Etiology: Murine cytomegalovirus (MCMV) is a herpesvirus of the subfamily betaherpesviridae. It is a double-stranded enveloped DNA virus with host specificity for mice.

Incidence:  Incidence of infection is rare.

Transmission:  MCMV is transmitted horizontally by contact with virus in saliva and urine.  Vertical transmission occurs under experimental conditions. Salivary glands are considered persistently infected, with virus shed in saliva even with the presence of circulating anti-viral immunoglobulin. Latent infection can occur in most parenchymal organs (spleen, kidney, heart, liver and lung) as well as in myeloid cells, so that transfer of tissue or blood cells might also be a source of virus to recipient mice.

Clinical Signs: Infected mice are asymptomatic, although experimentally, mouse genotypes and viral strains can influence virus infectivity and organ damage.

Pathology:   Depending on the route of inoculation, mice develop systemic viremia either from parenteral inoculation or following replication from the primary site of infection.  Virus replication occurs in salivary glands, liver, spleen, adrenal glands, lymph nodes, kidney, lung, intestine and pancreas.  Necrosis is common. Infected cells harbor intranuclear inclusions that fill the nucleus and may induce an enlarged or “megalic” appearance. Cytoplasmic basophilic inclusions bodies have also been described, especially in the ductal epithelium in the salivary glands and represent packages of virions.

Chronic infections display similar pathology typically in the salivary glands, but with minimal necrosis and accompanied by periductal lymphocytic infiltrates.

Diagnosis:  Serologic assays (MFI and IFA) are used to identify chronically infected mice.  For mice with active disease or in infected immunodeficient mice, PCR can amplify virus genome from tissue samples. Virus can also be isolated on primary or established fibroblast lines.