Pathogen Prevalence Estimates and Diagnostic Methodology Trends in Laboratory Mice and Rats from 2003 to 2020.

Rodents used in biomedical research are maintained as specific pathogen-free (SPF) by employing biosecurity measures that eliminate and exclude adventitious infectious agents known to confound research. The efficacy of these practices is assessed by routine laboratory testing referred to as health monitoring (HM). This study summarizes the results of HM performed at Charles River Research Animal Diagnostic Services (CR-RADS) on samples submitted by external (non-Charles River) clients between 2003 and 2020. Summarizing this vast amount of data has been made practicable by the recent introduction of end-user business intelligence tools to Excel. HM summaries include the number of samples tested and the percent positive by diagnostic methodology, including direct examination for parasites, cultural isolation and identification for bacteria, serology for antibodies to viruses and fastidious microorganisms, and polymerase chain reaction (PCR) assays for pathogen-specific genomic sequences. Consistent with comparable studies, the percentages of pathogen-positive samples by diagnostic methodology and year interval are referred to as period prevalence estimates (%PE). These %PE substantiate the elimination of once common respiratory pathogens, such as Sendai virus, and reductions in the prevalence of other agents considered common, such as the rodent coronaviruses and parvoviruses. Conversely, the %PE of certain pathogens, for example, murine norovirus (MNV), Helicobacter, Rodentibacter, and parasites remain high, perhaps due to the increasing exchange of genetically engineered mutant (GEM) rodents among researchers and the challenges and high cost of eliminating these agents from rodent housing facilities. Study results also document the growing role of PCR in HM because of its applicability to all pathogen types and its high specificity and sensitivity; moreover, PCR can detect pathogens in samples collected antemortem directly from colony animals and from the environment, thereby improving the detection of host-adapted, environmentally unstable pathogens that are not efficiently transmitted to sentinels by soiled bedding.

Serendipitous Discovery of a Novel Murine Astrovirus Contaminating a Murine Helper T-cell Line and Incapable of Infecting Highly Immunodeficient Mice.

The unexpected seroconversion of sentinel mice in our facility to murine T lymphotrophic virus (MTLV) positivity led to our identification of a novel murine astrovirus that we designated murine astrovirus 2 (MuAstV-2). During our investigation, MuAstV-2 was found to be a contaminant of the T helper cell line (D10. G4.1) that was used to generate the MTLV antigen that we included in the multiplex fluorometric immunoassay (MFIA) that we used for sentinel screening. We eventually determined that cross-reactivity with the astrovirus generated a positive result in the MTLV assay. A confirmatory immunofluorometric assay (IFA) using the same MTLV-infected cell line yielded a similar result. However, the use of antigen prepared from MTLV-infected neonatal mouse thymus did not reproduce a positive result, leading us to suspect that the seroreactivity we had observed was not due to infection with MTLV. A mouse antibody production test showed that mice inoculated with naïve D10. G4.1 cells and their contact sentinels tested positive for MTLV using cell-line generated antigen, but tested negative in assays using MTLV antigen produced in mice. Metagenomic analysis was subsequently used to identify MuAstV-2 in feces from 2 sentinel mice that had recently seroconverted to MTLV. Two closely related astrovirus sequences (99.6% capsid identity) were obtained and shared 95% capsid amino acid identity with the MuAstV-2 virus sequenced from the D10. G4.1 cell line. These viruses are highly divergent from previously identified murine astroviruses, displaying <30% capsid identity, yet were closely related to murine astrovirus 2 (85% capsid identity), which had recently been isolated from feral mice in New York City. A MuAstV-2 specific PCR assay was developed and used to eradicate MuAstV-2 from the infected colony using a test and cull strategy. The newly identified MuAstV2 readily transmits to immunocompetent mouse strains by fecal-oral exposure, but fails to infect NOD-Prkdcem26Cd52Il2rgem26Cd22/NjuCrl (NCG) mice, which have significantly impaired adaptive and innate immune systems. Neither immunocompetent nor immunodeficient mice showed any astrovirus-associated pathology. MuAstV-2 may provide a valuable model for the study of specific aspects of astrovirus pathogenesis and virus-host interactions.

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.

Efficacy of direct detection of pathogens in naturally infected mice by using a high-density PCR array.

We used a high-density array of real-time PCR assays for commonly reported rodent infectious agents (PRIA) to test naturally infected index mice and sentinel mice exposed by contact and soiled-bedding transfer. PRIA detected 14 pathogens--including viruses, bacteria, fur mites, pinworms, and enteric protozoa--in 97.2% of 28 pooled fecal samples, fur-perianal swabs, and oral swabs from 4 cages containing a total of 10 index mice. Among these pathogens, PRIA (like conventional health monitoring methods) failed to detect Mycoplasma pulmonis, Pasteurella pneumotropica, and Giardia spp. in all of the 9 contact and 9 soiled-bedding sentinels. PRIA demonstrated murine adenovirus and Cryptosporidium and Spironucleus spp. in contact but not soiled-bedding sentinels and detected Helicobacter and pinworms in fewer than half of the soiled-bedding sentinels. Of the 4 species of Helicobacter that species-specific PCR assays identified in index mice, only H. ganmani was found in soiled-bedding and contact sentinels. PRIA detected all of the pathogens in sentinels that were identified by conventional methods. Myobia musculi was detected by PCR in index and sentinel mice but missed by conventional parasitologic examinations. In summary, PRIA reproducibly detected diverse pathogens in heavily pooled specimens collected noninvasively from infected index mice antemortem. The inability of PRIA and conventional health monitoring methods (that is, parasitology, micro-biology, and serology) to demonstrate transmission of some pathogens to contact sentinels and the inefficient transmission of others to soiled-bedding sentinels underscores the importance of direct PCR testing to determine the pathogen status of rodents in quarantine and during routine colony surveillance.

Multiplexed fluorometric immunoassay testing methodology and troubleshooting.

To ensure the quality of animal models used in biomedical research we have developed a number of diagnostic testing strategies and methods to determine if animals have been exposed to adventitious infectious agents (viruses, mycoplasma, and other fastidious microorganisms). Infections of immunocompetent animals are generally transient, yet serum antibody responses to infection often can be detected within days to weeks and persist throughout the life of the host. Serology is the primary diagnostic methodology by which laboratory animals are monitored. Historically the indirect enzyme-linked immunosorbent assay (ELISA) has been the main screening method for serosurveillance. The ELISA is performed as a singleplex, in which one microbial antigen-antibody reaction is measured per well. In comparison the MFIA is performed as a multiplexed assay. Since the microspheres come in 100 distinct color sets, as many as 100 different assays can be performed simultaneously in a single microplate well. This innovation decreases the amount of serum, reagents and disposables required for routine testing while increasing the amount of information obtained from a single test well. In addition, we are able to incorporate multiple internal control beads to verify sample and system suitability and thereby assure the accuracy of results. These include tissue control and IgG anti-test serum species immunoglobulin (αIg) coated bead sets to evaluate sample suitability. As in the ELISA and IFA, the tissue control detects non-specific binding of serum immunoglobulin. The αIg control (Serum control) confirms that serum has been added and contains a sufficient immunoglobulin concentration while the IgG control bead (System Suitability control), coated with serum species immunoglobulin, demonstrates that the labeled reagents and Luminex reader are functioning properly.

Role of housing modalities on management and surveillance strategies for adventitious agents of rodents.

Specific pathogen-free (SPF) rodents for modern biomedical research need to be free of pathogens and other infectious agents that may not produce disease but nevertheless cause research interference. To meet this need, rodents have been rederived to eliminate adventitious agents and then housed in room- to cage-level barrier systems to exclude microbial contaminants. Because barriers can and do fail, routine health monitoring (HM) is necessary to verify the SPF status of colonies. Testing without strict adherence to biosecurity practices, however, can lead to the inadvertent transfer of unrecognized, inapparent agents among institutions and colonies. Microisolation caging systems have become popular for housing SPF rodents because they are versatile and provide a highly effective cage-level barrier to the entry and spread of adventitious agents. But when a microisolation-caged colony is contaminated, the cage-level barrier impedes the spread of infection and so the prevalence of infection is often low, which increases the chance of missing a contamination and complicates the corroboration of unexpected positive findings. The expanding production of genetically engineered mutant (GEM) rodent strains at research institutions, where biosecurity practices vary and the risk of microbial contamination can be high, underscores the importance of accurate HM results in mitigating the risk of the introduction and spread of microbial contaminants with the exchange of mutant rodent strains among investigators and institutions.

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.

Large-scale rodent production methods make vendor barrier rooms unlikely to have persistent low-prevalence parvoviral infections.

A recent article in Contemporary Topics in Laboratory Animal Science by Pullium and colleagues expressed the opinion that because no other source could be found for a parvoviral contamination detected in sentinel mice prior to deployment, the infection apparently came from the unspecified vendor, even though no antibodies were ever detected in mice within 3 weeks of arrival. As this opinion may be shared by others and expresses some of the deep frustration in trying to detect the source of parvoviral infection in facilities using cage-level bioexclusion housing, Charles River Laboratories (CRL) feels it important to contribute to scientific dialogue by claiming to be the unnamed vendor in the Pullium article and discussing why a parvoviral contamination in a CRL barrier room would be detected rapidly. We show that viral infections in CRL barrier rooms rapidly reach high prevalence and that such contaminations historically have been detected quickly, and we describe why we feel enhancements in current monitoring methods provide for even more rapid detection of parvoviruses. Furthermore, we present substantial evidence that the barrier rooms that served as the source of the customer-suspect sentinel mice remain free of all parvoviruses, in light of monitoring of hundreds of mice by all available techniques. Therefore, although an initial list of all possible sources of contamination prudently should include vendors, the evidence is overwhelming that this vendor was not the source of the parvoviral contamination discussed in the Pullium paper.

Bordetella bronchiseptica infection of rats and mice.

Bordetella bronchiseptica has long been associated with respiratory tract infections in laboratory research, food-producing, companion, and wildlife animal species. Its range of distribution also may include humans and contaminated inanimate environmental sources. Natural diseases due to B. bronchiseptica infections in laboratory rats and mice were described before many of the major pathogens of these hosts were discovered. To our knowledge, there are no recent reports of natural disease due to B. bronchiseptica in these species; as a result, some have questioned its role as a natural pathogen in murine hosts. We reviewed occurrence of natural B. bronchiseptica infections and present information gained from recent experimental infection studies in murine hosts. We also discuss the potential impact of natural B. bronchiseptica infections on research and methods of control.