Charles B. Clifford

Biology and Diseases of Rats

The laboratory rat, Rattus norvegicus, is within the order Rodentia and family Muridae. The genus Rattus contains at least 56 species (retrieved January 28, 2014, from the Integrated Taxonomic Information System online database http://www.itis.gov); however, the Norway rat, R. norvegicus, and the black rat, R. rattus, are the two species most commonly associated with the genus. Rattus rattus preceded R. norvegicus in migration from Asia to Europe and the Americas by several hundred years. The former species reached Europe in the 12th century, and the Americas in the 16th century; whereas, R. norvegicus emerged in the 18th century in Europe and in the 19th century in the Western Hemisphere. Globally, the Norway rat has largely displaced the black rat, probably because of the Norway rat’s larger size and aggressiveness. The domestication and introduction of the albino R. norvegicus is rooted by its use in Europe and America in the 1800s as prey for a sport (rat baiting) in which individuals would wager on which terrier dog would most swiftly kill the largest number of rats confined to a pit. Because of the large numbers of rats needed for this sport, wild rats were purpose-bred, and albinos were selected out by some people as a hobby (Robinson, 1965; Mayhew, 1851).

Contemporary prevalence of infectious agents in laboratory mice and rats

Periodic health screening of rodents used in research is necessary due to the consequences of unwanted infections. One determinant of the risk of infection for any given agent is its prevalence; other factors being equal, a prevalent agent is more likely than a rare one to be introduced to a research facility and result in infection. As an indicator of contemporary prevalence in laboratory populations of rats and mice, the rate of positive results in the samples received at a major commercial rodent diagnostic laboratory was compiled for this paper. Although samples from laboratory rodent vendors have been excluded, results are tabulated from samples from more than 500,000 mice and 80,000 rats submitted over several years from pharmaceutical, biotechnology, academic, and governmental institutions in North America and Europe, allowing meaningful determination of which agents are common in the research environment versus which agents are rare. In mice, commonly detected infectious agents include mouse norovirus, the parvoviruses, mouse hepatitis virus, rotavirus, Theilers murine encephalomyelitis virus, Helicobacter spp., Pasteurella pneumotropica, and pinworms. In rats, commonly detected infectious agents include ‘rat respiratory virus’, the parvoviruses, rat theilovirus, Helicobacter spp., P. pneumotropica, and pinworms. A risk-based allocation of health-monitoring resources should concentrate frequency and/or sample size on these high-risk agents, and monitor less frequently for the remaining, lower-risk, infectious agents.

Immune Competency of a Hairless Mouse Strain for Improved Preclinical Studies in Genetically Engineered Mice

Genetically engineered mouse models (GEMM) of cancer are of increasing value to preclinical therapeutics. Optical imaging is a cost-effective method of assessing deep-seated tumor growth in GEMMs whose tumors can be encoded to express luminescent or fluorescent reporters, although reporter signal attenuation would be improved if animals were fur-free. In this study, we sought to determine whether hereditable furlessness resulting from a hypomorphic mutation in the Hairless gene would or would not also affect immune competence. By assessing humoral and cellular immunity of the SKH1 mouse line bearing the hypomorphic Hairless mutation, we determined that blood counts, immunoglobulin levels, and CD4+ and CD8+ T cells were comparable between SKH1 and the C57Bl/6 strain. On examination of T-cell subsets, statistically significant differences in naïve T cells (1.7 versus 3.4 × 105 cells/spleen in SKH1 versus C57Bl/6, P = 0.008) and memory T cells (1.4 versus 0.13 × 106 cells/spleen in SKH1 versus C57Bl/6, P = 0.008) were detected. However, the numerical differences did not result in altered T-cell functional response to antigen rechallenge (keyhole limpet hemocyanin) in a lymph node cell in vitro proliferative assay. Furthermore, interbreeding the SKH1 mouse line to a rhabdomyosarcoma GEMM showed preserved antitumor responses of CD56+ natural killer cells and CD163+ macrophages, without any differences in tumor pathology. The fur-free GEMM was also especially amenable to multiplex optical imaging. Thus, SKH1 represents an immune competent, fur-free mouse strain that may be of use for interbreeding to other genetically engineered mouse models of cancer for improved preclinical studies. Mol Cancer Ther; 9(8); 2354–64. ©2010 AACR.

Old enemies, still with us after all these years

Abstract Although some previously common infections, such as Sendai virus and Mycoplasma pulmonis, have become rare in laboratory rodents in North American research facilities, others continue to plague researchers and those responsible for providing biomedical scientists with animals free of adventitious disease. Long-recognized agents that remain in research facilities in the 21st century include parvoviruses of rats and mice, mouse rotavirus, Theilers murine encephalomyelitis virus (TMEV), mouse hepatitis virus (MHV), and pinworms. The reasons for their persistence vary with the agent. The resilience of parvoviruses, for example, is due to their resistance to inactivation, their prolonged shedding, and difficulties with detection, especially in C57BL/6 mice. Rotavirus also has marked environmental resistance, but periodic reintroduction into facilities, possibly on bags of feed, bedding, or other supplies or equipment, also seems likely. TMEV is characterized by resistance to inactivation, periodic reintroduction, and relatively long shedding periods. Although MHV remains active in the environment at most a few days, currently prevalent strains are shed in massive quantities and likely transmitted by fomites. Pinworm infestations continue because of prolonged infections, inefficient diagnosis, and the survivability of eggs of some species in the environment. For all of these agents, increases in both interinstitutional shipping and the use of immunodeficient or genetically modified rodents of unknown immune status may contribute to the problem, as might incursions by wild or feral rodents. Elimination of these old enemies will require improved detection, strict adherence to protocols designed to limit the spread of infections, and comprehensive eradication programs.

Diagnostic Necropsy and Selected Tissue and Sample Collection in Rats and Mice

There are multiple sample types that may be collected from a euthanized animal in order to help diagnose or discover infectious agents in an animal colony. Proper collection of tissues for further histological processing can impact the quality of testing results. This article describes the conduct of a basic gross examination including identification of heart, liver, lungs, kidneys, and spleen, as well as how to collect those organs. Additionally four of the more difficult tissue/sample collection techniques are demonstrated. Lung collection and perfusion can be particularly challenging as the tissue needs to be properly inflated with a fixative in order for inside of the tissue to fix properly and to enable thorough histologic evaluation. This article demonstrates the step by step technique to remove the lung and inflate it with fixative in order to achieve optimal fixation of the tissue within 24 hours. Brain collection can be similarly challenging as the tissue is soft and easily damaged. This article demonstrates the step by step technique to expose and remove the brain from the skull with minimal damage to the tissue. The mesenteric lymph node is a good sample type in which to detect many common infectious agents as enteric viruses persist longer in the lymph node than they are shed in feces. This article demonstrates the step by step procedure for locating and aseptically removing the mesenteric lymph node. Finally, identification of infectious agents of the respiratory tract may be performed by bacterial culture or PCR testing of nasal and/or bronchial fluid aspirates taken at necropsy. This procedure describes obtaining and preparing the respiratory aspirate sample for bacterial culture and PCR testing.

Diagnosis of Ecto- and Endoparasites in Laboratory Rats and Mice

Internal and external parasites remain a significant concern in laboratory rodent facilities, and many research facilities harbor some parasitized animals. Before embarking on an examination of animals for parasites, two things should be considered. One: what use will be made of the information collected, and two: which test is the most appropriate. Knowing that animals are parasitized may be something that the facility accepts, but there is often a need to treat animals and then to determine the efficacy of treatment. Parasites may be detected in animals through various techniques, including samples taken from live or euthanized animals. Historically, the tests with the greatest diagnostic sensitivity required euthanasia of the animal, although PCR has allowed high-sensitivity testing for several types of parasite. This article demonstrates procedures for the detection of endo- and ectoparasites in mice and rats. The same procedures are applicable to other rodents, although the species of parasites found will differ.

Bacterial Infections of Laboratory Mice

Although epizootic bacterial diseases infrequently occur in immunocompetent laboratory mice, immunodeficient mice may suffer from such outbreaks and sporadic disease such as abscesses are still seen in all strains of mice. In both types of situation, the pathogenicity of the organism must be considered in the context of the genotype of the mouse. Effective prevention and control require an understanding of the ecology of the organism, including host specificity and whether or not an environmental reservoir may exist. This chapter discusses many of the contemporary bacterial infections of laboratory mice in the context of mouse genetic susceptibilities, the mouse microbiome and the ecology of the bacteria, and provides a critical review of the literature.

Parasitic Infections of Laboratory Mice

Internal and external parasites remain a problem in laboratory mouse colonies, despite the eradication of many other types of infections. The repeated presence of parasitic organisms in a mouse facility may be due to repeated introduction of parasitized animals, incomplete treatment of affected animals, uncharacterized aspects of a parasites life cycle or failure of environmental decontamination. Many rare parasites may be found in association with laboratory mice, but this chapter addresses only those parasites that are found quite regularly. The characteristics, diagnosis and treatment of these commonly encountered mouse parasites are discussed.