As a continuation of our studies on the pathogenesis of domestic animal schistosomiasis, eleven Suffolk X Border Leicester sheep were each infected with 10 000 S. mattheei cercariae, sixteen with 5000 cercariae, and thirteen acted as worm-free controls. Blood samples were taken weekly for packed cell volume determination, serum samples were collected weekly during the first 30 weeks and thereafter fortnightly for protein analysis, and samples from the rectum were obtained at similar intervals for faecal egg counts. At intervals, 4–8 animals from each group were placed in metabolism cages for periods of 2–3 weeks to monitor albumin and red cell turnover. The animals were necropsied at 13, 35 or 57 weeks after infection, their worms recovered by perfusion and various tissues were collected for histological examination and egg counting.

A most unexpected finding was the almost complete absence of clinical signs in most of the infected animals, the only manifestations of infection being a slight retardation in growth and a mild anaemia between the 2nd and 5th months. These findings were quite unlike previous ones and were remarkable in view of the large cercarial exposure doses used and the heavy worm and tissue egg counts these produced. However, the present infections differed in two respects from those described previously: there was an absence of eggs from the faeces with associated lack of intestinal bleeding, and also large numbers of worms died in the later stages of the infections. In our previous studies with S. mattheei we used the same parasite but the sheep used in the current study were infected after the parasite had undergone several passages in laboratory hamsters. This fortuitous reduction in the pathogenicity of the parasite provided comparative haematological and parasitological data indicating that the major factor in the aetiology of severe S. mattheei infection in sheep is intestinal bleeding caused by the passage of eggs through the bowel wall.