A lethal respiratory infection of rabbits with a strain of Semliki Forest virus (SFV).

Rabbits were highly susceptible to the inhalation of the L10.H6 strain of SFV; the smallest amount of virus capable of infecting by this route killed the animals within 14 days. There were certain unusual features about the pathogenesis of this disease. Virus reached the brain within 24 hr of infection and rapidly attained high titres throughout the CNS, but pathological changes were confined to the anterior part of the brain. The inflammatory response present in the brain during the early stage was rapidly suppressed. Although there was an early viraemic phase, no evidence for a primary focus of infection could be found in the lungs and the role of viraemia in invasion of the CNS by the virus is uncertain. Prior administration of antiserum protected against respiratory infection, but antiserum given after virus had reached the brain was also protective in the early stages of the disease. In some instances late administration of antiserum produced an apparent recovery which was followed by relapse. Inoculation i.v. of massive amounts of SFV after respiratory infection also prevented death.