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.

The parasitaemia and associated blood picture following infections of Nuttallia rodhaini in rats and mice, and N. microti in rats are described. After inoculation of the blood stages of these parasites, it is evident that only further blood stages occur in the host animal. Infections with both piroplasms follow a similar course, there being a prepatent period followed by a patent infection which reaches a peak and then disappears. The prepatent period is an artifact due to the rarity of the parasites in the peripheral blood. The patent infection of N. rodhaini in rats and mice rapidly reaches a peak of up to 90% R.B.C. infected. That of N. microti develops more slowly and rarely reaches 20%. Rats show age resistance to both piroplasmoses but none is shown by mice aged up to 1 year against N. rodhaini infection. Increase of the parasite inoculum dose decreases the prepatent period and also the time from infection to the peak. In rats there is a corresponding increase of the peak but in mice this remains constant. N. microti infections in rats are enhanced by splenectomy, the peak being higher but the time from infection to the peak not altering. The parasitaemia produced by both piroplasms results in a temporary anaemia and reticulocytosis. The anaemia is not prolonged which does not suggest autoimmune reactions. Both piroplasms preferentially invade reticulocytes as compared with mature erythrocytes although this is apparently reversed at the onset of reticulocytosis.

An explanation of the mechanism of the parasitaemia of the piroplasms in rats and mice is suggested.

This paper is a report of work carried out in part for a thesis submitted for the degree of Ph.D. in the University of London. The work was aided by an S.R.C. research studentship. I would like to thank Dr F. E. G. Cox for his supervision and also for performing the splenectomies.