The effect of immunosuppression on viral encephalitis, with special reference to cyclophosphamide.

Cyclophosphamide altered not only the pathological picture of virus encephalitis, but also enhanced the invasiveness of viruses and produced fatal forms of disease to which monkeys were otherwise resistant. Normal patas monkeys infected either i.c. or intranasally (i.n.) with louping ill developed clinical encephalitis from which they recovered, but when cyclophosphamide was administered the disease proved fatal. Normal rhesus monkeys inoculated i.n. with virulent western equine encephalitis virus developed neither clinical disease nor CNS lesions, but after treatment with cyclophosphamide they developed fatal encephalitis. The viruses of louping ill, Venezuelan equine encephalitis and Western equine encephalitis when given without an immunosuppressant usually gave rise to acute inflammatory CNS pathology, but when the monkeys were given cyclophosphamide, the inflammatory reaction was replaced by a degenerative process causing both neuronal necrosis and spongy degeneration.