Murine cytomegalovirus is present in both chronic active and latent states in persistently infected mice.

Cytomegalovirus induces serious disease in immunosuppressed individuals, often from an "activated" persistent infection. Whether the infection is chronically active or latent is unknown. Using murine cytomegalovirus (MCMV) in mice as a model system, we examined persistent infections in spleen, lung, and bone marrow of infected animals. At 28 days after infection, no virus could be recovered from any organs tested except salivary glands, and here, virus was cleared by 48 days. Virus could be retrieved at all times by cocultivation of spleen or lung with permissive cells. In addition, MCMV DNA was always present in spleen, lung, and bone marrow. After acute infection, RNA from the MCMV immediate early-1 (ie-1) gene was routinely found only in the lung. In spleen and bone marrow, only one sample from each organ examined at these times contained ie-1 RNA, and the RNA in these two samples was present at levels comparable to that found in acute infection. This suggests that the virus had reactivated. The ie-1 RNA found in the lung was present at a much lower RNA:DNA ratio than that found at early times. Taken together, these results indicate that persistent MCMV exists simultaneously in both chronic active and latent states.