Prevalence of Enterotropic and Polytropic Mouse Hepatitis Virus in Enzootically Infected Mouse Colonies

Mouse hepatitis virus (MHV) causes the most prevalent viral infection in contemporary laboratory mouse colonies. According to their primary replication site, different strains of MHV segregate into two overlapping biotypes, enterotropic and polytropic. These two groups vary greatly in disease pattern, pathogenicity, immune response, and duration of infection. Historically, the polytropic MHV strains represent the extensively studied prototype strains that have minimal enterotropism, whereas enterotropic MHV strains have been less characterized. Anecdotal reports suggest that most MHV strains encountered today belong to the enterotropic biotype. We have identified 15 isolates of MHV from 19 independent enzootically infected mouse colonies. Sequencing of a variable region of the nucleoprotein (N) gene of each isolate confirmed that all were independent genetic variants. The principal tissue tropism of the new isolates was determined by experimental inoculation of infant mice and examination of intestine, liver, spleen, and brain for lesions. Nine isolates infected only intestine; four isolates infected intestine and liver; one isolate infected intestine, liver, and brain; and one isolate infected liver. These results confirm that the enterotropic MHV biotype is predominant in contemporary laboratory mouse colonies. The MHV biotype features need to be taken into consideration when dealing with MHV infections.