The vector competence of Culicoides spp. for attenuated Bluetongue virus vaccine strains and studies on reversion to virulence

Summary

Bluetongue virus was first identified from Australia in 1977, and since then 8 different serotypes have been recognised in this country at one time or another. So far, two of these serotypes (1 and 21) have moved from the Northern Territory to the east coast of the continent where they have come close to the commercial sheep raising areas, and continue to be more or less active in cattle from year to year.

Pathogenicity studies with Australian bluetongue virus serotypes have shown several serotypes capable of causing severe clinical disease and deaths in sheep. Recent vector competence and virus distribution studies have independently indicated that Culicoides brevitarsis and C. wadai are the principal vectors of bluetongue virus in eastern Australia. The former has a seasonal distribution that can extend well into sheep producing areas. Should virulent serotypes cross into eastern Australia then the potential exists for outbreaks of severe clinical disease in sheep.

An option for veterinary authorities to control an outbreak of disease is to vaccinate at-risk sheep and possibly in contact cattle. Live attenuated viruses were prepared and evaluated under project CS 129 and although they appear efficacious they had undesirable side effects. A disadvantage already demonstrated under that project was that modified viruses were teratogenic. In ewes vaccinated in the first and second trimesters of pregnancy, up to 50% of foetuses were lost and up to 13% of surviving foetuses were born with mild to severe hydrancephaly and were non viable.