Transmission probabilities of mouse parvovirus 1 to sentinel mice chronically exposed to serial dilutions of contaminated bedding

Intermittent serodetection of mouse parvovirus (MPV) infections in animal facilities occurs frequently when soiled bedding sentinel mouse monitoring systems are used. We evaluated induction of seroconversion in naïve single-caged weanling ICR mice (n = 10 per group) maintained on 5-fold serially diluted contaminated bedding obtained from SCID mice persistently shedding MPV1e. Soiled bedding from the infected SCID mice was collected, diluted, and redistributed weekly to cages housing ICR mice to represent chronic exposure to MPV at varying prevalence in a research colony. Sera was collected every other week for 12 wk and evaluated for reactivity to MPV nonstructural and capsid antigens by multiplex fluorescent immunoassay. Mice were euthanized after seroconversion, and DNA extracted from lymph node and spleen was evaluated by quantitative PCR. Cumulative incidence of MPV infection for each of the 7 soiled bedding dilution groups (range, 1:5 to 1:78125 v/v]) was 100%, 100%, 90%, 20%, 70%, 60%, and 20%, respectively. Most seropositive mice (78%) converted within the first 2 to 3 wk of soiled bedding exposure, correlating to viral exposure when mice were 4 to 7 wk of age. Viral DNA was detected in lymphoid tissues collected from all mice that were seropositive to VP2 capsid antigen, whereas viral DNA was not detected in lymphoid tissue of seronegative mice. These data indicate seroconversion occurs consistently in young mice exposed to high doses of virus equivalent to fecal MPV loads observed in acutely infected mice, whereas seroconversion is inconsistent in mice chronically exposed to lower doses of virus.Abbreviations: mfi, median fluorescent intensity; MFI, multiplex fluorescent immunoassay; MPV, mouse parvovirus; NS1, nonstructural protein 1; qPCR, quantitative PCR; SCID, severe combined immunodeficiency; VP2, viral capsid protein 2Mouse parvovirus (MPV) is among the most prevalent infectious agents detected in contemporary laboratory mouse colonies^2,7,10 and can have deleterious effects on research because of in vitro and in vivo immunomodulatory effects, tumor suppression, and contamination of cell cultures and tissues originating from mice.^11-13 The potential for MPV transmission among mice in research facilities is enhanced by its environmental stability,⁶ potential to induce persistent infection in mice,⁸ and difficult eradication from infected laboratory mouse colonies. Despite the availability of highly sensitive and specific diagnostic assays,^9,14,15 detection of MPV infections in contemporary laboratory mouse colonies remains problematic, with intermittent detection even under conditions of enzootic colony infections. The widespread use of sentinel mice exposed to soiled bedding as the primary detection system, a relatively short period of viral transmission postinfection in immunocompetent mice, and a fairly high viral dose required to induce productive infection are considered key factors that result in intermittent detection of MPV contamination in mouse colonies. As a result, MPV infections present important and costly challenges to contemporary laboratory animal research facilities.Several studies have investigated the horizontal transmission of MPV to sentinel mice. Experimentally infected SENCAR mice transmitted MPV1a to naïve sentinels both by direct contact and soiled bedding exposure, predominantly during the first 3 wk after inoculation.¹⁷ Similarly, experimentally infected Swiss Webster mice transmitted MPV1d within 2 wk to sentinels by direct contact or through various amounts of soiled bedding.¹⁸ Interestingly, transmission to sentinel mice appeared to be enhanced in mice maintained in individually ventilated caging as compared with static microisolation caging in the cited study. Naturally infected BALB/c mice when 1 mo old, but not when 2, 3, and 6 mo old, transmitted MPV to direct contact sentinels.¹⁶ Recent studies completed in our laboratory¹ indicate that C.B-17/Icr-Prkdc^scid mice inoculated with MPV1e as neonates persistently shed high levels of virus in their feces over several months. Undiluted contaminated bedding collected at any time point during this period consistently transmitted MPV1e to weanling C3H sentinel mice exposed for 2 wk. Similarly inoculated neonatal BALB/c mice shed high levels of virus, with transmission to sentinels, for only 2 wk after inoculation.¹ In all of these reports, the period of exposure of sentinel mice to soiled bedding was limited (2 wk or less), with no repeated exposure opportunities, as might be expected under field conditions with an infected colony. In the present study, we simulated a typical sentinel monitoring program and determined whether chronic exposure to various concentrations of MPV1-contaminated bedding, reflective of a broad range of disease prevalence scenarios within any given affected room, can induce seroconversion in sentinel mice.