Learning deficits and activity changes: A partial laboratory model in postencephalitic rats for studies of brain damage

Rats ranging from 8–21 days of age were inoculated in the right cerebral hemisphere with St Louis encephalitis virus, Japanese B encephalitis virus, Murray Valley encephalitis virus. Mumps virus however, was injected at age one day. When very young rats were injected with these viral preparations, fatal encephalitis usually developed. Rats injected at ages of more than two weeks showed little or no clinical signs of infection. When inoculated at ages between these extremes, clinical disease developed in most rats but many survived. Physically most of these surviving animals appeared to be normal, but when tested at maturity in a rather complex maze, they showed a marked reduction in learning ability. Tilting-cage-type activity measurements taken overnight with the Murray Valley encephalitis group also indicated that most of the postinfectious rats were hyperactive. These data imply that such a laboratory model might be related to similar hyperactivity and learning disabilities seen in certain types of minimally brain damaged children. A viral-sex interaction often seen in earlier studies is also noted in our animals injected at younger ages.