Pseudorabies virus, Iowa strain ("mad itch"), after passage through guinea pig brain, fails to produce infection in guinea pigs when injected subcutaneously unless enormous doses are employed. Such virus is still pathogenic for rabbits when given subcutaneously and for rabbits and guinea pigs intracerebrally. Comparison of the amounts of virus present in the brains of rabbits and guinea pigs following fatal cerebral infection shows that the latter contain, per gram, only approximately one-tenth the amount of virus in the former. Comparing the resistance of the two species to subcutaneously administered pseudorabies virus it has been found that rabbits are approximately 100 times more susceptible than guinea pigs. Over and above the working of these two factors, guinea pig passage appears to achieve some actual attenuation of virus when tested by subcutaneous inoculation into guinea pigs.

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