Coping with parvovirus infections in mice: health surveillance and control

Parvoviruses of mice, minute virus of mice (MVM) and mouse parvovirus (MPV), are challenging pathogens to eradicate from laboratory animal facilities. Due to the impediment on rodent-based research, recent studies have focused on the assessment of re-derivation techniques and parvoviral potential to induce persistent infections. Summarizing recent data, this review gives an overview on studies associated with parvoviral impact on research, diagnostic methods, parvoviral persistence and re-derivation techniques, demonstrating the complex nature of parvovirus infection in mice and unfolding the challenge of controlling parvovirus infections in laboratory animal facilities.     

Humoral immunity and protection of mice challenged with homotypic or heterotypic parvovirus.

BACKGROUND AND OBJECTIVES: Two serotypes of autonomously replicating parvoviruses infect laboratory mice. Genome regions coding for the nonstructural proteins of minute virus of mice [MVM] and mouse parvovirus [MPV] are almost identical, whereas capsid-coding sequences are divergent. We addressed these questions: Does humoral immunity confer protection from acute infection after challenge with homotypic or heterotypic parvovirus, and if it confers protection against acute MPV infection, does it also protect against persistent MPV infection? METHODS: Infant mice without maternal antibody or antibody to MVM or MPV and young adult mice given normal mouse serum or antibody to MVM or MPV were challenged with homotypic or heterotypic virus. In situ hybridization with target tissues was the indicator of infection. RESULTS: Humoral immunity failed to confer protection against acute heterotypic parvovirus infection. In passive transfer studies, MPV DNA was observed occasionally in lymph nodes, intestine, or the spleen of MPV-challenged mice given homotypic antibody and kept for 6 or 28 days. Variable proportions of mice given MPV antibody and homotypic challenge had viral DNA in lymphoid tissues 56 days after virus inoculation. CONCLUSION: A mouse or colony that has sustained infection with MVM or MPV is probably fully susceptible to infection with the heterotypic virus.     

Infectious Diseases in Wild Mice (Mus musculus) Collected on and around the University of Pennsylvania (Philadelphia) Campus

Laboratory mice serve as important models in biomedical research. Monitoring these animals for infections and infestations and excluding causative agents requires extensive resources. Despite advancements in detection and exclusion over the last several years, these activities remain challenging for many institutions. The infections and infestations present in laboratory mouse colonies are well documented, but their mode of introduction is not always known. One possibility is that wild rodents living near vivaria somehow transmit infections to and between the colonies. This study was undertaken to determine what infectious agents the wild mice on the University of Pennsylvania (Philadelphia) campus were carrying. Wild mice were trapped and evaluated for parasites, viruses, and selected bacteria by using histopathology, serology, and PCR-based assays. Results were compared with known infectious agents historically circulating in the vivaria housing mice on campus and were generally different. Although the ectoparasitic burdens found on the 2 populations were similar, the wild mice had a much lower incidence of endoparasites (most notably pinworms). The seroprevalence of some viral infections was also different, with a low prevalence of mouse hepatitis virus among wild mice. Wild mice had a high prevalence of murine cytomegalovirus, an agent now thought to be confined to wild mouse populations. Helicobacter DNA was amplified from more than 90% of the wild mice (59% positive for H. hepaticus). Given the results of this study, we conclude that wild mice likely are not a source of infection for many of the agents that are detected in laboratory mouse colonies at the University of Pennsylvania.Abbreviations: EDIM, epizootic diarrhea of infant mice; MAV, mouse adenovirus; MCMV, murine cytomegalovirus; MFIA, multiplex fluorescent immunoassay; MHV, mouse hepatitis virus; MNV, murine norovirus; MPV, mouse parvovirus; MVM, minute virus of mice; TMEV, Theiler mouse encephalomyelitis virusLaboratory mice constitute the most popular animal models used in biomedical research today. Like all animals, even mice housed in so-called ‘barrier’ facilities are subject to infection. The infectious agents and organisms present in laboratory mouse colonies on the University of Pennsylvania campus are known and documented by the University Laboratory Animal Resources Diagnostic Services Unit. Sentinel mice that are housed on soiled bedding from resident mouse cages are screened onsite at 3 quarterly intervals for fur mites and pinworms and for a panel of viral infections: mouse hepatitis virus (MHV); epizootic diarrhea of infant mice (EDIM) virus; minute virus of mice (MVM); mouse parvovirus (MPV); Theiler mouse encephalomyelitis virus (TMEV); and Sendai virus. Comprehensive bacteriology and parasitology assessments are performed on all sentinels once yearly during the fourth quarter. In addition, these sentinels are screened serologically for 18 viral infections, Mycoplasma pulmonis, cilia-associated respiratory bacillus, and Encephalitozoon cuniculi and by PCR for Helicobacter spp. and M. pulmonis. Mesenteric lymph nodes from sentinels monitoring barrier-maintained colonies are also screened once yearly by PCR for MPV. In addition, University Laboratory Animal Resources maintains a quarantine facility for rodents received from nonapproved sources (sources other than selected commercial breeding facilities). Mice entering the quarantine facility are housed in semirigid isolators, and contact sentinels are tested for all of the agents included in the fourth quarter comprehensive health assessment described, including PCR for MPV.Wild mice (Mus musculus) could serve as a source of infection or infestation in laboratory mouse colonies, although little is known about the prevalence of infectious diseases in wild mouse populations in Philadelphia. However, we have surveyed wild mouse populations in other geographic areas.^1,9 Significant seroprevalence of MHV, EDIM, murine cytomegalovirus (MCMV), parvovirus, and thymic virus (murid herpesvirus 3), in addition to the presence of many types of parasites and bacteria including Myocoptes spp., Myobia spp., Radfordia spp., Spironucleus spp., Giardia spp., Pasteurella pneumotropica, Pseudomonas spp., and Leptospira spp. were found in wild populations of mice from farms in southeastern Connecticut.¹ Studies of wild mouse (Mus domesticus) populations in the cereal-growing region of southeastern Australia revealed a high serologic prevalence of MHV, EDIM, and MCMV, as well as significant seroprevalence of mouse adenovirus (MAV), MPV, and reovirus type 3.⁹The goal of the current study was to expand preliminary data obtained from wild mice trapped in the University City district of Philadelphia in 2005 (which are included with the current results from a 2007 survey). These data document the prevalence of various infectious agents and parasites commonly found in populations of wild mice on the University of Pennsylvania campus in Philadelphia and are discussed in the context of infectious disease outbreaks in campus vivaria over the past 5 y.     

Detection of mouse parvovirus in Mus musculus gametes, embryos, and ovarian tissues by polymerase chain reaction assay

We used primary and nested polymerase chain reaction (PCR) assays to determine the presence of mouse parvovirus (MPV) in mouse sperm, oocytes, preimplantation embryos, and ovarian tissues collected from MPV-infected mice. The primary PCR assay detected MPV in 56% of the sperm samples. MPV was not eliminated by passing sperm samples through a Percoll gradient. After Percoll treatment, MPV was still present in 50% of the samples according to primary PCR assay. Oocyte samples that did not undergo extensive washing procedures had detectable MPV in 7% of the samples based on the primary PCR assay, but nested PCR assay detected higher (28%) infection rate. However, MPV was not detected in oocytes that underwent extensive washing procedures, as assessed by either primary or nested PCR assay. Although primary PCR did not detect MPV in embryos, a nested PCR assay determined that 50% of the embryos were positive for the virus. In addition, ovarian tissues were collected from 3 different mouse colonies with enzootic MPV infection. Ovarian tissue collected from 129CT, 101/R1, and Sencar mice had high incidence (38%, 63%, and 65%, respectively) of MPV infection on the basis of nested PCR amplification. These results demonstrate that mouse gametes, embryos, and ovarian tissues may be contaminated with MPV and therefore caution is necessary when infected germplasm is used for assisted reproductive technologies such as embryo transfer, establishing embryonic stem cell lines, in vitro fertilization, ovary transplantation, and intracytoplasmic sperm injection.     

Development of ELISA using recombinant antigens for specific detection of mouse parvovirus infection.

Nucleotide sequences of mouse parvovirus (MPV) isolate, named MPV/UT, and mouse minute virus (MMV) were analyzed and used for expressing recombinant proteins in E. coli. ELISA tests using recombinant major capsid protein (rVP2) and recombinant major non-structural protein (rNS1) as antigens were developed and their performance in serologic detection of rodent parvovirus infection was assessed. MPV-rVP2 and MMV-rVP2 ELISAs reacted specifically with anti-MPV and anti-MMV mouse sera, respectively. MMV-rNS1 antigen had a wide reaction range with antisera to rodent parvoviruses including MPV, MMV, Kilham rat virus (KRV) and H-1 virus. All mice oronasally infected with MPV were seropositive at 4 weeks post-infection in screening by ELISAs using MPV-rVP2 and MMV-rNS1 antigens, but were negative by conventional ELISA using whole MMV antigen. A contact transmission experiment revealed that transmission of MPV occurred up to 4 weeks post-infection, and all cage mates were seropositive in screening with MPV-rVP2 and MMV-rNS1 ELISAs. These results indicate that MPV-rVP2 and MMV-rVP2 are specific ELISA antigens which distinguish between MPV and MVM infection, while MMV-rNS1 antigen can be used in generic ELISA for a variety of rodent parvoviruses. The higher sensitivity of MPV-rVP2 ELISA than conventional ELISA for detecting seroconversion to MPV in oronasally infected mice as well as in cage mates suggests the usefulness of MPV-rVP2 ELISA in quarantine and microbiological monitoring of MPV infection in laboratory mice.     

Embryo transfer rederivation of C.B-17/Icr-Prkdc(scid) mice experimentally infected with mouse parvovirus 1

We determined whether embryos derived from C.B-17/Icr-Prkdc(scid) (SCID) mice infected with mouse parvovirus (MPV) 1b and mated to MPV-naive B6C3F1 mice would transmit virus to naive recipient female mice and rederived progeny. Viral DNA was detected by quantitative PCR (qPCR) in lymphoid tissues, gonad, sperm, and feces of all MPV1b-inoculated SCID mice. Viral DNA was detected in 1 of 16 aliquots of embryos from infected male SCID mice and in 12 of 18 aliquots of embryos from infected female SCID mice. All recipient female mice implanted with embryos from infected SCID male mice and their progeny were negative by serology and qPCR. In contrast, 3 of 5 recipient female mice implanted with embryos from infected SCID female mice and 14 of 15 progeny mice from these recipients were seropositive by multiplex fluorescent immunoassay (MFI) for MPV capsid antigen (rVP2). All of these mice were negative by MFI for parvovirus nonstructural protein antigen (rNS1) and by qPCR, with the exception of 1 recipient female mouse that displayed weak rNS1 seroreactivity and low levels of MPV DNA in lymphoid tissues. Seroreactivity to rVP2 declined over time in all progeny mice from infected SCID female mice until all were seronegative by 20 wk of age, consistent with maternal antibody transfer. Given that the high levels of MPV contamination detected in our experimentally infected SCID mice are unlikely in naturally infected immunocompetent mice, these data indicate that embryo transfer rederivation is effective for the eradication of MPV from infected colonies.     

Antemortem detection of mouse parvovirus and mice minute virus by polymerase chain reaction (PCR) of faecal samples

Parvoviruses remain one of the most common viral infections seen in laboratory mouse colonies. The purpose of this study was to develop an antemortem polymerase chain reaction (PCR) assay to detect mice infected with mouse parvovirus-1 (MPV) and mice minute virus (MMV) using faecal samples. The MMV PCR assay consistently detected as few as 100 plasmid copies of MMV in faecal samples, while the MPV PCR assay detected as few as 10 plasmid copies of MPV. Faecal pellets from infected mice held at room temperature from 1 to 7 days tested positive by MMV and MPV PCR, respectively. This demonstrates that parvovirus DNA is stable in faecal samples kept at room temperature. PCR assays were also used to follow the length of MMV and MPV shedding in faeces from SENCAR mice, which were endemically infected with multiple agents. MMV faecal shedding was detected in 60-70% of the mice 5-7 weeks old, and by 13 weeks of age, faecal samples from all mice were negative for MMV. MPV faecal shedding was detected in 90-100% of the mice 5-11 weeks old; however, by 19 weeks of age, faecal samples from all mice were negative for MPV. These findings confirm that faecal shedding occurs for a limited time and suggest that 5-9-week-old mice are the most appropriate age group in endemically infected mice for faecal testing by MMV and MPV PCR.     

Reliability of soiled bedding transfer for detection of mouse parvovirus and mouse hepatitis virus

Serologic monitoring of sentinel mice exposed to soiled bedding is a common method of detecting viral infections in mice. Because bedding transfer protocols vary, the sensitivity of this method has not been documented sufficiently. We examined the reliability of bedding transfer during various stages of infection with mouse parvovirus (MPV) and mouse hepatitis virus (MHV). Most mice exposed to bedding contaminated with MPV 0, 3, or 7 d previously seroconverted, whereas only mice exposed to bedding contaminated with MHV 4 h previously seroconverted, thus confirming the differing stabilities of these viruses. Index mice were inoculated with 30 times the infectious dose 50 (ID50) of MPV or 300 ID50 of MHV. At 3 d, 1 wk, and 2 wk postinoculation (PI), we transferred 25, 50, or 100 ml of bedding to cages of sentinel mice. Viral infection and shedding by index mice was confirmed by serology and fecal polymerase chain reaction assay. Transfer of soiled bedding between mice in static cages induced seroconversion of sentinel mice most reliably during peak viral shedding (1 wk PI for MPV and 3 d PI for MHV). Soiled bedding transfer between mice in individually ventilated cages induced a higher prevalence of sentinel seroconversion to MPV and MHV than that after transfer between mice in static cages. Our findings indicate that although soiled bedding transfer is an effective method for detecting MHV and MPV under optimal conditions, the method is less than 100% reliable under many conditions in contemporary mouse facilities.     

Detection of rodent parvoviruses by use of fluorogenic nuclease polymerase chain reaction assays

Polymerase chain reaction (PCR) assays have proven useful for detection of rodent parvoviruses in animals and contaminated biological materials. Fluorogenic nuclease PCR assays combine PCR with an internal fluorogenic hybridization probe, eliminating post-PCR processing and potentially enhancing specificity. Consequently, three fluorogenic nuclease PCR assays were developed, one that detects all rodent parvoviruses, one that specifically detects minute virus of mice (MVM), and one that specifically detects mouse parvovirus 1 (MPV) and hamster parvovirus (HaPV). When rodent parvoviruses and other rodent DNA viruses were evaluated, the rodent parvovirus assay detected only rodent parvovirus isolates, whereas the MVM and MPV/HaPV assays detected only the MVM or MPV/ HaPV isolates, respectively. Each assay detected the equivalent of 10 or fewer copies of target template, and all fluorogenic nuclease PCR assays exceeded the sensitivities associated with previously reported PCR assays and mouse antibody production testing. In addition, each fluorogenic nuclease PCR assay detected the targeted parvovirus DNA in tissues obtained from mice experimentally infected with MVM or MPV. Results of these studies indicate that fluorogenic nuclease PCR assays provide a potentially high-throughput, PCR-based method to detect rodent parvoviruses in infected mice and contaminated biological materials.     

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.     

Microbiological monitoring of laboratory mice and biocontainment in individually ventilated cages: a field study

Over recent years, the use of individually ventilated cage (IVC) rack systems in laboratory rodent facilities has increased. Since every cage in an IVC rack may be assumed to be a separate microbiological unit, comprehensive microbiological monitoring of animals kept in IVCs has become a challenging task, which may be addressed by the appropriate use of sentinel mice. Traditionally, these sentinels have been exposed to soiled bedding but more recently, the concept of exposure to exhaust air has been considered. The work reported here was aimed firstly at testing the efficiency of a sentinel-based microbiological monitoring programme under field conditions in a quarantine unit and in a multi-user unit with frequent imports of mouse colonies from various sources. Secondly, it was aimed at determining biocontainment of naturally infected mice kept in an IVC rack, which included breeding of the mice. Sentinels were exposed both to soiled bedding and to exhaust air. The mice which were used in the study carried prevalent infectious agents encountered in research animal facilities including mouse hepatitis virus (MHV), mouse parvovirus (MPV), intestinal flagellates and pinworms. Our data indicate that the sentinel-based health monitoring programme allowed rapid detection of MHV, intestinal flagellates and pinworms investigated by a combination of soiled bedding and exhaust air exposure. MHV was also detected by exposure to exhaust air only. The IVC rack used in this study provided biocontainment when infected mice were kept together with non-infected mice in separate cages in the same IVC rack.     

Efficacy of three microbiological monitoring methods in a ventilated cage rack

The use of individually ventilated caging (IVC) to house mice presents new challenges for effective microbiological monitoring. Methods that exploit the characteristics of IVC have been developed, but to the authors' knowledge, their efficacy has not been systematically investigated. Air exhausted from the IVC rack can be monitored, using sentinels housed in cages that receive rack exhaust air as their supply air, or using filters placed on the exhaust air port. To aid laboratory animal personnel in making informed decisions about effective methods for microbiological monitoring of mice in IVC, the efficacy of air monitoring methods was compared with that of contact and soiled bedding sentinel monitoring. Mice were infected with mouse hepatitis virus (MHV), mouse parvovirus (MPV), murine rotavirus (agent of epizootic diarrhea of mice [EDIM]), Sendai virus (SV), or Helicobacter spp. All agents were detected using contact sentinels. Mouse hepatitis virus was effectively detected in air and soiled bedding sentinels, and SV was detected in air sentinels only. Mouse parvovirus and Helicobacter spp. were transmitted in soiled bedding, but the efficacy of transfer was dependent on the frequency and dilution of soiled bedding transferred. Results were similar when the IVC rack was operated under positive or negative air pressure. Filters were more effective at detecting MHV and SV than they were at detecting MPV. Exposure of sentinels or filters to exhaust air was effective at detecting several infectious agents, and use of these methods could increase the efficacy of microbiological monitoring programs, especially if used with soiled bedding sentinels. In contemporary mouse colonies, a multi-faceted approach to microbiological monitoring is recommended.     

Temporal transmission studies of mouse parvovirus 1 in BALB/c and C.B-17/Icr-Prkdc(scid) mice

Fecal shedding and transmission of mouse parvovirus 1 (MPV) to naive sentinels, breeding mates, and progeny were assessed. Neonatal SCID and BALB/c mice inoculated with MPV were evaluated over 24 wk; several mice from each strain were mated once during this period. Fecal MPV loads for each cage were determined weekly by quantitative polymerase chain reaction (PCR) analysis, and all mice were evaluated by quantitative PCR analysis of lymphoid tissues and seroconversion to MPV antigens in immunocompetent mice. Results indicated persistently high fecal shedding of MPV in SCID mice throughout the evaluation period sufficient to allow transmission to sentinels, naive breeding partners, and the progeny of infected male mice and naive partners. Lymphoid tissue viral loads in the progeny of infected female SCID mice were high at weaning but low at 6 wk of age. Infected BALB/c mice shed high levels of MPV in feces for 3 wk postinoculation, with seroconversion only in sentinels exposed during the first 2 wk postinoculation. Thereafter the feces of infected BALB/c mice and the lymphoid tissues of sentinels, naive breeding partners, and progeny intermittently contained extremely low levels of MPV DNA. Although pregnancy and lactation did not increase viral shedding in BALB/c mice, MPV exposure levels were sufficient to induce productive infection in some BALB/c progeny. These data indicate that the adaptive immune response suppresses, but does not eliminate, MPV shedding; this suppression is sufficient to inhibit infection of weanling and adult mice but allows productive infection of some progeny.     

Decontamination of a barrier facility using microisolator cages and provisional partitioning

In 2000, the authors found endemic infections of mouse hepatitis virus, minute virus of mice, Syphacia obvelata, and Myobia musculi among mice in a large barrier facility at the University of Mainz. To eliminate the infections, they subdivided the facility into two distinct hygiene units. However, architectural constraints made it impossible to completely separate the HVAC systems of both hygiene units and to establish adequate personnel locks. To compensate for these suboptimal barrier conditions of the two newly established units, the authors replaced the open-top caging and open-servicing system with filter-top cages that were manipulated in cage-changing stations. The authors then depopulated the two units in series, independently eliminating the contaminated mice and restocking the units with SPF animals. In spite of the high infection pressure and the suboptimal barrier conditions, the authors had only a single case of recontamination.     

A Comparison of Mouse Parvovirus 1 Infection in BALB/c and C57BL/6 Mice: Susceptibility,Replication, Shedding,and Seroconversion

This study characterized the effects of challenge with a field isolate of mouse parvovirus 1 (MPV1e) in C57BL/6NCrl (B6) and BALB/cAnNCrl (C) mice. We found that C mice were more susceptible to MPV1e infection than were B6 mice; ID₅₀ were 50 to 100 times higher after gavage and 10-fold higher after intraperitoneal injection in B6 as compared with C mice. To evaluate the host strain effect on the pathogenesis of MPV1e, B6 and C mice were inoculated by gavage. Feces and tissues, including mesenteric lymph nodes (MLN), ileum, spleen and blood, were collected for analysis by quantitative PCR (qPCR) to assess infection and fecal shedding and by RT-qPCR to evaluate replication. Peak levels of MPV1e shedding, infection, and replication were on average 3.4, 4.3, and 6.2 times higher, respectively, in C than in B6 mice. Peaks occurred between 3 and 10 d after inoculation in C mice but between 5 and 14 d in B6 mice. Multiplexed fluorometric immunoassays detected seroconversion in 2 of 3 C mice at 7 d after inoculation and in all 3 B6 mice at 10 d. By 56 d after inoculation, viral replication was no longer detectable, and fecal shedding was very low; infection persisted in ileum, spleen, and MLN, with levels higher in C than B6 mice and highest in MLN. Therefore, the lower susceptibility of B6 mice, as compared with C mice, to MPV1e infection was associated with lower levels of infection, replication, and shedding and delayed seroconversion.Abbreviations: B6, C57BL/6; C, BALB/c; MFI, median fluorescence intensity; MFIA, multiplexed fluorometric immunoassay; MLN, mesenteric lymph node; MMV, mouse minute virus; MPV, mouse parvovirus; NS1, nonstructural protein 1; qPCR, quantitative PCR; r, recombinant; Rn, normalized reporter value; VP2, virus capsid protein 2Parvoviruses are small (20 to 28 nm), nonenveloped icosahedral single-stranded DNA viruses that infect a diverse range of vertebrate and arthropod species. Much of what is understood about the biology and pathogenesis of autonomous parvoviruses has been derived from studies of the original murine parvoviral isolates, particularly the prototypic and immunosuppressive strains of mouse minute virus (MMV).^9,13,32 Because autonomous parvoviruses have a requirement and predilection for proliferating cells to replicate, they are primarily teratogenic pathogens. In contrast, rodent parvovirus infections of older animals are usually asymptomatic, because the cells that divide in mature animals, such as enterocytes, lymphoreticular cells, and hematopoietic cells, are largely spared.^2,47,48 The most common parvovirus of laboratory mice, mouse parvovirus 1 (MPV1), was first isolated²⁹ from mouse T-lymphocyte cultures that had lost viability or the ability to proliferate when stimulated. In contrast to MMV,^10,27,40 MPV1 has not been shown to cause disease in newborn or immunodeficient mice^19,45 but nevertheless has been reported to modulate the immune response of infected mice.^30,31Adventitious infections of laboratory mice with MPV1 and other parvoviruses continue to occur regularly, despite biosecurity improvements that have successfully excluded once-common pathogens such as Sendai virus.^22,37,39 One reason for the continued occurrence of these infections is that nonenveloped parvovirus virions are environmentally stable and resistant to disinfection.^18,49 Furthermore, related to their tendency to persist in host tissues even after seroconversion and their predilection for dividing cells, parvoviruses have been among the most frequent viral contaminants of transplantable tumor lines and other rodent-derived biologic reagents.^34,35 Inoculation of parvovirus-contaminated biologic reagents into experimental animals can contribute to the incidence of parvoviral outbreaks. Currently, mouse populations typically are housed in microisolation cages and are monitored for MPV1 infections through the use of soiled bedding sentinels. An MPV1 infection of the principal animals may not be transmitted to sentinels when the prevalence of infection is low, as is often the case after contamination, or when the sentinels are comparatively resistant to infection because of their genetic background or age.^7,16,17 However, a recent study found that sentinel age did not affect the likelihood of MPV1 infection.¹⁷The C57BL/6 (B6) mouse strain is popular in biomedical research and is commonly used as the background strain for spontaneous and genetically engineered mutations. We and others have noted that B6 mice are less likely to be MPV1 seropositive than are mice of other strains and stocks, even in facilities where MPV1 is widespread.⁴⁴ There has been speculation that B6 mice might not seroconvert when infected with MPV1. However, data reported here and by others^7,15 indicate that B6 mice are less likely to seroconvert because they are comparatively resistant to MPV1 infection; when they become infected, they do seroconvert. The current study evaluated whether resistance of B6 mice to infection with MPV1, as compared with BALB/c (C) mice, varies with virus inoculation route and correlates with differences in the time course and levels of viral infection, replication, and shedding and of humoral immunity.Most studies of MPV1 in mice have been performed with the cultivable MPV1a strain.^{7,19,30,31,45} Cultivable murine parvoviruses are known to differ from wildtype strains genetically and in their cell tropisms, pathogenicity, and transmissibility in vivo. For example, MPV1d, a noncultivable field isolate, was more readily transmitted to sentinels than was MPV1a.¹¹ We therefore chose to perform the current experiments with MPV1e,^3,4 a representative field strain that we originally isolated from an adventitiously infected barrier colony⁴⁴ and that has been propagated only in mice.     

A serological survey on 15 murine pathogens in mice and rats

A serological survey for 15 murine pathogens was performed on 269 mouse sera collected from 21 conventional and 12 barrier colonies, and on 376 rat sera collected from 21 conventional and 23 barrier colonies. Animals having an antibody against at least one of the antigens were contained in 81.0% of conventional and 16.7% of barrier mouse colonies and also in 81.0% of conventional and 43.5% of barrier rat colonies. Main contaminants were mouse hepatitis virus and Sendai virus in mice, and Sendai virus and pneumonia virus of mice in rats. Results also indicated that antibodies to Toolan's H-1, minute virus of mice and PVM were positive in mice from a considerable number of colonies and those to Kilham rat virus, Mycoplasma pulmonis and Toolan's H-1 were sometimes detected in rats, suggesting prevalences of these pathogens in mice and rats in Japan.     

Successful sanitation of an EDIM-infected mouse colony by breeding cessation

Despite decreasing prevalence, rotavirus infections still rank among the most important viral infections in colonies of laboratory mice. Although the disease is characterized by low mortality and a relatively short and mild clinical period, the infection has the potential to alter the outcome of experiments substantially. For animal facilities, it is therefore essential to eradicate the virus. Here we report a successful sanitation of a rotavirus-infected mouse colony in an animal facility. Despite a high ratio of transgenic and partially immunodeficient strains, a permanent eradication of the virus was achieved by euthanasia of highly susceptible mice, a prolonged breeding cessation in areas containing immunocompromised mice and a strict hygienic management. The management of a rotavirus infection reported here is a feasible and inexpensive opportunity for sanitation that benefits from maintaining most of the animal population, even in today's mouse colonies comprising mainly transgenic mice with unknown or compromised immune status.     

Molecular characterization of a newly recognized mouse parvovirus.

Mouse parvovirus (MPV), formerly known as orphan parvovirus, is a newly recognized rodent parvovirus distinct from both serotypes of minute virus of mice (MVM). Restriction analysis of the MPV genome indicated that many restriction sites in the capsid region were different from those of MVM, but most sites in the nonstructural (NS) region of the genome were conserved. MPV resembled MVM in genome size, replication intermediates, and NS proteins. Replication intermediates in infected cells were the same for MPV and MVM, including packaging of the 5-kb minus (V) strand. Furthermore, the MPV NS proteins were the same size as and present at the same ratio as the MVM(i) proteins in infected cells. Cloning and sequencing of the MPV genome revealed a genome organization closely resembling that of MVM, with conservation of open reading frames, promoter sequences, and splice sites. The left terminal hairpin was identical to that of MVM(i), but the right terminus was not conserved. Also, the MPV genome was unique in that it contained 1.8 copies of the terminal repeat sequence rather than the 1 or 2 copies found in other parvoviruses. The predicted amino acid sequence of the NS proteins of MPV and MVM(i) were nearly identical. In contrast, the predicted amino acid sequence of the capsid proteins of MPV was different from sequences of other parvoviruses. These results confirm that MPV is a distinct murine parvovirus and account for the antigenic differences between MPV and MVM.     

Successful rederivation of contaminated immunocompetent mice using neonatal transfer with iodine immersion

There is an ongoing need to eradicate intercurrent disease from research mouse colonies. Commonly used surgical methods, however, are expensive and time-consuming. The purpose of this study was to determine the percentage of litters that could be rederived from infected mouse colonies by neonatal transfer. We immersed neonatal mice in a dilute iodine solution and transferred them to disease-free foster mothers within 48 h of birth. Donor and foster mothers were evaluated for pathogens by serology and fecal polymerase chain reaction (PCR) assay. Of 55 donor mothers, 100% were positive serologically and 59% were positive by fecal PCR for one or more tested organisms, including mouse hepatitis virus, Theiler's murine encephalomyelitis virus, mouse rotavirus, and Helicobacter hepaticus. At 4 to 6 weeks after neonatal transfer, 95% of foster mothers (which served as sentinels for the transferred pups) tested free of pathogens, the exceptions being one case of mouse parvovirus 1 and two of Helicobacter spp. We suggest that cross-fostering is a viable low-cost method for rederivation of mouse colonies contaminated with pathogens such as mouse hepatitis virus, Theiler's murine encephalomyelitis virus, mouse rotavirus, and H. hepaticus.     

Simple Databases to Monitor the Generation and Organisation of Transgenic Mouse Colonies

The generation and analysis of transgenic mice has become an important tool to progress our understanding of human and mouse gene function and its association with human genetic diseases. Animal models, based on genetically modified mice, both standard transgenic and knock-out animals, are increasingly being used world-wide. Monitoring of transgenic mouse production and transgenic mouse colonies is required to efficiently manage the resources that are available. Here, I describe three independent FileMaker databases (transgenics, mymouse and cages) that have been developed to track the generation of transgenic mice, the organisation of transgenic mouse colonies and the distribution of mice in cages. These three databases are freely available for academic use.