It is extremely contagious and is transmitted primarily via aerosol, direct contact, fomites, and, experimentally, via transplantable tumors and transplacental passage. Susceptibility, tissue tropism, clinical signs, and pathologic lesions are dependent on several host, environmental, and pathogen factors. Approximately 25 strains or isolates of MHV have been described and have been classified as either respiratory or enterotropic. Recently, an outbreak of a highly hepatotropic strain of MHV was reported from a breeding colony of nude mice in Taiwan. The presence or absence of the MHV receptor, a glycoprotein in the carcinoembryonic antigen family of the Ig superfamily, may determine tissue tropism.
Intestinal involvement is usually absent.
Pulmonary involvement is uncommon. While polytropic strains have historically been considered more common, this situation is thought to have reversed. Lesions are present for only 7 to 10 days following infection, are dependent upon strain of virus, and are characterized by multifocal necrosis. Additionally, multinucleate syncytial giant cell formation occurs and may be associated with fragmentation and rearrangement of the Golgi apparatus. Lesions due to polytropic strains may be observed in the olfactory mucosa, brain, lungs, and liver, while lesions due to enterotropic strains are generally, though not always, confined to the intestinal tract.