We have previously described the characteristics of a relatively non-pathogenic laboratory strain of S. mattheei, attenuation of which was apparently caused by passage in hamsters. We now show that chronic infection with this avirulent strain largely protects sheep from the manifestations of acute schistosomiasis when challenged with a virulent strain of S. mattheei.

Four sheep were each infected with 10 000 cercariae of the avirulent strain and, together with four worm-free sheep, challenged 63 weeks later with 10 000 S. mattheei cercariae of a pathogenic strain. Four more sheep acted as uninfected controls. Following challenge, the animals were weighed and bled weekly for PCV and serum protein determinations, and egg counts were carried out fortnightly on faeces taken from the rectum. Red cell and albumin turnover were monitored for two weeks immediately before challenge and for a similar period before necropsy, when the adult worms were recovered by perfusion and tissues sampled for histopathology and egg counting.

The unvaccinated sheep developed severe disease 6—12 weeks after exposure characterised by marked anaemia, hypoalbuminaemia and hyper-gamma globulinaemia coinciding with the passage of blood-stained faeces and progressive inappetence. In the vaccinated sheep, there was and even earlier rise in gamma globulins, but the other clinico-pathological changes were generally slower to develop and much milder in severity. The parasitological data showed that although this was partly due to a reduction in the establishment of the challenge worm population the main factor was probably a reduction in the fecundity of these worms.