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. Lesions caused by either strain tend to be more severe and widespread in immunocompromised mice.
Most infections follow one of three clinical patterns.
No carrier state exists, although in a recent study viral RNA was detected in the liver up to 60 days after infection. Adults are asymptomatic, and their young become asymptomatically infected by the time passively transferred maternal immunity wanes at weaning. Adult infections are again usually asymptomatic. As the infection becomes established in the colony, the epizootic pattern is replaced with the enzootic pattern. Numerous reports document effects of natural or experimental infection with MHV on host physiology and research.