In this study, we analyzed MNV infection in mice derived from animal facilities in Japan, the US, and three breeders (Japan) using serological and molecular techniques. We show that MNV infection is widespread among both conventional and SPF mice in Japan, as well as commercial SPF mice. In a previous study, serological analysis revealed that 53.4% of conventional mice are positive for MNV, 48% positive for mouse hepatitis virus, and 31.2% positive for M. pulmonis
(Kitagawa et al., 2010
). Consistent with this report, we show that MNV is the most prevalent infection in mouse laboratory colonies, and the mice we tested, with the exception of one case, were negative for the antibodies for hepatitis virus and M. pulmonis
). This report is the first on MNV infection in SPF mice in Japan and commercial mice.
The frequency of MNV infection as detected by RT-PCR was lower than that detected by serological study using ELISA. We detected MNV antibodies in all five mice tested 4 weeks after the first MNV sequences were detected by RT-PCR Using RT-PCR, MNV sequences were detected consistently in the feces of only two mice. Thus, a survey for MNV infection might require the use of serological methods, such as ELISA. The first norovirus to infect mice, MNV-1, caused death in severely immunocompromised mice lacking recombination-activating gene 2 (RAG2) and signal transducer and activator of transcription 1 (STAT-1; RAG2/STAT1-/-
) and mice lacking both the alpha/beta interferon (IFN-α/β) and the IFN-γ receptors (IFN-αβγR-/-
; Karst et al., 2003
; Mumphrey et al., 2007
). In this study, the mice infected with MNV exhibited no clinical signs, though the tested mice, with the exception of the mice from breeders, were genetically modified, including compromised immune systems. Recently, MNVs (except MNV-1) were associated with asymptomatic infection and shedding in both normal and genetically modified mice (Hsu et al., 2006
; Muller et al., 2007
). The origin of MNV in laboratory mouse strains is unknown. Phylogenetic analysis of the partial capsid sequences of MNV in this study showed that the isolated virus strains comprised the reported MNV subgroups, though MNV-1 and Ku29 isolated from the SPF facility were divergent form the main group. However, bootstrap values for the position of Ku29 were not high (<70%). Thus, further study is needed to address whether the Ku29 isolate belongs to a new MNV genotype. These results indicate that attenuated (persistent) strains of MNV, such as MNV-2, -3, and -4, are spread around the world, making the viruses difficult to detect in animals without active screening (Muller et al., 2007
; Barron et al., 2011
). Using ELISA, we did not detect MNV antibodies in stocked serum from mice transported from domestic animal facilities before 2009 (data not shown). These results suggest that MNV has been spreading through 50% of animal facilities in Japan in only 2 years.
Finally, our results suggest that MNV infections have spread widely throughout the animal facilities in Japan, not only conventional mice colonies, but also SPF mice colonies. The findings warrant further studies to elucidate the spread of MNV infection in SPF colonies, including breeders, in Japan. These results also suggest that SPF laboratory mice, including commercial mice, which might require considerable attention as an animal model for human norovirus infection.