Ibaraki Virus, an Agent of Epizootic Disease of Cattle Resembling Bluetongue

Ibaraki virus multiplied and induced cytopathic effects in primary cell cultures of bovine, sheep and hamster kidney and chick embryo, and cultures of BHK21‐WI2 cells of baby hamster kidney origin and mouse fibroblastic L cells, but did not in primary cultures of horse and swine kidney cells and HeLa cell cultures. The virus was readily passaged serially in 4 to 5‐day‐old eggs using the yolk sac inoculation and incubation at 33.5 C. The viral growth was better in eggs incubated at 33.5 C than 37 C, and in younger eggs, with high yields in yolk, yolk sac and embryo. The virus was passaged serially in newborn mice by the intracerebral route. The virus multiplied in the brain of mice of any age, but younger mice supported better viral growth and developed encephalitis. As the age of mice increased, the morbidity and mortality became lower, no deaths being observed in 2 to 3‐week‐old mice. These observations in cell cultures, embryonated eggs and mice emphasize the similarity of Ibaraki virus to bluetongue virus. No evidence was obtained that young adult rabbits and weanling guinea pigs are susceptible to Ibaraki virus. The virus seemed to have little if any pathogenicity but infectivity of a low grade for sheep, while the virus is capable of inducing clinical illness, even severe in some instances, in cattle. This is in contrast to bluetongue virus which is highly pathogenic for sheep and much less so for cattle. Serial passages in embryonated eggs and suckling mice resulted in attenuation for cattle of Ibaraki virus.