Attenuated variants of eastern equine encephalomyelitis virus: pathomorphological, immunofluorescence and virological studies of infection in Syrian hamsters.

A virulent strain of eastern equine encephalomyelitis virus produced in hamsters a lethal infection with severe lesions of nerve cells predominatly in the anterior parts of the brain. Parenchyma necroses occurred in the liver and lymphoid organs. Infectious virus and viral antigen were found in all the organs examined with the greatest accumulation in the brain. Attenuated variants of the virus produced in most hamsters an inapparent infection. In some animals without clinical signs, focal inflammatory-dystrophic lesions were found in the central nervous system (CNS) and the liver, as were immunomorphological changes in the lymphoid tissue. In the latter infectious virus could be detected for 6 days after inoculation (p.i.), whereas in the brain only viral antigen could be found up to 14 days. At 3 and 6 months p.i., no persistence of attenuated virus in the brain and lymphoid tissues could be established by organ culture and co-cultivation methods. Nor was virus antigen detected. No pathomorphological changes or proliferative-hypertrophic astrocyte reaction were found.