Skin penetration by cercariae of the bird schistosome Austrobilharzia terrigalensis: the stimulatory effect of cholesterol

Abstract

An artificial skin of hardened gelatine was used to examine the factors affecting penetration of skin by cercariae of the bird schistosome, Austrobilharzia terrigalensis.The percentage of cercariae able to penetrate through a gelatine membrane was increased by a factor of 3–4 by coating the membrane with a thin layer of lipid collected from the surface of chicken skin.The free sterol fraction, isolated from chicken skin surface lipid by thin-layer chromatography, stimulated penetration to the same extent as whole skin lipid.Cholesterol was detected in the sterol fraction by mass spectrometry and pure cholesterol had the full stimulating effect on cercarial penetration.Skin lipid, from which free sterols had been removed, lost the stimulatory effect on cercariae, but full activity was recovered by adding cholesterol to the sterol-free lipid. Fractions of skin lipid containing free fatty acids or triglycerides, wax esters and sterol esters similarly failed to stimulate penetration.These results establish that penetration of A. terrigalensis cercariae is greatly stimulated by the free sterols present in the surface lipid of chicken skin but cholesterol may not be the only active sterol. Cholestanol and the plant sterols campesterol and β-sitosterol were also detected in chicken skin surface lipid. These sterols were not tested for activity on cercariae because samples free from cholesterol could not be obtained.Some cercariae were able to penetrate plain gelatine membranes not coated with cholesterol but small amounts of free sterol were detected in the gelatine itself. This sterol could not be completely removed by prolonged solvent extraction and consequently it is not known whether any cercariae are able to penetrate in the complete absence of sterols.Temperature had a marked effect on penetration of cercariae; lowering the temperature from 40 to 25 °C reduced the number of successful penetrants by a factor of 4.I am grateful to Miss V. Bowen for excellent technical assistance and to Dr J. McLeod of the Research School of Chemistry, Australian National University, who performed and interpreted the mass spectrometry. This investigation was partly supported by a research grant from the World Health Organization.