in diameter, became infected with significantly smaller numbers of third stage larvae than larger snails (4, 8, 12 and 16 mm). No significant differences in the intensity of infection were evident between the larger size groups. The number of third stage larvae recovered from snails was directly related to the number of first stage larvae to which the snails were exposed. The mean percentage recovery per snail was more or less constant regardless of the infective dose. First stage larvae showed a slight reduction in their survival rate within 7 and 4 days, at 24 degrees C and 26 degrees C respectively, after which a sharp decrease in their survival rate occurred. However, the infectivity of larvae was progressively reduced from the second day at these two temperatures. The mean intensity of infection in snails was independent of whether the infection was by mass or individual exposure. The average number of first stage larvae entering a snail was greatest during the first half hour of exposure, this decreased considerably during the second half hour, and thereafter the number of larvae which entered a snail was low. It was concluded that 26 degrees C was the optimal temperature for infection and that the presence or absence of light had no effect on the infection.