Inhibition of in vitro lymphoproliferative responses by in vivo passaged rat 13762 mammary adenocarcinoma cells. II. Evidenceth Kilham rat virus is responsible for the inhibitory effect

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.

Implications of infectious agents on results of animal experiments

Report of the Working Group on Hygiene of the Gesellschaft für Versuchstierkunde–Society for Laboratory Animal Science (GV-SOLAS)

GV-SOLAS Working Group on Hygiene: Werner Nicklas (Chairman), Felix R. Homberger, Brunhilde Illgen-Wilcke, Karin Jacobi, Volker Kraft, Ivo Kunstyr, Michael Mähler, Herbert Meyer & Gabi Pohlmeyer-Esch

1 Management of Immunocompromised and Infected Animals

This chapter discusses the management of immunocompromized and infected animals. The microbiological quality of laboratory animals is a direct result of colony management practices, and monitoring provides an after-the-fact assessment of the adequacy of those practices. In the case of immunocompromised animals or in infection experiments, however, monitoring for a comprehensive list of micro-organisms is reasonable. The testing of animals usually starts with necropsy and blood sampling for serology, followed by microscopic examination for parasites and sampling of organs for bacteriology, pathology, and, in rare cases, virological examinations. Biological materials represent a high risk, if they originate from or have been propagated in animals. In particular, tumors, viruses, or parasites that are serially passaged in animals often pick up pathogens, and therefore a high percentage of these are contaminated. It has been shown in mice and rats that all preimplantational stages can be revitalized successfully upon freezethaw procedures. For long-term storage, eight-cell stages have been recommended in the chapter, while two-cell stages were considered to be less suitable. One embryo batch (inbred strain) derived from a single pedigree donor pair may be regarded as a prospective breeding nucleus, if one fertile breeding pair is obtained upon revitalization. Assuming an average revitalization rate of 20% (fertile breeders), one embryo batch should contain a minimum number of 10 embryos to obtain at least one breeding pair with a 50% chance of revitalization.

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.

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.

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.

Expression of recombinant parvovirus NS1 protein by a baculovirus and application to serologic testing of rodents

A recombinant baculovirus containing the NS1 gene of minute virus of mice was constructed. Optimal expression of the recombinant NS1 protein (rNS1) was achieved by infecting Trichoplusa ni High Five cells at a multiplicity of 10 and incubating them for 72 h postinfection. An enzyme-linked immunosorbent assay (ELISA) with rNS1 as the antigen was evaluated for serologic testing of laboratory rodents. The rNS1 ELISA proved to be a more sensitive method for the detection of antibodies to recently recognized rodent parvovirus species (mouse orphan parvovirus and rat orphan parvovirus) and prototypic parvovirus species (minute virus of mice, Kilham's rat virus, and H-1) than were conventional parvovirus ELISAs that use whole parvovirus virions.

Detection of newly recognized rodent parvoviruses by PCR

Several autonomous parvovirus isolates distinct from the prototypic rodent parvoviruses have recently been identified. These include variants of a mouse orphan parvovirus (MOPV) and a hamster isolate designated hamster orphan parvovirus (HOPV). In this study, a PCR primer set specific for these newly identified rodent parvoviruses was designed on the basis of DNA sequence comparisons of these isolates with other autonomous parvoviruses. The specificity of the primer set was determined by testing viral preparations of seven different parvoviruses and eight other viruses known to infect rodents. The PCR assay amplified the expected 260-bp product only in the presence of DNA from MOPV, HOPV, or LuIII a parvovirus of unknown species origin. The assay was able to detect as little as 10 pg of MOPV viral DNA or 1 pg of HOPV viral DNA, and it was able to detect MOPV in tissues from naturally infected mice and HOPV in tissues from experimentally infected hamsters. In contrast, the 260-bp product was not amplified from tissues of MOPV-negative mice or mock-infected hamsters. Our findings indicate that this PCR assay provides a rapid, specific, and sensitive method for the detection of MOPV in mice, HOPV in hamsters, and MOPV and HOPV in cell culture systems and that it may also be useful for the detection of LuIII contamination of cell culture systems.

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 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.

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.

Parvovirus replication

The members of the family Parvoviridae are among the smallest of the DNA viruses, with a linear single-stranded genome of about 5 kilobases. Currently the family is divided into three genera, two of which contain viruses of vertebrates and a third containing insect viruses. This review concentrates on the vertebrate viruses, with emphasis on recent advances in our insights into the molecular biology of viral replication. Traditionally the vertebrate viruses have been distinguished by the presence or absence of a requirement for a coinfection with a helper virus before productive infection can occur, hence the notion that the dependoviruses (adeno-associated viruses [AAV]) are defective. Recent data would suggest that not only is there a great deal of structural and genetic organizational similarity between the two types of vertebrate viruses, but also there is significant similarity in the molecular biology of productive replication. What differs is the physiological condition of the host cell that renders it permissive. Healthy dividing cells are permissive for productive replication by autonomous parvoviruses; such cells result in latent infection by dependoviruses. For a cell to become permissive for productive AAV replication, it must have been exposed to toxic conditions which activate a latent AAV genome. Such conditions can be caused by helper-virus infection or exposure to physical (UV light) or chemical (some carcinogens) agents. In this paper the molecular biology of replication is reviewed, with special emphasis on the role of the host and the consequences of viral infection for the host.

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.

Potential detrimental effects of rodent viral infections on long-term experiments

Healthy animals are of paramount importance in obtaining meaningful, reliable scientific results. Viral infections of rodents often have a significant impact on various types of biomedical research. Laboratory animal specialists and researchers must be aware of the possible consequences associated with the use of infected animals. The objective of the paper is a discussion of the frequently encountered viral infections that can complicate or invalidate the interpretation of results by altering the host's response.

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

The pathogenesis of rat virus (RV) infection was studied in random-bred Sprague-Dawley rats after oronasal inoculation of a recent RV isolate designated RV-Yale (RV-Y). RV-Y was pathogenic for rats inoculated as infants (2 days) whereas rats inoculated as juveniles (4 weeks) had asymptomatic infection and no lesions. Rats inoculated as infants developed pantropic infection accompanied by hepatic necrosis, granuloprival cerebellar hypoplasia and hemorrhagic encephalopathy. Virological and serological studies showed that virus could persist in inoculated rats for at least 35 days and for at least 28 days after seroconversion was first detected. Immunohistochemical results indicated that RV-Y infects tissues conducive to virus excretion including kidney and lung. RV-Y also was found in genital tissues of some rats. Athymic juvenile rats inoculated intraperitoneally with RV-Y had a poor humoral immune response and harbored infectious virus for at least 3 weeks, whereas infection in euthymic control rats was detected for 1 week. These studies indicate that RV-Y can persists in the presence of humoral immunity and suggest that transmission of infection could occur for a substantial period after seroconversion. They also suggest that immunodeficient rats have increased susceptibility to persistent infection.

DNA sequence of the lymphotropic variant of minute virus of mice, MVM(i), and comparison with the DNA sequence of the fibrotropic prototype strain

The sequence of molecular clones of the genome of MVM(i), a lymphotropic variant of minute virus of mice, was determined and compared with that of MVM(p), the fibrotropic prototype strain. At the nucleotide level there are 163 base changes: 129 transitions and 34 transversions. Most nucleotide changes are silent, with only 27 amino acids changes predicted, of which 22 are conservative. Notable differences between the MVM(i) and MVM(p) genomes which may account for the cell specificities of these viruses occur within the 3' nontranslated regions. The differences discussed include the absence of a 65-base-pair direct in MVM(i), the presence of only two polyadenylation sites in MVM(i) compared with four in MVM(p), and sequences that bear a resemblance to enhancer sequences. Also included in this paper is an important correction to the MVM(p) sequence (C.R. Astell, M. Thomson, M. Merchlinsky, and D. C. Ward, Nucleic Acids Res. 11:999-1018, 1983).

Interaction of minute virus of mice with differentiated cells: strain-dependent target cell specificity is mediated by intracellular factors

The prototype strain of minute virus of mice and the immunosuppressive strain are unable to grow lytically in each other's murine host cell type. To characterize these strain-dependent virus-host cell interactions further, we have compared the early events of both productive and restrictive infections. Each virus binds to specific receptors on the surface of both productive and restrictive cell types. Competition experiments show that both viruses recognize the same receptor on each cell type. Penetration and uncoating are presumed to be similar in both productive and restrictive infections, since incoming viral genomes are converted to parental replicative form DNA independent of the final outcome of the virus-host cell interaction. In contrast to the majority of other systems studied to date, these differences in minute virus of mice target cell specificity are not mediated at the cell surface, but by the interaction of a strain-specific viral determinant with intracellular host factors that are expressed in particular cell types as a function of differentiation. These cellular factors catalyze a step in viral replication which occurs after the initiation of viral DNA synthesis, but before the detectable expression of the viral capsid polypeptide genes.

Reciprocal productive and restrictive virus-cell interactions of immunosuppressive and prototype strains of minute virus of mice

Viral and cellular factors responsible for parvovirus target cell specificity have been examined for two serologically indistinguishable strains of the minute virus of mice which infect mouse cells of dissimilar differentiated phenotype. Both the prototype strain and the immunosuppressive strain grow in and form plaques on monolayers of simian virus 40-transformed human fibroblasts, a finding that has allowed the comparison of several aspects of their virus-host cell interactions. Although closely related by antigenic and genomic criteria, both the prototype strain and the immunosuppressive strain are restricted for lytic growth in each other's murine host cell, that is, in T cells and fibroblasts, respectively. The host range of each virus variant appears to be specified by a genetic determinant that is stably inherited in the absence of selection. In the restrictive virus-host interaction lytic growth is limited to a small or, in some cases, undetectable subset of the host cell population, and the majority of the infected cells remain viable, continuing to grow at the normal rate without expressing viral antigens. The susceptible host cell phenotype is dominant in T lymphocyte x fibroblast fusion hybrids, implying that different cell types express different developmentally regulated virus helper functions that can only be exploited by the virus variant that carries the appropriate strain-specific determinant.