Investigations of reasons for the avirulence of the A7 strain of Semliki Forest virus in adult mice.

A strain of Semliki Forest virus (A7) which is avirulent in adult mice killed baby mice in a similar manner to strain V13 which was also virulent for adult mice. In the muscle and brains of baby mice A7 and V13 replicated and produced haemagglutinating activity similarly. Our previous suggestion that defective interfering particles were present in the brains of A7-infected adult mice appears not to be so. The interference formerly detected was due to an inhibitor present in brain tissue of adult and baby mice both normal and infected. Homogenates of A7-infected adult brain produced normal RNA species in BHK cells and not those characteristic of defective interfering particles. Organ culture experiments indicated that avirulence of A7 was not due to lack of release of virus from infected adult brain cells. Also, A7 as well as V13 was detected and was probably replicating in all parts of the brain and spinal cord that were sectioned and examined. Evidence is presented that suggests that the reason for the avirulence of A7 for adult mice compared with its virulence for baby mice may relate to a lower ability to replicate in brain tissue per se rather than to interaction with host defence mechanisms.