Murine Astrovirus Infection and Transmission in Neonatal CD1 Mice

Murine astrovirus (MuAstV) is a recently identified, widespread infection among laboratory mice. Our goal was to determine the duration of MuAstV infection, susceptibility of pups, and efficacy of soiled-bedding sentinels and environmental monitoring. Eight CD1 dams and their litters of 3-d-old pups and 8 CD1 dams and their litters of 13-d-old mice were inoculated orally with MuAstV. Neither dams nor offspring demonstrated any clinical signs, and MuAstV had little to no effect on weight gain in pups. MuAstV RNA was detected in feces from 15 of the 16 dams through postnatal day (PND) 21, and 9 dams were still shedding MuAstV at PND 42. MuAstV RNA was highest in intestines of mice. Low levels of MuAstV RNA were sporadically detected in the spleen, liver, and kidney. MuAstV was detected in 97% of feces from 3- to 9-wk-old mice born to infected dams. Several weanlings became pregnant, and intestines from their pups were MuAstV-negative at PND 0 through 5. Weekly swabs of cages housing MuAstV-infected mice were MuAstV-positive through PND 42. Swabs of the rear exhaust manifold of the ventilated rack were MuAstV-positive at 21 through 56 d after inoculation. In addition, 98% of sentinels that received soiled bedding from dams and their litters and 83% of sentinels that received soiled bedding from weaned mice were MuAstV-positive. Feces from most sentinels exposed to soiled bedding that had been stored for 1, 2 or 3 wk before addition of the sentinels were MuAstV-positive.

Effect of immunodeficiency on MPV shedding and transmission

C57BL/6 (B6) mice briefly shed low levels of MPV, and transmission is inefficient. To determine whether deficits in B or T cells or in interferon γ on a B6 background increased the duration of MPV shedding or transmission, B-cell-deficient (Igh), interferon-γ-deficient (Ifnγ), B- and T-cell-deficient (Rag), and B6 mice were inoculated with MPV. At 1 and 2 wk postinoculation (wpi), 11% to 94% of mice shed MPV. From 4 to 18 wpi, 80% to 100% of Rag mice and 0% of B6 and Ifnγ mice shed MPV; Igh mice sporadically shed MPV through 20 wpi. MPV was transmitted from B6 mice and Ifnγ mice at 2 to 4 wpi. Rag and Igh mice transmitted MPV to sentinels at all or most time points, respectively, between 2 to 16 wpi. Once transmission ceased from B6, Ifnγ, and Igh mice, breeding trios were setup and showed that MPV was transmitted to offspring in only one cage of Igh mice. In another experiment, MPV shedding ceased from B6, CD8-deficient (CD8), CD4-deficient (CD4), and T-cell-receptor-deficient (TCR) mice by 2, 6, 8, and 8 wpi, respectively. MPV was transmitted to sentinels only at 1 to 4 wpi. Mesenteric lymph nodes collected from 61% to 100% of B6, Ifnγ, TCR, CD4, CD8, and Rag mice were MPV DNA-positive. In conclusion, MPV transmission did not differ between mice deficient in T cell functions or Ifnγ and B6 mice. In contrast, B-cell deficiency posed an increased risk for MPV transmission in mice.

Maternal antibodies or nonproductive infections confound the need for rederivation

After rederivation of a mouse parvovirus (MPV)-contaminated transgenic mouse strain, serology and PCR testing of the surrogate dam showed it to be infected with mouse parvovirus strain 1 (MPV-1). The rederived pups (n = 3) also were MPV-positive, according to serology. Despite MPV seropositivity, fecal PCR tests of the pups were negative, as were serologic results from direct-contact sentinels. Only one rederived pup survived, and this male was bred successfully. None of its mates or progeny seroconverted to MPV. At 14.5 mo of age, the rederived male mouse was euthanized; tissues were collected and submitted for MPV testing; both serologic tests and PCR analysis of mesenteric lymph nodes were MPV-negative. One explanation for the rederived pups' MPV seropostivity is passive transfer of maternal antibodies or a nonproductive MPV infection. This case illustrates that although routine serological testing of surrogate mothers and pups is appropriate, any positive results should be further investigated by using transmissibility testing (fecal PCR or contact sentinels or both) prior to repeat rederivation.

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 x 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 x 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.

Mucosal immunity: the immunology of breast milk

The mammary glands represent one part of the mucosal immune system, a definable, subunit of humoral and cellular immune functions in man that appears to have developed particular qualities well suited to guard our interface with the environment. As our understanding of secretory immunoglobulins and lymphocyte migration patterns continues to develop, the immunologic components found in breast milk appear increasingly likely to play a specific immunologic role in the protection of the nursing infant. The biologic basis for the observed protective effect of breast-feeding is reviewed with an emphasis on the mechanisms involved in the development and maintenance of mucosal immunity in general.