Immunization of sheep against Schistosoma bovis using an irradiated schistosomular vaccine

S. bovis is an economically important parasite of cattle and possibly of sheep and other domestic stock in many African and Mediterranean countries. Control of this infection by chemotherapy and mollusciciding is impractical and this has stimulated interest in vaccination. Previous work in sheep with the Central and South African species S. mattheei has led to the development of an effective vaccine which incorporates irradiated schistosomula produced by artificial transformation of cercariae. In the present experiments a similar S. bovis vaccine has been used in sheep. Fifteen 5 month old Border Leicester × Suffolk crossbred wethers were divided into three groups, two were vaccinated with either 10000 or 20000 6 krad irradiated S. bovis schistosomula, and the third kept as unvaccinated controls. All sheep were challenged with 6700 normal S. bovis cercariae.

Worm recoveries and tissue egg counts at necropsy showed that vaccination with 10000 irradiated schistosomula produced a reduction of 71% in the mean worm burden resulting from challenge of the controls and the mean densities of eggs in the tissues were reduced by 75–82%. Immunization with 20000 irradiated schistosomula was not more effective. Histopathological observations showed that both groups of vaccinated sheep had much milder lesions in the intestines than the non-vaccinated controls. There was also a clear reduction in the severity of liver lesions in 4 of the 5 sheep given the lower dose of irradiated schistosomula but the livers of sheep vaccinated with 20000 irradiated schistosomula had severe and widespread lesions due mainly to a massive accumulation of eosinophils and other inflammatory cells around the vessels.