The comparative sensitivity of sheep and chicken embryos to bluetongue virus and observations on viraemia in experimentally infected sheep.

The virus titre in sheep blood samples received from BT-suspected cases in the field was assayed in sheep and in chicken embryos. These infected blood samples represented 3 different BT virus types: 4, 10 and 16. Three identical experiments were performed, one with each of the 3 different virus types. Ten-fold dilutions of the infected blood samples were prepared and 1 ml of each blood dilution was inoculated IV into series of 10 to 12-month old susceptible sheep; at the same time 0.1 ml of each dilution was inoculated IV into series of 10 to 13-day-old chicken embryos. The virus titre was found to be similar when assayed in the two host systems. There was no correlation between the amount of virus inoculated and the severity of symptoms in the inoculated sheep. The virus content in daily blood samples collected from the experimental sheep was assayed by IV inoculation of CE. Virus was isolated from all the reacting sheep and was detected sometimes as early as 1 day PI and as late as 30 days PI. A high titre of log10 4.0 to 7.0 per 1 ml of blood was recorded during several consecutive days before and after the onset of clinical signs. There seemed to be an inverse ratio between the amount of virus inoculated and the number of days the virus persisted in the bloodstream. The neutralisation index in day 22 serum samples was 3.5 to 4.5. Virus was isolated from some of the reacting sheep on the day that these antibody levels were recorded. Since the comparative simultaneous titrations of BT virus in sheep and in CE yielded similar results, the IV inoculation of CE is advocated as the routine method to be employed for laboratory diagnosis of this disease.