The pathogenesis of rat virus infection in infant and juvenile rats after oronasal inoculation

Viral Disease

This chapter discusses infections of rats with viruses in the following 14 virus families: Adenoviridae, Arenaviridae, Coronaviridae, Flaviviridae, Hantaviridae, Hepeviridae, Herpesviridae, Paramyxoviridae, Parvoviridae, Picornaviridae, Pneumoviridae, Polyomaviridae, Poxviridae, and Reoviridae . Serological surveys indicate that parvoviruses, coronaviruses, cardioviruses, and pneumoviruses are the most prevalent in laboratory rats. A new polyomavirus and a new cardiovirus that cause disease in laboratory rats are described. Metagenomic analyses of feces or intestinal contents from wild rats have detected viruses from an additional nine virus families that could potentially cause infections in laboratory rats.

Identification and genomic characterization of a novel rat bocavirus from brown rats in China

Despite recent discoveries of novel animal bocaparvoviruses, current understandings on the diversity and evolution of bocaparvoviruses are still limited. We report the identification and genome characterization of a novel bocaparvovirus, rat bocaparvovirus (RBoV), in brown rats (Rattus norvegicus) in China. RBoV was detected in 11.5%, 2.4%, 16.2% and 0.3% of alimentary, respiratory, spleen and kidney samples respectively, of 636 brown rats by PCR, but not in samples of other rodent species, suggesting that brown rats are the primary reservoir of RBoV. Six RBoV genomes sequenced from three brown rats revealed the presence of three ORFs, characteristic of bocaparvoviruses. Phylogenetic analysis showed that RBoV was distantly related to other bocaparvoviruses, forming a distinct cluster within the genus, with ≤55.5% nucleotide identities to the genome of ungulate bocaparvovirus 3, supporting its classification as a novel bocaparvovirus species. RBoV possessed a putative second exon encoding the C-terminal region of NS1 and conserved RNA splicing signals, similar to human bocaparvoviruses and canine bocaparvovirus. In contrast to human, feline and canine bocaparvoviruses which demonstrates inter/intra-host viral diversity, partial VP1/VP2 sequences of 49 RBoV strains demonstrated little inter-host genetic diversity, suggesting a single genetic group. Although the pathogenicity of RBoV remains to be determined, its presence in different host tissues suggests wide tissue tropism. RBoV represents the first bocaparvovirus in rodents with genome sequenced, which extends our knowledge on the host range of bocaparvoviruses. Further studies are required to better understand the epidemiology, genetic diversity and pathogenicity of bocaparvoviruses in different rodent populations.

Humoral immune responses against minute virus of mice vectors

BACKGROUND

Owing to their oncolytic properties, autonomous rodent parvoviruses and derived vectors constitute potential anti-tumor agents.

METHODS

Humoral immune responses to minute virus of mice (MVMp) were characterized. In particular, the generation of neutralizing antibodies on subsequent therapeutic virus applications was evaluated in a mouse melanoma model. Mice bearing subcutaneous melanomas were injected intratumorally with virus and re-injected 10 days later in a second tumor on the other flank. Four days after the first or second injection, the tumors and lymph nodes were analyzed by RT-PCR for gene expression.

RESULTS

Injection of MVMp in tumor-bearing B6 mice resulted in viral gene expression in tumors and draining lymph nodes. A repeated virus administration did not lead to detectable viral transcription if it was preceded by a virus infection 10 days earlier. This protection correlated with the induction of virus-neutralizing antibodies following the first virus application. The restrictions on viral gene expression after a consecutive MVMp injection could be alleviated in subsequent applications by the use of viruses consisting of MVMp genomes packaged into capsids of a related parvovirus. Neutralizing antibody induction was irrespective of the route of administration and of the presence of a tumor and persisted at significant levels at least up to 26 weeks after the viral infection. MVMp infection of B6 mice stimulated the generation of IgM and IgG anti-viral antibodies, the latter mainly of the T-helper (Th) 1-dependent IgG2, and the T-cell-independent IgG3 subclasses.

CONCLUSIONS

Neutralizing antibodies impede the effectiveness of a subsequent virus administration, but can be overcome by pseudotyping.

Viral Disease

This chapter discusses the infections caused by DNA viruses and also RNA viruses. The chapter focuses on the detection, diagnosis, risk assessment, and decision-making regarding viral infections. Several infections caused by DNA viruses are parvoviruses, rat cytomegalovirus, poxviruses, adenovirus, and papovavirus. Several RNA viruses and infections caused by these viruses are coronaviruses, paramyxoviruses, rotavirus and reovirus, and picornaviruses. Monitoring for viral infections should cover at least three venues: animals in established breeding and experimental colonies, animals held in entry quarantine, and animal tissues and products destined for in vivo use. For established colonies, monitoring should be tailored to local conditions. Effective monitoring should encompass sampling on a pre-arranged schedule, which can be intensified if evidence or suspicion of viral infection emerges. Because viral infections of rats can spread insidiously, early detection and epidemiologic “staging” should employ a detection matrix that includes clinical observation, appropriate sampling, and sensitive and specific diagnostic testing.

Health surveillance of specific pathogen-free and conventionally-housed mice and rats in Korea

The present study contains information about proper microbiological monitoring of laboratory animals' health and the standardization of microbiological monitoring methods in Korea. Microbiological quality control for laboratory animals, composed of biosecurity and health surveillance, is essential to guard against research complications and public health dangers that have been associated with adventitious infections. In this study, one hundred and twenty-two mice and ninety rats from laboratory animal breeding companies and one animal facility of the national universities in Korea were monitored in 2000-2003. Histopathologically, thickening of the alveolar walls and lymphocytic infiltration around the bronchioles were observed in mice and rats from microbiologically contaminated facilities. Cryptosporidial oocysts were observed in the gastric pits of only conventionally-housed mice and rats. Helicobacter spp. infection was also detected in 1 of 24 feces DNA samples in mice and 9 of 40 feces DNA samples in rats by PCR in 2003, but they were not Helicobacter hepaticus. This paper describes bacteriological, parasitological, and virological examinations of the animals.

Immune responses to the major capsid protein during parvovirus infection of rats

Rat virus (RV) is a common parvovirus of laboratory rodents which can disrupt rat-based research. Prenatal or perinatal infection can be pathogenic or lead to persistent infection, whereas infection of adult rats is typically self-limiting. Effects on the host immune system have been documented during RV infection, but little is known about immune responses necessary for viral clearance. Our studies were conducted to identify humoral and cellular responses to the predominant capsid protein, VP2, during experimental infection of adult rats. We observed VP2-specific proliferation, gamma interferon production, and an immunoglobulin G2a humoral response that is maintained for at least 35 days following RV infection. These results strongly suggest the induction of virus-specific Th1-mediated immunity.

Persistent rat virus infection in smooth muscle of euthymic and athymic rats

Rat virus (RV) infection can cause disease or disrupt responses that rely on cell proliferation. Therefore, persistent infection has the potential to amplify RV interference with research. As a step toward determining underlying mechanisms of persistence, we compared acute and persistent RV infections in infant euthymic and athymic rats inoculated oronasally with the University of Massachusetts strain of RV. Rats were assessed by virus isolation, in situ hybridization, and serology. Selected tissues also were analyzed by Southern blotting or immunohistochemistry. Virus was widely disseminated during acute infection in rats of both phenotypes, whereas vascular smooth muscle cells (SMC) were the primary targets during persistent infection. The prevalence of virus-positive cells remained moderate to high in athymic rats through 8 weeks but decreased in euthymic rats by 2 weeks, coincident with seroconversion and perivascular infiltration of mononuclear cells. Virus-positive pneumocytes and renal tubular epithelial cells also were detected through 8 weeks, implying that kidney and lung excrete virus during persistent infection. Viral mRNA was detected in SMC of both phenotypes through 8 weeks, indicating that persistent infection includes virus replication. However, only half of the SMC containing viral mRNA at 4 weeks stained for proliferating cell nuclear antigen, a protein expressed in cycling cells. The results demonstrate that vasculotropism is a significant feature of persistent infection, that virus replication continues during persistent infection, and that host immunity reduces, but does not eliminate, infection.

Common Diseases

Many pathogens have been reported to cause disease in the laboratory rat. This chapter concentrates on the pathology of the more common pathogens of the laboratory rat. Based on serologic surveys, parvo viruses are some of the most common viral pathogens in wild and laboratory rat. In general, there are three main serogroups, including Rat virus (RV), H-1 virus, and Ratparvovirus (RPV). Both RPV and RV are tropic for many of the same tissues and they both may result in a persistent infection. However, RPV is antigenically and genetically distinct from RV, and it apparently does not cause clinical signs or lesions in infant rats. M. pulmonis causes natural disease in rats and mice. The infection in young rats is usually clinically silent. In older rats, there are nonspecific clinical signs such as snuffling, chromodacryorrhea, and face and ear rubbing. Several bacteria of the genus Streptococcus can cause clinical disease in rats. All of the streptococci of concern in rats are Gram-positive cocci, and are catalase-negative, nonfermentative, and generally nonmotile. Cilia-associated respiratory bacillus has been identified in rats. In rats, infection is usually asymptomatic although nonspecific clinical signs, such as weight loss and dyspnea, may be observed.

Rat parvovirus type 1: the prototype for a new rodent parvovirus serogroup

A newly recognized parvovirus of laboratory rats, designated rat parvovirus type 1a (RPV-1a), was found to be antigenically distinct. It was cloned, sequenced, and compared with the University of Massachusetts strain of rat virus (RV-UMass) and other autonomous parvoviruses. RPV-1a VP1 identity with these viruses never exceeded 69%, thus explaining its antigenic divergence. In addition, RPV-1a had reduced amino acid identity in NS coding regions (82%), reflecting phylogenetic divergence from other rodent parvoviruses. RPV-1a infection in rats had a predilection for endothelium and lymphoid tissues as previously reported for RV. Infectious RPV-1a was isolated 3 weeks after inoculation of infant rats, suggesting that it, like RV, may result in persistent infection. In contrast to RV, RPV-1a was enterotropic, a characteristic previously associated with parvovirus infections of mice rather than rats. RPV-1a also differed from RV in that infection was nonpathogenic for infant rats under conditions where RV infection causes high morbidity and mortality. Thus, RPV-1a is the prototype virus of an antigenically, genetically, and biologically distinct rodent parvovirus serogroup.

Parvovirus minute virus of mice strain i multiplication and pathogenesis in the newborn mouse brain are restricted to proliferative areas and to migratory cerebellar young neurons

Newborn BALB/c mice intranasally inoculated at birth with a lethal dose of the immunosuppressive strain of the parvovirus minute virus of mice (MVMi) developed motor disabilities and intention tremors with a high incidence by the day 6 postinfection (dpi). These neurological syndromes paralleled the synthesis of virus intermediate DNA replicative forms and yield of infectious particles in the brain, with kinetics that peaked by this time. The preferred virus replicative sites in the brain were established early in the infection (2 dpi) and at the onset of clinical symptoms (6 dpi) and were compared with major regions of cellular proliferative activity found after intraperitoneal injection of bromodeoxyuridine 24 h before encephalons were subjected to immunohistochemistry detection. At 2 dpi, viral capsid antigen was located in the laterodorsal thalamic and the pontine nuclei but not in the extensive proliferative regions of the mouse brain at this postnatal day. At 6 dpi, however, the neurotropism of the MVMi was highlighted by its ability to target the subventricular zone of the ventricles, the subependymal zone of the olfactory bulb, and the dentate gyrus of the hippocampus, which are the three main germinal centers of the cerebrum in mouse postbirth neurogenesis. Unexpectedly, in the cerebellum, the MVMi capsid antigen was confined exclusively to cells that have undergone mitosis and have migrated to the internal granular layer (IGL) and not to the proliferative external granular layer (EGL), which was stained with antiproliferative cell nuclear antigen antibody and is the main target in other parvovirus infections. This result implies temporal or differentiation coupling between MVMi cycle and neuroblast morphogenesis, since proliferative granules of the EGL should primarily be infected but must migrate in a virus carrier state into the IGL in order to express the capsid proteins. During migration, many cells undergo destruction, accounting for the marked hypocellularity specifically found in the IGL and the irregular alignment of Purkinje cell bodies, both consistent histopathological hallmarks of animals developing cerebellar symptoms. We conclude that MVMi impairs postmitotic neuronal migration occurring in the first postnatal week, when, through the natural respiratory route of infection, the virus titer peaks in the encephalon. The results illustrate the intimate connection between MVMi neuropathogenesis and mouse brain morphogenetic stage, underscoring the potential of parvoviruses as markers of host developmental programs.

Efficient induction of persistent and prenatal parvovirus infection in rats

Parvoviruses are prevalent and disruptive infectious agents of laboratory rats. Risks to rat-based research from infection are increased by the persistence of virus in immune rats and by prenatal transmission of infection. The mechanisms leading to viral persistence and prenatal infection are poorly understood and have been difficult to study for lack of reliable and humane induction methods. We report here protocols for inducing persistent and prenatal infection without causing clinical disease using the UMass strain of rat virus (RV), a common rat parvovirus. Infant rats inoculated by the oronasal route at 6 days of age had greater than 90% prevalence of persistent infection. RV-UMass also induced intrauterine infection in pregnant rats inoculated by the oronasal route. Inoculation of dams at gestation day 9 frequently caused severe disease in the fetuses whereas inoculation at gestation day 12 caused primarily asymptomatic fetal infection that persisted post partum RV-UMass infection facilitates study of parvoviralhost interactions that are relevant to laboratory rats and which also may improve understanding of persistent and prenatal human parvovirus infection.

Vertical transmission to embryo and fetus in maternal infection with rat virus (RV)

The influence of maternal rat virus (RV) infection on rat embryogenesis and fetus was examined by viral reisolation, immunostaining and PCR analysis. Vertical transmission caused by the UT-1 strain of RV depended on the stage of gestation when maternal infection occurred. When females were infected at the pre-mating point, the number of fetuses was smaller than that normally obtained, possibly due to infection at the stage of the hatched blastocyst, but almost all of the fetuses obtained were free from infection and developed normally. The incidence of transplacental infection was the highest when pregnant females were infected in the middle of the gestation stage, and some of the fetuses died. In pregnant females which were infected late in the gestation stage, all fetuses developed normally. Some of them were infected transplacentally and harbored the infectious virus. Much attention should be paid to performing reliable rederivation of RV-infected rat colonies by hysterectomy and embryo transfer.

Detection of H-1 parvovirus and Kilham rat virus by PCR

H-1 virus and Kilham rat virus (KRV) are autonomous parvoviruses which generally cause subclinical infections in rats and can cause persistent infections in cell cultures. In this study, primer sets specific for either H-1 or KRV were designed on the basis of DNA sequence comparisons of the rodent parvoviruses. The specificities of the H-1 and KRV-specific primer sets were determined by testing viral preparations of seven different parvoviruses and nine other viruses known to infect rodents. The H-1-specific PCR assay amplified the expected 254-bp product only in the presence of H-1 viral DNA and was able to detect as little as 100 fg of H-1 viral DNA. The KRV-specific PCR assay generated the expected 281-bp product only when KRV viral DNA was used as the template and was able to detect as little as 10 pg of KRV viral DNA. Each assay was able to detect its respective virus in tissues from rats experimentally infected with H-1 or KRV. In contrast, no product was amplified by either assay with tissues from mock-infected rats. Our findings indicate that these PCR assays provide rapid, specific, and sensitive methods for the detection of H-1 or KRV infection in rats and cell culture systems.

Characterization of mouse parvovirus infection by in situ hybridization

Infection of young adult BALB/cByJ mice with mouse parvovirus-1, a newly recognized, lymphocytotropic, nonpathogenic parvovirus, was examined by in situ hybridization. Virus appeared to enter through the small intestine and was disseminated to the liver and lymphoid tissues. Strand-specific probes detected virion DNA in a consistently larger number of cells than replicative forms of viral DNA and/or viral mRNA. The number of signal-positive cells in the intestinal mucosa, lymph nodes, spleen, and thymus increased through day 10 after oral inoculation but decreased after seroconversion. Positive cells were still detected, however, in peripheral lymphoid tissues of mice examined at 9 weeks postinoculation. The results underscore the need to assess potential effects of persistent mouse parvovirus-1 infection on immune function in mice.

Characterization of acute rat parvovirus infection by in situ hybridization

In situ hybridization and virus titration were used to characterize early stages of rat virus (RV) infection of rat pups after oronasal inoculation. Results suggest that virus enters through the lung and that early viremia leads rapidly to pantropic infection. Cells derived from all three germ layers were infected with RV, but those of endodermal and mesodermal origin were the predominant targets. Infection of vascular endothelium was widespread and was associated with hemorrhage and infarction in the brain. Convalescence from acute infection was accompanied by mononuclear cell infiltrates at sites containing RV DNA. Viral DNA was also detected in endothelium, fibroblasts and smooth muscle myofibers four weeks after inoculation. Further examination of these cells as potential sites of persistent infection is warranted.

Induction of type I diabetes by Kilham's rat virus in diabetes-resistant BB/Wor rats

Type I diabetes mellitus is an autoimmune disease resulting from the interaction of genetic and environmental factors. A virus that was identified serologically as Kilham's rat virus (KRV) was isolated from a spontaneously diabetic rat and reproducibly induced diabetes in naive diabetes-resistant (DR) BB/Wor rats. Viral antigen was not identified in pancreatic islet cells, and beta cell cytolysis was not observed until after the appearance of lymphocytic insulitis. KRV did not induce diabetes in major histocompatibility complex-concordant and discordant non-BB rats and did not accelerate diabetes in diabetes-prone BB/Wor rats unless the rats had been reconstituted with DR spleen cells. This model of diabetes may provide insight regarding the interaction of viruses and autoimmune disease [corrected]

Persistent rat parvovirus infection in individually housed rats

The duration of infection with rat virus (RV), an autonomous rodent parvovirus, was examined at multiple intervals over 6 months in rats inoculated by the oronasal route at 2 days of age or 4 weeks of age and individually housed after weaning to prevent cross-infection. Infectious virus was recovered by explant culture from 32 of 80 rats inoculated as pups and was detected as late as 6 months after inoculation. Rats inoculated as juveniles developed acute infection, but virus was not detected beyond 7 weeks after inoculation. Tissues from rats in both age groups were surveyed for RV DNA by Southern blotting using a double-stranded DNA probe made from a 1700 bp cloned fragment of RV spanning map units 0.19-0.52. Band patterns representative of acute infection (juvenile rats) were consistent with the replicating form of RV DNA, whereas patterns representative of persistent infection (rats inoculated as pups) were suggestive of defective or non-productive viral replication.

Modulation of lethal and persistent rat parvovirus infection by antibody

Two day-old athymic (rnu/rnu) and euthymic (rnu/+) rat pups nursing immune or non-immune dams were inoculated oronasally with the Yale strain of rat virus (RV-Y). All athymic and euthymic pups (57/57) from immune dams remained clinically normal, whereas 51 of 66 athymic and euthymic pups from non-immune dams died within 30 days. Infectious RV was detected by explant culture in 12 of 15 surviving pups of both genotypes from non-immune dams 30 days after inoculation, but in none of the 57 surviving pups from immune dams. RV-Y DNA was detected by Southern blotting in kidneys of surviving athymic pups from non-immune dams but was not detected in pups from immune dams. Euthymic pups from immune dams appeared not to produce endogenous antibody to RV after virus challenge, whereas euthymic pups from non-immune dams produced high-titered RV immune serum. Pups of both genotypes given immune serum prior to or with RV were fully protected from disease and persistent infection, whereas pups given immune serum 24 hours after RV were partially protected. These studies show that RV antibody offers significant protection against lethal and persistent RV infection.

Characterization of newly isolated rat viruses from asymptomatic laboratory rats

We surveyed the extent of rat virus (RV) infections in Japan and isolated new RV strains. The new strains were similar to the prototype RV strain in stability, morphology in electron microscopy and structural polypeptides. There were slight differences, however, in hemagglutination activity and the antigenicity.

Persistence of rat parvovirus in athymic rats

Euthymic (SD or outbred rnu/+) and athymic (rnu/rnu) rats were inoculated oronasally or intraperitoneally with the RV-Y strain of rat virus when they were 2 days or 4 weeks old. Clinical signs of infection in athymic infants were similar to those in euthymic infants, but significantly more athymic infants died. Some infants developed anemia and thrombocytopenia. After inoculation of infants. RV-Y was detected in surviving euthymic rats for 7 weeks and in surviving athymic rats for at least 10 weeks. After oronasal inoculation of 4 week-old rats no clinical illness was observed. RV-Y persisted less than 6 weeks in juvenile euthymic rats but at least 12 weeks in athymic juvenile rats. Intraperitoneal inoculation of juveniles resulted in infection for at least 6 weeks. The antibody response of athymic rats to RV-Y was significantly reduced compared to that of euthymic rats. These studies indicate that T cell deficiency increases the severity and duration of RV infection and imply that T cells are required for the full expression of resistance to RV infection. They also suggest that RV-Y induced anemia could serve as a model for human parvovirus-induced anemia.

Persistence of rat virus in seropositive rats as detected by explant culture. Brief report

Rat virus (RV) was detected by explant culture for up to 14 weeks in rats inoculated as infants and for up to 7 weeks in rats inoculated as juveniles, although both groups were seropositive by 3 weeks post-inoculation.