Effects of Pelleting, Irradiation, and Autoclaving of Rodent Feed on MPV and MNV Infectivity

Murine norovirus (MNV) and mouse parvovirus (MPV) are among the most common adventitial viruses seen in laboratory mice, and infections arise in barrier facilities despite rigorous biosecurity programs. Some authors have implicated nonsterilized feed as a source of MPV in rodent facilities, but none have conclusively documented viral particles in the feed. In this study, we hypothesized that both viruses can resist the pelleting process but not subsequent irradiation or autoclaving, thus revealing a potential source of outbreaks in rodent facilities. To test this hypothesis, we contaminated powdered feed with 10-fold concentrations of MNV and MPV and fed it to both Swiss Webster (SW) and C57BL/6NTac (B6) mice to determine a 'powdered ID₅₀' according to seroconversion over a 28-d period. We repeated the experiment by using powdered feed that we contaminated with increasing viral doses (as no. of powdered ID₅₀) and subsequently pelleted; from these results, we determined a 'pelleted ID₅₀.' Finally we assessed the effect of irradiation and autoclaving on contaminated pellets by using the same experimental design. The powdered ID₅₀ was relatively low and identical in both mouse strains (2.51 × 10² pfu) for MNV but higher in B6 (copy number, 3.20 × 10⁶) than SW (3.98 × 10⁴ copies) for MPV. As hypothesized, mice were infected by contaminated rodent feed despite the pelleting process. Indeed, pelleting resulted in a 1- to 2-log increase in ID₅₀ in both strains for MNV and MPV. Irradiation and autoclaving of infected pellets effectively prevented seroconversion of mice exposed to all doses of MNV, whereas a single mouse seroconverted at the highest dose of MPV (1.35 × 10⁷ copies). These data suggest that both MNV and MPV remain infectious after conditions reproducing the rodent chow pelleting process and that nonsterilized rodent chow might be a source of viral outbreaks.