Seasonal Fluctuation in the Infestation of Planorbis trivolvis with Larval Trematodes

trematodes was made in the study of schistosomiasis in Egypt by Manson-Bahr and Fairley (1920) who kept a record of the cercariae found in the snails which they dissected over a period of one year. They found the cercariae of the human blood flukes, Schistosoma haematobium and S. mansoni, present in the snails at all seasons of the year. The highest percentage of infested snails occurred during the month of December. They note also that the infestations found in the autumn months were all mature, whereas in the spring months immature cercariae were the rule rather than the exception. Their observations on the other larval trematodes found in the snails indicate the same general trend, for the snails were infested throughout the year, with the highest percentage occurring in December. Their study was made in a part of the world in which drought has a great influence on the snail population and where there is not a marked fluctuation in temperature from season to season.