Kinetics of Transmission, Infectivity, and Genome Stability of Two Novel Mouse Norovirus Isolates in Breeding Mice

Murine noroviruses are a recently discovered group of viruses found within mouse research colonies in many animal facilities worldwide. In this study, we used 2 novel mouse norovirus (MNV) wildtype isolates to examine the kinetics of transmission and tissue distribution in breeding units of NOD.CB17-Prkdc^scid/J and backcrossed NOD.CB17-Prkdc^scid/J × NOD/ShiLtJ (N1) mice. Viral shedding in feces and dissemination to tissues of infected offspring mice were monitored by RT-PCR over a 6-wk period postpartum. Histologic sections of tissues from mice exposed to MNV were examined for lesions and their sera monitored for the presence of antibodies to MNV. Viruses shed in feces of parental and offspring mice were compared for sequence homology of the Orf2 gene. Studies showed that the wildtype viruses MNV5 and MNV6 behaved differently in terms of the kinetics of transmission and distribution to tissues of offspring mice. For MNV5, virus transmission from parents to offspring was not seen before 3 wk after birth, and neither isolate was transmitted between cages of infected and control mice. Susceptibility to infection was statistically different between the 2 mouse strains used in the study. Both immunodeficient NOD.CB17-Prkdc^scid/J mice and NOD.CB17-Prkdc^scid/J × NOD/ShiLtJ offspring capable of mounting an immune response shed virus in their feces throughout the 6-wk study period, but no gross or histologic lesions were present in infected tissues. Progeny viruses isolated from the feces of infected offspring showed numerous mutations in the Orf2 gene for MNV5 but not MNV6. These results confirm previous studies demonstrating that the biology of MNV in mice varies substantially with each virus isolate and mouse strain infected.Abbreviations: MNV, murine norovirus; MLN, mesenteric lymph nodes; NOD-scid, NOD.CB17-Prkdc^scid/J; VP1, viral protein 1The recent discovery of murine-specific noroviruses¹⁵ has stimulated concern in the laboratory animal health community regarding the potential for this group of viruses to cause disease in breeding colonies of mice or to negatively impact research with mice from norovirus infected colonies. Current knowledge of the biology of noroviruses in mice (MNV) is constrained by the limited number of virus isolates and mouse strains studied. One study¹⁵described the biologic and physicochemical properties of the original MNV1 isolated from mice deficient in a specific innate immune function. More recently, this innate immune deficiency has been mapped to STAT1 regulation of IFNαβ secretion.²¹Previous work¹⁵ demonstrated that inoculation of MNV1 into mouse strains deficient in the acquired immune response (129 RAG 2^?/?, B6 RAG1^?/?) resulted in the development of persistent infections with no evidence of disease, whereas inoculation of fully immunocompetent mice (129S6/SvEvTac) resulted in rapid elimination of MNV1, with viral RNA undetectable in the viscera by 3 d after inoculation. More recently, infections of outbred immunocompetent mouse strains with 3 wildtype isolates of MNV obtained from different geographic areas of the United States have been described.¹¹ Virus was detected in the feces and tissue of infected mice throughout the 8-wk study, suggesting that some isolates of MNV may persistently infect immunocompetent mice.The purpose of the present investigation was to extend the current knowledge of MNV by using 2 isolates of the virus in mouse strains that have not been previously used as infection models for MNV. We examined natural virus transmission from infected breeders to offspring, kinetics of infection within litters of infected breeding mice, and the pathogenesis of infection in breeding colonies of mice. In addition, we examined the effect of virus passage from parents to offspring on genomic stability of these 2 viral isolates. Exposure of offspring of immunodeficient mice and immunocompetent mice to the 2 different isolates of MNV resulted in different patterns of virus transmission, susceptibility to infection and kinetics of infection as shown by the progressive spread of virus within litters and in intestinal and extraintestinal tissues. MNV was shed persistently in the feces of all mice tested regardless of immune status, and viral progeny isolated from offspring mice contained genome sequence differences from the parent virus in the Orf2 gene, an area of the MNV genome known to be susceptible to mutations.