The stimulus provided to cercariae by the skin fat of their definitive hosts is not the same for Schistosoma mansoni and Austrobilharzia terrigalensis. Cercariae of the former parasite are stimulated by unsaturated fatty acids, those of the latter by cholesterol.
A reinvestigation of the penetration stimulant factors present in crude egg lecithin, which contains both cholesterol and free fatty acids as impurities, has shown that the latter are responsible for the stimulus in S. mansoni. The phospholipid fraction is inactive as is pure cholesterol. A little excitation of cercariae is produced by the monoglyceride fraction. These results fully confirm earlier findings.
Of surface active agents other than lecithin, stimulatory activity was found only in one product which possessed carboxyl groups. Of the C18 acids examined, stearic (18:0) is inactive, oleic (18:1) slightly active, linoleic (18:2) and linolenic (18:3) acids highly active. Oleic acid was shown by GC-mass spectrometry to be the most abundant acid in crude egg lecithin and is probably the main penetration stimulus present.
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