Langat virus, passaged i.c. twice in suckling hamsters, became virulent for adult hamsters and caused progressive subacute encephalitis with a high degree of mortality spread out over the period of 3 months that the animals remained under observation.
The lesions in the brain during the first few days after infection were purely inflammatory in character, but from the second week onwards became also degenerative and subacute and gave rise to widespread granulomatous perivascular cellular infiltrations and a specific astrocytic transformation and proliferation. The unchecked progress of the pathological process led to severe sclerosis and atrophy of many parts of the brain, especially of the hippocampus.
The use of an immunosuppressant did not alter the mortality after i.c. inoculations but enhanced the virulence and invasiveness of Langat virus after intradermal inoculations. In addition, immunosuppression gave rise to an unprecedented increase in the severity of the astrocytic reaction.