Pathogenesis and immunology of ruminant schistosomiasis in the Sudan

Schistosoma bovis in the Kosti area (Sudan) was almost 90% in cattle up to 2 years old but fell to around 30% by 10 years of age. Infection incidence followed a markedly seasonal pattern of transmission. Infected animals either lost weight or gained less weight than controls. This was associated with the development of severe anaemia and hypoalbuminaemia, which in turn were due to massive blood and plasma losses through the gut mucosa. The epizootiological work suggested that cattle can acquire resistance to S. bovis as a result of repeated field infections. This was tested by artificially challenging field-infected cattle, together with animals of similar age and breed from a non-enzootic area. Animals reared in an enzootic environment clearly become solidly resistant to a cercarial challenge sufficient to cause fatal disease in cattle from non-enzootic areas. The mechanisms responsible for this phenomenon are being studied. Immunization with irradiated vaccines was tested in the laboratory. A marked reduction in the faecal egg counts, worm burdens, tissue egg counts and clinical manifestations of the vaccinated compared with non-vaccinated cattle and sheep indicated significant protection. Vaccination efficacy was also tested under field conditions. A group of calves was immunized with irradiated S. bovis schistosomula and a second group served as controls. These animals were released into the field in the enzootic area of Kosti and followed up for 10 months. The results showed 80% reduction in the faecal egg counts and about 70% protection in terms of reduced worm and tissue egg counts in vaccinated compared with non-vaccinated calves. Further evidence of resistance was shown by the lower mortality rate in the former animals. Vaccination against schistosomiasis can be of practical benefit to livestock