Rodent Models

MHC and T, B and NK Cell Functions - Both B and T cells are able to recognize antigens. B cells are responsible for humoral, or serum, immunity by producing immunoglobulins, or Igs. These Igs are divided into five chief classes-IgG, IgM, IgA, IgD, and IgE-each with special properties. T cells, making up about 70 percent of all lymphocytes, are responsible for cellular immunity, meaning they attack and kill antigens directly. T cells do not themselves make antibodies but they help regulate the production of antibodies by the B cells. There are four types of T lymphocytes-helper, cytotoxic, memory, and suppressor.

Differentiation of B and T cells into a vast variety of cloned cell types, each responding to a specific antigen, involves two phases-the primary or antigen-independent phase and the secondary or antigen-dependent phase. During the primary phase, stem cells proceed through stages of differentiation to generate vast amounts of B or T cell clones, each with unique antigen receptors. The immune system generates an incredibly diverse range of gene sequences, or antigen-binding specificities for antibodies.

The secondary (antigen-dependent) phase involves only B cells, which can recognize an infinite number of antigens (but each individual B cell recognizes only one antigen). When a particular antigen binds to the antigen receptors on the appropriate B cell, that B cell is triggered to proliferate into a large clone of cells, all responsive to the specific antigen. In this clonal selection process, some of the cloned B cells become long-lived memory cells and others differentiate into plasma cells secreting antibodies.

Another type of immune cell was discovered in 1975 -- the natural killer (NK) cell, which looks like a lymphocyte but contains granules resembling granulocytes. NK cells apparently recognize some feature of the target cells, either directly or via receptors that attach to the tails of antibodies on the target cell's surface. As a result, NK cells act by releasing the contents of their granules to kill the target cells or by recruiting the help of other immune cells.
Back to Top

Immunodeficiency Mutations in Mice - More that 4,000 genes of the mouse have now been assigned to specific chromosomal locations. Many genes were first identified following spontaneous mutations that produced distinctive physical characteristics. Single-gene mouse mutants have provided highly useful experimental research models. Two key single-gene, naturally occurring mutations are the nude (nu) and the severe combined immunodeficiency (SCID); both are important research models for the study of xenografts, transplanted tissues and tumors from foreign species. Other single-gene mutations commonly used as research models include the beige (bg) and the X-linked immune deficiency (xid).

The nu mutation was first reported in 1966 in a closed stock of mice in a laboratory in Glasgow, Scotland. It was not until 1968, however, that it was discovered that the homozygous nude mouse also lacked a functional thymus, i.e., it was athymic. The mutation produces a hairless state, generating the name "nude." The other, unique defect of nude mice is the failure of the thymus to develop normally to maturity. The thymus remains rudimentary and produces reduced numbers of mature T cells. This means nude homozygotes (animals with identical mutant genes at corresponding chromosome loci) do not reject allografts and often do not reject xenografts (tissue from another species). The discovery that human neoplasms (tumors) could be grown in nude mice was immediately recognized as an important research tool. Thus, the spontaneous mutation of nu among laboratory mice was a serendipitous development that led to the nude mouse becoming the first animal model of a severe immunodeficiency. In the decades since, the nude mouse has been widely utilized by researchers studying factors regulating transplantable human tumor growth and cancer metastasis.

The first successful transplantation of a human malignant tumor to nude mice was reported in 1969. Nude mice have been used extensively in studies of the tumorigenicity of in vitro cultured cells. Nude mice are also widely utilized in evaluating anticancer agents prior to human clinical trials.

SCID Mice: Accident of Nature - Serendipity also played a role in the discovery of another important mutant strain of immunodeficient mice, which lacks both B and T cells, called severe combined immunodeficiency (SCID) mice. During routine lab tests on the immune system in mice, Dr. Melvin J. Bosma of the Fox Chase Cancer Center in Philadelphia discovered the strain in 1980. The first SCID mice were an accident of nature, the product of chance matings of apparently normal mice that carried a recessive mutant gene now called SCID. Some of the offspring inherited a complete pair of SCID genes and were born with the SCID defect.

"We were conducting antibody studies when we found that some of these mice lacked antibody," recalled Dr. Bosma. "The disease seemed to affect cellular immunity, too. These animals had tiny lymph nodes and the thymus was about one-tenth normal size. It took about 3 years to determine and demonstrate that these mice had a severe immune deficiency disease similar to that called SCID that afflicts some human children."

Dr. Bosma's laboratory bred these mice with each other to produce the original SCID colony. At first, the SCID mouse attracted interest because it was the first known animal model for human SCID, a congenital syndrome that is usually fatal to human babies. The SCID mouse is also an excellent model for studying the relationship between immune defects and cancers of the lymph system. The Fox Chase researchers found that histologic abnormalities in SCID were remarkably uniform, because they all share the same underlying genetic defect. Dr. Bosma and his colleagues also noted that, like nude mice, the normal immune function of SCID mice could be genetically reconstituted by "seeding" with lymphocytes from bone marrow of normal mice. But because the SCID model lacks both B and T cells, it presents much greater potential for studies of selective reconstitution of immune cell populations.

The action of the SCID mutation in blocking lymphocyte development is not absolute, however. As they mature, some adult SCID mice generate a few clones of functional B and T cells. These SCID mice are said to be "leaky," meaning that low levels of B and T cells are detectable. "By 10 to 14 months of age, virtually all [C.B-17] SCID mice are leaky," says Dr. Bosma. "Those with detectable B cells also invariably contain T cells. This implies that the development and growth in numbers of B cells in SCID mice may be totally dependent on T cells and-perhaps-vice versa." However, other researchers now report that another strain of SCID mice appears to be virtually devoid of the leaky phenotype, the ICR SCID mouse.
Back to Top

Beige and Xid Mutations - Two other single-gene immunodeficient mouse models have less severely compromised immune systems than the nude and SCID models. These are models with the beige (bg) and the X-linked immunodeficiency (xid) mutations. Mice with the X-linked recessive mutation have been widely used in studies of B cell development and maturation. Homozygous xid females and xid males do not respond to thymus-independent antigens and also fail to respond to specific thymus-dependent antigens. No defects in T cell functions, such as graft rejection, are noted in xid mice.

The beige model, named for its hair color, has been used extensively for studies of selective NK cell deficiency. A defect in the NK-cell function in bg/bg mice blocks the normal process of degranulation, leading to impaired antibody-dependent and antibody-independent cytolysis of tumor cells. Mice with the bg mutation show greater-than-normal susceptibility to infection by pyrogenic (fever-producing) bacteria. Beige mice have been used in studies of hematopoietic cell differentiation.
Back to Top

Multiple-Gene Immunodeficiencies -Researchers have been able to intercross various immunodeficient mice models to selectively investigate the effects of combined mutations. For example, mice that are bred to be simultaneously homozygous for both xid and nu mutations are severely deficient in both B cells and T cells. But thymus grafts in these doubly homozygous mice promote B cell development, indicating that the maturation of xid-mediated B cells past the early developmental stages is T cell-dependent.

The ability to selectively breed mice models combining various immune deficiency mutations is invaluable to researchers. A prominent example is the triple-deficient (bg-nu-xid) model. By controlling the inheritance of the three mutations, researchers can devise experimental systems designed to bear the effects resulting from any single mutation or in various combinations of the mutations in the same experiment.

Importance of Genetic Background - When working with a specific immunological mutation in rodents, it is important to select the appropriate genetic background (stock or strain of mouse) upon which it should be maintained. Experienced researchers say the ideal choice for mutant maintenance is an inbred strain. Once the mutant gene is established on a selected inbred strain through several generations of backcrossing (generally 10 or more), the resulting offspring are considered a congenic strain-a strain that is genetically identical to its confrere inbred strain except for the mutant gene (and, perhaps, closely linked genes). By comparing the immunodeficient congenic strain with the partner-inbred strain, investigators can study the functions demonstrated by the gene in question.

The Swiss nude immunodeficient strain is a general-purpose outbred model that is economical and easy to maintain. The C57BL/6 nude is also a general-purpose strain suitable for a wide range of studies requiring an immunodeficient research animal. The investigator can select an inbred model, in which all animals are genetically identical, or an outbred model, which has animals representing a diverse gene pool. Outbred models, such as the Swiss nude, are more economical to produce because Swiss females have good nurturing instincts and abilities, thus producing larger litters with more robust pups.

The double-mutant C.B-17-scid-beige model is deficient in B, T and NK cells, making it valuable for cancer research because one has removed another layer of immunity-the [NK] population of cells that kill tumors. Another immunodeficient model, the athymic (nude) rat has the same (or very similar) nu mutation as the nude mouse, but because of the rat's larger size it is a better research model for investigations requiring extensive surgery.
Back to Top

SCID-hu Mouse Chimera Has Functioning Human Immune Cells - Soon after biomedical researchers began working with immunodeficient mice models in the 1960s, some scientists wondered whether these mice-which readily accepted human cancer tumor transplants-could someday accept transplants from the human immune system to replace their own crippled immune systems. Such a mouse model of the human immune system would be a momentous step forward in using laboratory animals as realistic living models for investigations of human diseases, including AIDS.

In September of 1988 two groups of researchers almost simultaneously announced that they had succeeded in transplanting elements of the human immune system into SCID mice. They had used totally different approaches in creating their human-mouse chimeras.

At Stanford Institutional, Dr. J. Mike McCune and Dr. Irving Weissman implanted 300 SCID mice with tissue taken from human fetal thymus. In some of the animals they also transplanted human fetal tissue from the liver and lymph nodes, along with the thymus tissue, into the kidneys of the SCID mice. In fetal development of mammals, stem cells that will eventually become part of bone marrow are initially produced in the liver. Progenitor T cells originate in the thymus and then enter the lymph nodes as mature and functioning T cells.

The implanted human fetal tissues soon produced mature human T cells in the Stanford SCID mice. The mice that received fetal lymph tissue also developed mature human B cells. The chimera, named the SCID-hu, immediately proved useful in studies of drug toxicity and efficacy that would not be appropriate in human subjects because of uncertainties about safety in early stages of testing. Dr. McCune showed in 1989 that the AIDS drug AZT suppressed HIV replication in SCID-hu mice. His study, requiring only a few weeks in mice, produced results similar to others that took 5 years of clinical trials with human AIDS patients.

Another team of researchers, at the Medical Biology Institute in La Jolla, Calif., reported they also used SCID mice as hosts of human immune transplants. But the La Jolla researchers, headed by Dr. Donald Mosier, used a simpler approach. They injected human peripheral blood leukocytes (PBLs) into the mice. Human PBLs contain B and T cells. Almost immediately after injection, the mice began replicating the human B and T cells.

This chimera, called the hu-PBL-SCID, also was able to produce human tetanus antibodies when injected with tetanus toxin, further demonstrating that its immune system was functioning as though it was naturally human.

This human-mouse chimera, the SCID-hu, dramatically demonstrates sciences' ability to create mouse models that are more precise analogs of the human condition. It offers great potential for a broad range of research with human applications, including the study of HIV and other viral infections, immune system development, congenital diseases and other diseases.
Back to Top

Opportunities in Transgenic Technology - Interest in immunodeficient animals has mushroomed in recent years with advances in molecular biology and genetics that allow researchers to manipulate the genome of mice to eliminate or add genes and even replace selected genes. Science can actually engineer laboratory mice to meet specific research needs and protocols. Creating new, genetically engineered animal research models involves two transgenic techniques-microinjection of cloned genes randomly inserted into the host DNA and gene targeting or homologous recombination between cloned DNA and one of the identical copies of the sequence normally present in the chromosome. The more complete names of these two transgenic techniques are:

classical pronuclear microinjection: introduction of foreign DNA into embryonic pronuclei resulting in random integration and expression and
embryonic stem (ES) cell-mediated gene targeting: introduction of genetically modified ES cells into recipient embryos resulting in the ablation (knockout) or modification of a specific genetic expression.

Transgenic animals are designed to exhibit either a gain of function (expression of a novel cell-surface receptor) or a loss of function (knockout of a cellular function). Classical pronuclear microinjection techniques have been used for 15 years to create mouse models, which express unique phenotypes. The major flaw in the pronuclear microinjection models has been the random nature of transgene integration locus and copy number. Expression patterns may vary significantly in a series of lines expressing identical transgenes. Modifiers of expression such as age, sex and health status further confound the process, increasing potential for variability.

By using ES cell gene knockout technology, an investigator can produce an animal model in which expression (or the lack of expression) is highly predictable. A clone of cultured ES cells is selected in which a specific DNA sequence in the mouse genome has been modified (usually inactivated). Transformation of cultured cells with foreign DNA is relatively simple and most commonly is achieved using a procedure called electroporation. All transgenic models, whether targeted or untargeted, still may present unpredictable expression patterns due to incomplete knockout of the targeted gene, redundancy within the genome or unanticipated genetic interactions, such as down-regulation of other genes.

Despite some unpredictability questions, transgenic knockout technology can produce research animals that are "custom designed" to meet the specific needs of an investigator's experimental protocol. Knockout technology, or homologous recombination, is also a valuable tool for determining functions of specific genes. Dr. Mario R. Capecchi of the Institutional of Utah School of Medicine, one of the pioneers of gene targeting, explains the concept of targeted gene replacement: If we suspected a particular gene were involved in brain development, we could generate mouse embryos in which the normal gene was "knocked out"-that is, completely inactivated. If this inactivation caused newborn mice to have a malformed cerebellum, we would know that the gene in question was essential to forming that part of the brain. Back to Top

Transgenic Animals Are `Extraordinary' - Transgenic technologies, which are rapidly expanding, provide potential advantages over naturally occurring mutations in developing animals specifically tailored to mimic elements of the human system or human diseases. The knockout technique has already produced mouse models of many human diseases, including retinoblastoma, p53 antioncogene deficiency, Gaucher's disease and others.

Knockout mice may display the most severe types of human inherited disorders because they lack expression from the corresponding genes of the mouse. Some knockout models are particularly fascinating because they live with inactivated genes that formerly were believed to be essential for survival. The expanding ability to produce double (and multiple) gene knockout animal models further enhances investigators' abilities to elucidate the functions of specific combinations of genes and to more accurately model features of the human immune system and human diseases.In only about 10 years, genetically engineered mice have brought dramatic and exciting advances to biomedical research. "These mice are, quite simply, extraordinary," says Dr. Joseph Perch, vice president for grants and special programs at the Howard Hughes Medical Institute in Chevy Chase, MD.

Due to rapid advances in genetic engineering and increasing use of the technology by laboratories, it is evident that these extraordinary models will lead to further significant developments in knowledge of the immune system's broad array of components and their complex functions. Obviously, transgenic animal research models will contribute to important advances in a wide range of biomedical research.

Social and Ethical Concerns - Safety - The advent of biotechnology, more so than any other technological innovation, has made it clear to scientists that they can no longer dismiss ethical concernsin society as irrelevant to the scientific enterprise, on the grounds that, by its very nature, science is and ought to be "value-free" or "value-neutral". More pragmatically, it is evident that society is extremely exercised about the genetic engineering of animals, and that a failure on the part of scientists to enter into the dialogue concerning the ethical issues raised by transgenics is likely to leave the development of principles governing the field to those who shout the loudest.

In the first place, it is necessary to separate out genuine moral issues from spurious ones, which nonetheless command and galvanize a great deal of public attention. For example, religious groups complain that creation of transgenic animals violates divine law, or fails to show proper respect for life. Other critics argue that humans were not "meant" to create life. Still others see species as inviolable, or assert that humans should not alter "nature". Such objections have stirred public skepticism and fear regarding biotechnology. When scientists fail to exhaustively discuss - and deal with - the genuine moral issues growing out of genetic engineering of animals, one is inexorably led to a version of Gresham's Law, wherein, instead of bad money driving good money out of circulation, bad (or spurious) ethical and social concerns drive genuine and legitimate ethical and social concerns out of the social mind. Thus it is imperative that researchers planning to develop or use transgenic animals be fully cognizant of, and prepared to address, the genuine social and moral questions raised by such activities, lest the sensationalistic, baseless concerns inform and shape the public response to biotechnology.

The first genuine bioethical issue relevant to the creation and use of transgenic animals in biomedical research concerns the possible danger to humans and/or other animals, which might be presented by the animals in question.

Perhaps the most dramatic real case of such a concern is exemplified by the mouse model for AIDS created at the National Institute of Allergy and Infectious Disease of the National Institutes of Health. HIV- 1, the pathogen causing human AIDS, naturally infects only humans and chimpanzees, and chimpanzees remain asymptomatic. In an attempt to create an "animal model" for the disease, researchers introduced the HIV genome into mouse embryos by microinjection and propagated these mice by breeding. Although the purpose of the model was primarily to study viral latency, F1 progeny resulting from the mating of one of the founder animals, its nontransgenic mates developed symptoms similar in some respects to human AIDS. Furthermore, the tissues of these animals produced infectious HIV particles.

Obviously, the creation of mice capable of harboring infectious HIV virus represents a genuine and legitimate social concern of both a moral and prudential nature regarding biosafety. The moral question is, of course, whether one ought to produce such a potentially dangerous organism. The prudential question is, given the decision that the benefit outweighs the risk, and therefore that such an animal should be created (leaving aside separate moral questions regarding animal suffering), how does one reduce the risk to an acceptable minimum?

These sorts of concerns are legitimate and understandable and encapsulate questions which any researcher planning to develop or utilize transgenic organisms should carefully address. Indeed, potential dangers from a transgenic organism may be far more subtle than just transmission of the disease it is designed to host. For example, if one were genetically engineering for resistance to a given pathogen in an animal, one could conceivably be selecting new variants among the natural mutations of that microbe to which the modified animal would not be resistant. One possible example of this sort of reaction has recently been discussed above regarding the SCID-hu mouse developed as a model for AIDS. These animals are genetically engineered to possess a human immune system, and are then infected with the AIDS virus. Some researchers suggest that, in such a mouse, the AIDS virus could become more virulent and infectious through new routes of transmission in virtue of interacting with native mouse viruses. This could conceivably wreak havoc both with animals and humans. Genetically engineered animals could conceivably damage extant ecosystems if such animals are not confined. Thus, it would seem morally incumbent both to do careful cost-benefit analysis before creating any transgenic animal, and also to build in an additional safety mechanism by demanding significant balance of benefit over cost to compensate, at least in some measure, for unknown or unrecognized dangers. Michael Crichton's recent novel, Jurassic Park, provides an ingenious, scientifically based, albeit fanciful account of how small, unanticipated problems in genetic engineering can amplify into major catastrophes along lines describable by the mathematical theory of chaos.

Thus the first ethical requirement relevant to creating transgenic animals is making a case that benefits clearly outweigh risks, with significant allowance made for unforeseeable risk. If this demand is met, the next ethical requirement is the demand that one control for the remaining risks.

Fortunately, the research community has been extremely sensitive to the theoretical dimen-sions of laboratory biosafety. Researchers utilizing transgenic animals for disease-related study should familiarize themselves with the principles encoded in the CDC-NIH publications Guidelines for Research Involving Recombinant DNA Molecules and Biosafety in Microbiological and Biomedical Laboratories. These volumes describe four increasingly stringent levels of biological containment (Biosafety levels 1 to 4).

In the case of the AIDS mice mentioned above, the microinjected embryos were inserted into surrogate female mice, which were transferred to a stainless steel glove box within a BL4 facility. All transgenic animals were maintained in the glove box closed system for the duration of the experiment. Indeed, one can characterize the containment procedures for this experiment as BL4+, for, in addition to standard BL4 procedures, the containment boxes were surrounded by bleach-filled moats and traps to provide "overkill" assurances that the danger was contained.
Back to Top

RAT - USES IN RESEARCH - Rats have been regarded as vile, insidious vermin, damned for their role in the Black Plague of the Middle Ages, and for the injury and destruction they have caused to humans and property throughout the ages. However, one species of rat, Rattus norvegicus, has been used in research since the mid 1800s, in what Lindsey described as an ascendancy from the gutter to a place of nobility through its contributions to human health and well being. In an ever-evolving process since the first crude uses of rats in research, investigators have sought, through trial and error, to develop appropriate husbandry, care, and use techniques to mini-mize or eliminate the impact of variables such as nutrition and disease on research results. Early ignorance of basic needs translated into inappropriate or substandard care, which affected the health and well being of the animals first and foremost, but also affected the quality and reliability of the research data that was generated. Advances were made in understanding the requirements for and provision of adequate nutrition, in recognition and control/elimination of latent infectious diseases, and in providing sanitary environments to minimize contamination of and disease in the rats.

The rat is the second most commonly used animal species in biomedical research and testing. Rats comprise 21% of all animals used, a somewhat deceptively low figure, but when coupled with mice, these two species account for 88% of all animals used in research and testing. Rats possess a number of characteristics which make them ideal animal models: they are readily available from many commercial and private sources, they possess genetic unifor-mity, they are inexpensive to purchase and maintain, they are easy to handle, they are adaptable to novel situations and environments, they have well-defined physiologic param-eters, they have known microflora, some have spontaneous diseases useful in modeling, and their short life-span affords an opportunity to study long-term effects of experimental treat-ments on health and well-being.

The American College of Laboratory Animal Medicine published a review of and refer-ences for several research uses of the rat in studies involving gnotobiology, dental research, embryology and teratology, toxicology, oncology, gerontology, cardiovascular research, immunology, immunogenetics, and infectious disease research. Andrews et al. edited a two-volume set on spontaneous animal models of human disease which includes additional information and references pertaining to uses of rats.
Back to Top

HAMSTERS AND GERBILS - There are more than 15 species of hamsters, but the one used most frequently in biomedical research is the Syrian (golden) hamster, Mesocricetus auratus. Commercially available golden hamsters were derived from only three or four littermates collected in Syria in 1930. The Mongolian gerbil, Meriones unguiculatus, has been used in research in Europe since the mid to late 1800s. Dr. Victor Schwentker is credited for introducing the Mongolian gerbil to the U.S. in 1954.

Both hamsters and gerbils are hardy creatures, and do not exhibit the wide spectrum of spontaneous overt and latent diseases common to rats and mice. Their good general health, their susceptibility to induced disease conditions, the low cost of production and maintenance, and literature available on the biology and physiology of these species make them useful animal models.

HAMSTER - USES IN RESEARCH - Hamsters account for 0.6% (approximately 500,000 used per year) of the total number of animals used in research annually. The American College of Laboratory Animal Medicine published a review of and references for several of the research uses of the hamster in studies involving antibiotic-associated colitis, behavioral and neuroscience research, dental research (caries and periodontal disease), endocrine research, genetics, hibernation and cold adapta-tion, immunology, infectious disease research, oncology (cheek pouch immunoprivileged transplant site, natural and induced tumors, viral oncogenesis), radiobiology (radioresistance), reproductive physiology, gerontology, tissue culture preparation, teratology, and toxicology.

GERBIL - USES IN RESEARCH Approximately 70,000 to 80,000 gerbils are used annually in research. Gerbils are used in studies involving aerospace medicine, aging, anatomy (gross, microscopic, ultrastructural), auditory research, behavior, cancer research (immunogenetics, transplantation, oncogenesis), dental research (caries, periodontal disease), endocrinology, genetics, hematology, infectious disease research, metabolism, neurology (spontaneous and induced seizures), nutrition, pharmacology/toxicology, radiobiology (radioresistance), reproduction, and stroke research. Additional areas of research involving gerbils have included investigations of experimental atherosclerosis and temperature regula-tion. Gerbils are readily susceptible to infection with Giardia duodenalis, making them a good bioassay model for evaluating or studying the infection in humans or in environmental monitoring. Gerbils of all ages are susceptible to the human infection, whereas in mice only suckling and weanling age animals are minimally susceptible.
Back to Top

GUINEA PIGS Guinea pigs are significantly less utilized for research than the murine rodents, mice and rats, and so it is somewhat of a mystery why the expression "guinea pig" came to mean "research animal subject". Historically, guinea pigs were first widely used in coat color genetics. Of course, being dependent on vitamin C in the diet, they are ideal subjects for investigations of this nutrient. Guinea pigs are not useful for studies that require any but terminal blood samples nor intravenous administration of substances, as they do not have easily accessible peripheral veins. Guinea pigs are a very good source of serum complement, and have been maintained by testing laboratories for this substance.

Guinea pigs are subject to anaphylactic shock and death from bronchospasm due to histamine release. However, they are widely used for skin-testing procedures; a test substance can be applied to the back and the animal wrapped to avoid scratching or biting of the test area. Of course, these animals must be individually caged to prevent cagemates from tearing bandages. Guinea pigs are used in otological research, as the hearing range and structures (except for size, of course) of the ear are similar to those of humans. Hormonal effects during pregnancy are similar to those of humans, and they may therefore model some aspects of human pregnancy.
Back to Top

WILD RODENTS Traditionally, animals used in laboratory research are mice, rats, guinea pigs, hamsters, gerbils, rabbits, dogs, cats, pigs, and certain primate species; however, there are over 41,700 species of vertebrates. The potential for any of these species to be used as research subjects increases as our knowledge of physiological and ecological processes expands.

Examples of the use of nontraditional laboratory animals in biomedical research are common. Woodchucks (Marmota monax) are used as models to study obesity, energy balance, hepatitis, and hepatocellular carcinomas and the opossum (Didelphis virginiana) as a model for endocarditis.

One species used increasingly in toxicological and epidemiological research, as well as in ecological, behavioral, and genetic studies, is the deer mouse (Peromyscus maniculatus). Wild deer mice are among the most abundant small mammals in North America. They range from Alaska to central Mexico, from Newfoundland to Virginia, and from Atlantic to Pacific oceans. Deer mice are absent only in the southeastern-most states, where they are replaced by related species. More than 60 formally described subspecies of deer mice occupy a wide variety of forest, prairie, and desert habitats from sea level to elevations of 14,000 ft. or more. In nature, deer mice nest in logs and stumps or in shallow burrows beneath rocks and clumps of vegetation. They subsist on seeds supplemented with occasional crickets or other insects. Owls, snakes, foxes, weasels, and other small carnivores find these rodents a staple food source, and thus they play an important role in natural ecosystems.

Deer mice were first bred in captivity about 1916 by Francis Sumner at the Scripps Institution at La Jolla, California. For the next half-century, a handful of investigators continued to collect and establish breeding stocks of these animals, which were used primarily to study genetic structure of small mammal populations.

In 1962, the deer mouse colony at the Institutional of South Carolina was established. Over the next 3 decades, more than 30 distinct "wild-type" and mutant genetic stocks were acquired, forming the nucleus of the Peromyscus Genetic Stock Center (PGSC). The function of the PGSC is to provide the scientific community with genetically defined animals and to improve the deer mouse as a model for biological research. The deer mouse has distinct advantages for certain kinds of studies. Many of the advantages stem from the fact that this native species can be used as a laboratory standard for contrasting wild counterparts. For example, deer mice can be utilized to monitor environmental pollution using exposed wild animals compared with laboratory-bred controls. A representative study is that of Schauber and others (1997), in which effects of insecticide ingestion were assayed in deer mice. Because deer mice are native, laboratory observations can be extrapolated to natural populations for ecological investigations. The existence of numerous deer mouse races and more than 50 closely related Peromyscus species, all in North America, also make the deer mouse a favorite subject for fine scale systematics studies in this country. Because many genetic variants in deer mice occur at loci homologous with those in the laboratory house mouse and rat, they are useful in parallel studies with more conventional laboratory species. Deer mice are appealing, alert creatures with large eyes and ears. The wild-type animal is sharply bi-colored with a rich brown-gray above and white below. Handled properly by experienced persons, even with bare hands, the majority bites only rarely.

Additionally, they are slightly smaller than most laboratory mice, and odor is negligible. Another major advantage is the ample genetic polymorphism in natural Peromyscus populations and various laboratory stocks that can be screened for specific variants of interest. For example, an outbred stock of deer mice was the source of the alcohol dehydrogenase "null" variant extensively employed in ethanol metabolism research.

Two major disadvantages of deer mice as laboratory animals are the unavailability of highly inbred, genetically homogeneous strains and difficulty in handling due to their quickness and jumping ability. Both of these problems are being approached through controlled breeding.

The most active research areas utilizing deer mice and related species are ecology and epidemiology. Peromyscus have been implicated in 2 human diseases of current interest: (1) Deer mice (P. maniculatus) are carriers of the pathogen producing the recent hantaviral pulmonary syndrome (Four Comers disease) outbreak in the southwestern United States. (2) The deer mouse and the congeneric white-footed mouse are known hosts for the larval stage of the tick (Ixodes), which transmits the Lyme disease spirochete (Borrelia). These species are extensively employed in laboratory studies of the conditions described.

Peromyscus are also used in aging research, since they are often long lived compared with house mice (Mus). However, less than 25% of research activity with deer mice and their allies is for strictly "biomedical" research, in which more traditional laboratory animals are usually preferred. Deer mice and other Peromyscus have long been considered ideal for evolutionary research at the morphological, biochemical, cytogenetic, and molecular levels. Deer mice are a frequent species of choice for studies of biological rhythms, and neurochemistry.

Field and laboratory research on a wide variety of species is conducted to learn more about the species and its biology and ecology. Many studies are conducted in the natural environ-ment where they are found, and others bring animals into the laboratory for study. Scientific societies in North America have developed guidelines for conducting research on the species of their concern. Field methods for mammals have been published by the American Society of Mammalogists. The Wildlife Society includes field research guidelines in its Research and Management Techniques for Wildlife and Habitats. The Canadian Council on Animal Care and the Universities Federation for Animal Welfare have published manuals that include guidelines for a wide range of species. The U.S. government has adopted "U.S. Government Principles for the Utilization and Care of Vertebrate Animals Used in Testing, Research, and Training".
Back to Top

Animal Model & Biomedical Problem	Specific Disease
Mouse Species & Genetic and developmental defects	Anemia, hereditary Athymic Autosomal trisomies Chediak-Higashi syndrome Copper malabsorption, X-linked Exencephaly Hereditary asplenia L call mutant Megacolon, aganglionic Megaloblastic anemia Polycystic kidney disease Testicular feminization
Mouse Species & Neoplastic disease	Adenocarcinoma, DES Adenoma, salivary Angiosarcoma, liver Carcinoma, cervix Carcinoma, embryonal Hodgkin's disease Leukemia, myelogenous Malignant tumor transplant Mammary tumor Ovarian tumor Preneoplastic lymphoid hyperplasia Teratoma and teratocarcinoma
Mouse Species & Metabolic/ nutritional disease	Amyloidosis Diabetes mellitus Gammopathies, monoclonal Gestational diabetes Globoid cell leukodystrophy Glucose-6-phosphate dehydrogenase deficiency Histidinemia Hyper- and hypotension Hypervitaminosis A Hypophosphatemia (rickets) Mast cell deficiency Methylmercury poisoning Niemann-Pick Disease Nonobese diabetic Ochratoxicosis Ornithine transcarbamylase deficiency Paraproteinemia, idiopathic Thalassemia, alpha
Mouse Species & Degenerative Disease	Adenosis, vagina/cervix Autoimmune disease Biliary obstruction Diverticulosis, oviduct Dysbaric osteonecrosis Proliferative glomerulonephritis Immunosuppression Macroglobulinemic neuropathy Menkes's disease Motor neuron disease Pulmonary fibrosis, bleomycin Pulmonary fibrosis, solvents/oxygen Reye's syndrome Salpingitis Vitiligo
Mouse Species & Infectious disease	Avian reovirus Capillaria hepatica Cytomegalovirus Encephalomyocarditis Epstein-Barr virus-dependent lymphoproliferative disease Filariasis Giardiasis Hepatitis, reovirus Helicobacteriosis Influenza B Listeriosis Lymphocytic choriomeningitis Meningoencephalitis, amebic Meningoencephalitis, Angiostrongylus Scrapie Theiler's encephalomyelitis Trypanosomiasis Yersinia infection
Rat Species & Genetic/ developmental defect	Amnionic fluid deficiency Fetal colon implants Fetal lung growth Hereditary hyperbilirubinemia Hydrocephalus Hydronephrosis Intrauterine growth retardation Megacolon, aganglionic
Rat Species & Neoplastic disease	Adenocarcinoma, colon Adenocarcinoma, intestine Adenocarcinoma, prostate Aflatoxin carcinogenesis Angiosarcoma, hepatic Carcinoma, bladder Carcinoma, esophagus Carcinoma, pancreas Carcinoma, kidney Carcinoma, squamous cell, lung Carcinoma yolk sac Interstitial cell tumor Lymphoblastic leukemia Malignant histiocytoma Medullary carcinoma, thyroid Neurogenic tumors, N-nitrosourea Osteosarcoma, Moloney sarcoma virus Pituitary tumors Urothelial tumors
Rat Species & Metabolic/ nutritional disease	Adrenal apoplexy Alcoholic fatty liver Anemia Cirrhosis Diabetes insipidus Diabetes mellitus Ethanol dependence Fructose-induced lesions Hepatic necrosis, halothane induced Hypervitaminosis A Hypothyroidism Lead encephalopathy Lipotrope deficiency Mucopolysaccharidosis Obesity Ochratoxicosis Osteopetrosis Phenylketonuria Skeletal muscle, defective glucose/glycogen Striatal lesions, kainic acid induced Urolithiasis Vasculitis, pulmonary, glucan induced
Rat Species & Degenerative disease	Aneurysm, cerebral Arthritis Autoimmune thyroiditis Duodenal ulcer Hypertension, induced Hypertension, spontaneous Hypertrophy, right ventricle Immunosuppression Ligation, cerebral artery Myocardial infarction Optic disc swelling Periodontitis Retinal degeneration Silica-induced pulmonary lipoproteinosis Thromboembolism Uterine vessel ligation
Rat Species & Infectious disease	Pneumocystis pneumonia Venezuelan equine encephalitis
Guinea Pig Species & Neoplastic disease	Transplantable leukemia
Guinea Pig Species & Metabolic/ nutritional disease	Hypervitaminosis A Hypovitaminosis C Mannosidosis Ulcerative colitis
Guinea Pig Species & Degenerative disease	Allergic optic neuritis Antitubular BM nephritis Inflammatory bowel disease Optic disc swelling
Guinea Pig Species & Infectious disease	Genital herpes Entamoeba histolytica Pichinde virus Tuberculosis
Hamster Species & Genetic/ developmental defect	Autoimmunity
Hamster Species & Neoplastic disease	Benzo(a)pyrene-induced tumors Carcinoma, larynx Cholangiocarcinoma Pancreatic tumors Spontaneous carcinoma, lung Tumors of respiratory tract
Hamster Species & Metabolic and nutritional disease	Hypervitaminosis A Diabetes mellitus
Hamster Species & Degenerative disease	Cardiomyopathy Thrombosis, atrial
Hamster Species & Infectious disease	Besnoitiosis, chronic Scrapie Syphilis Transmissible mink encephalopathy
Gerbil	Aural cholesteatoma Lead neuropathy Stroke
Squirrel	Porphyria
Woodchuck	Hepatocellular carcinoma Viral hepatitis
Opossum	Endocarditis