ILAR Journal Online, Volume 39(4) 1998: Opportunistic Infections in Laboratory Rats and Mice

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ILAR Journal V39(4) 1998
Opportunistic Infections in Laboratory Rats and Mice

Future Directions in Rodent Pathogen Control
David G. Baker

David G. Baker, D.V.M., M.S., Ph.D., Dipl. ACLAM, is Director and Associate Professor in the Division of Laboratory Animal Medicine, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana.

INTRODUCTION

The use of laboratory rodents is, and will continue to be, essential in biomedical research. However, like all living things, laboratory rodents have their associated "baggage" of other organisms, whether paying passengers, silent stowaways, or mutineers and pirates of the goods at hand. In other words, we should expect laboratory rodents to have characteristic health flaws. The field of laboratory animal science and medicine concerns, among many things, understanding and controlling the pathogens that are laboratory rodents' small but influential cohabitants. In this brief presentation, I review the history of rodent pathogen control and offer my expectation of the field in the future. Finally, I suggest a few practical ways in which laboratory animal scientists can further contribute to biomedical research and the betterment of society by facilitating greater understanding of the effects of microbes on laboratory rodents.

HISTORICAL PERSPECTIVE

The struggle against pathogens of laboratory rodents has been perceptively reviewed by Weisbroth (1996), who roughly divided the last 100 yr of research involving laboratory animals into 3 periods. During the first period of domestication (from 1880 to 1950), many rodent species were brought indoors to become much-used research subjects. Their transfer to an indoor environment, together with the many improvements made in laboratory animal care, resulted in a great reduction in the range and prevalence of pathogens, particularly those requiring intermediate hosts or other vectors and those generally associated with an outdoor environment.

The second period of gnotobiotic derivation (from 1960 to 1985; Weisbroth 1996) was marked by the development of organized laboratory animal science and medicine, resulting in part from a recognized need to address the continuing problem of laboratory animal diseases. During this period, cesarean derivation was developed and greatly facilitated the reduction and elimination of several pathogens. Advances of this period in animal husbandry and facility operation also contributed to this end.

The third period (from 1980 to the present) has been the period of eradication of the indigenous murine viruses (Weisbroth 1996). During this time, particular pathogens have disappeared or have been found less and less often. Examination of works published over the past 20 yr concerning common pathogens of laboratory animals confirms this assertion (Baker 1998; Carthew and others 1978; Lussier 1988). Reductions in pathogen number and burden have been accomplished primarily through serological testing of animals for antibodies to specific pathogens (most often viruses) and subsequent elimination or cesarean rederivation of antibody-positive colonies. Again, credit is also due to continued advances in animal husbandry methods and to increased knowledge and awareness in the discipline of laboratory animal science.

Presented another way, these 3 periods might be considered periods of the dead animal, the sick animal, and the antibody-positive animal. So how will the next period look? What is the ultimate goal? Would laboratory rodents, free of all other life forms, be ideal or even relevant? How much of what has been done will need to be repeated? Finally, what will be the role of health monitoring in the future? Let us briefly examine the information below in an attempt to answer these questions.

A NEW VIEW OF MICROBIAL INFECTION

Microbes found in or on laboratory (and other) animals have often been designated as pathogens, opportunists, or commensals, of which the last 2 have been most numerous. Such designations have certainly provided some reference for significance when finding specific microbes. However, such designations may not entirely suffice in the future. As the type of microbe found in laboratory rodents continues to shift from opportunist and/or pathogen to presumed commensal, it may be appropriate to reevaluate the relevance of traditional designations. The historical reliance on pathogenicity for microbial significance should be augmented with an emphasis on more subtle physiological effects, since such effects will continue to become more the real issue. In the future, rodents will show clinical signs of infection less commonly and will die of disease rarely, therefore our focus should shift to the next worst thing---covert disruption of research.

A recent review of the effects of microbial infections on laboratory mice and rats highlighted a large number of subtle, recently discovered effects of organisms considered in many cases to be of negligible pathogenicity (Baker 1998). I am aware of no case in which an organism ceases to be a true commensal, neither harming nor benefiting the host, and begins to exert some effect. Infection with mouse adenovirus-2 has long been assumed to be commensal. However, recent evidence suggests that it may affect local cytokine production (Einarsson and others 1996). One could ask whether cytokine modulation is truly compatible with a commensal state. Given the profound influence of cytokine profiles on direction of the immune system (Weckmann and Alcocer-Varela 1996), it would appear that cytokine modulation may not be completely benign and may indeed interfere with specific research goals. Likewise, infection of mice with Syphacia obvelata has recently been shown to interfere with an established murine model of autoimmunity, although it is unknown at what parasite burden such interference occurs (Agersborg and others 1998). Another difficult case is that of Staphylococcus aureus, a common opportunistic inhabitant of the skin, which can profoundly affect host physiology (Baker 1998). It is currently unknown at what point S. aureus load, or advantage of location, begins to exert some host effect, even in the initial absence of clinical disease.

A LOOK INTO THE FUTURE

Defining Our Expectation

Several events can be anticipated regarding pathogens of laboratory mice and rats. First, pathogens of historical significance will continue to appear, seemingly out of nowhere. Although such appearances may become less common than they currently are---continuing the recent trend---we should not assume that they will disappear altogether. Feral rodents will remain as reservoirs of infectious agents that will, on occasion, gain access to laboratory colonies by the entry of contagious animals and/or their bodily discharges into the animal facility. Although we may be on the asymptotic portion of a theoretical graph of "outbreaks" versus "time," the curve may never intersect the X axis in conventional rodent colonies.

Second, additional effects of currently known organisms will be reported as new research uses are found for traditional laboratory animals, new questions are asked, and new technologies are applied to those questions. As implied above, the field of cytokine biology provides an example of how new questions and technologies reveal previously unknown microbial effects. Because cells have a finite number of response mechanisms, microbial infection will surely trigger some generic responses. Examination of microbial effects on common cellular and subcellular mechanisms will likely yield valuable information on both the mechanisms themselves and the microbes eliciting them.

Third, microbes will continue to change as new pathogens are discovered and reported. Weisbroth (1996) has suggested using the term "post-indigenous" to describe a cluster of emerging conditions that share several common features such as recent recognition, poor description, a clinically mild or inapparent state, and (in some cases) human (rather than rodent) origin. Although the term post-indigenous may give the impression that many of these microbes were not previously associated with laboratory rodents, this conclusion is not necessarily accurate. Of course, one must wonder why microbes were not recognized sooner. The reason may never be known but may partially relate to the potential role of humans as reservoirs of some recently reported microbes (Weisbroth 1996). Human sources should be considered since neither the concept nor conditions of human-rodent microbe transmission are entirely new. Humans and laboratory rodents have been closely associated for several years. Many organisms, including some coliforms, S. aureus, and others, are known to colonize both humans and rodents. It is possible that with the increased mobility of both human society and laboratory rodents, subtle shifts in the microbial populations colonizing humans have occurred, and those changes are being reflected in the microbial populations of laboratory rodents. This issue deserves attention and may prove to be a fertile area of investigation.

Identifying the Goal of Microbe Exclusion

Rather than dogmatically proclaiming that the goal of microbial exclusion should be the elimination of all microbes from rodent colonies, I assert that efforts should be directed toward defining research needs versus microbial status. It is unrealistic to think that all microbes can be excluded from animal colonies without resorting to extremely expensive barrier systems, which themselves are not fail-safe. Although such management may be necessary for specific research needs over specific periods, it is not and will not be universally necessary to conduct valid research, as described below.

We know that axenic animals are different from gnotobiotic or otherwise microbially inhabited animals (Heidt and Vossen 1992). The exclusion of all but axenic animals from research use is therefore neither prudent nor economically feasible. For most research purposes, "apparently healthy" animals will continue to be suitable. However, their value will be increased with better microbial definition.

Evaluating the Usefulness of Past Findings

Some researchers might feel concern over the validity of past work and fear the potential need to repeat large bodies of experiments due to the implications of new discoveries. In veterinary school, I was taught that less than half of what is published in the scientific literature is true. Despite the actual figure, many reasons underlie this teaching, including unaccounted microbial infection. We already know that later discoveries of the effects of microbes, for example Mycoplasma pulmonis, have invalidated some previously reported findings (NRC 1991).

The refinement of knowledge is a natural occurrence in science that can be expected to continue and should be encouraged. New questions, technologies, and approaches are continually challenging the status quo. We should be willing to apply these methods to the question of microbial effects and not fear what we will find, such as the necessity to "repeat everything." It should be obvious by the benefits produced by biomedical science that much of what has been published utilizing laboratory rodents has been useful and need not be repeated. However, we should encourage consideration of microbial status in all experimentation, and especially in studies examining cellular or subcellular mechanisms.

Considering the Role of Health Monitoring Programs

As fewer microorganisms cause clinical disease, the importance of relevant health monitoring programs remains. As more is known of the effects of microbes on host physiology, it may be necessary to take a new approach to the makeup of commercially available health surveillance panels. Rather than offering a menu based on pathogenicity, diagnostic panels could be designed with more relevant goals in mind. For example, panels could be directed toward "health clearance," for entry of new animals into a facility; and toward "research effects," for existing animals in clean conventional facilities. Results from the latter panels might provide laboratory animal medicine professionals valuable information on the microbial populations within the colony.

Given the large number of available microbe-specific tests, it may be difficult to establish appropriate and cost-effective panels. Past efforts have been successful in eliminating many rodent viruses, mainly because serology facilitated elimination. In the future, standard methods of detection, including bacterial culture, histology, and serology, will remain useful. In addition, newer methods such as polymerase chain reaction may be used increasingly to identify newer and/or "commensal" pathogens that may not elicit an antibody response. Whether commensal microbes elicit detectable antibody responses has not been examined. In addition, comprehensive health monitoring systems of the future will continue to include environmental surveillance and may also incorporate newer diagnostic methods, such as the recent use of polymerase chain reaction to monitor air intake filters (Henderson and others 1998).

Encouraging Maximal Awareness of Microbial Effects

As anyone who has tried to keep up with new information can attest, science is moving at a rapid pace. As laboratory animal professionals, we have a responsibility to educate the research community on the known and potential effects of microorganisms on research. Although the veracity of the experimental system is the primary responsibility of the investigator, we can contribute substantially.

To facilitate our educational efforts, laboratory animal professionals should promote the development of a data base of information related to microbial effects on host physiology. The information should be updated regularly and made electronically accessible. Such data could be organized in a manner similar to that on, for example, laboratory animal housing, husbandry, and welfare, which is currently compiled by the Animal Welfare Information Center (WWW.NAL.USDA.GOV/AWIC/). In support of this goal, federal funding agencies should continue to support research related to the effects on host physiology of both the established as well as the newly discovered microorganisms. Emphasis should be placed on delineating the effects on cellular and subcellular mechanisms, such as cell signaling pathways and gene regulation. Finally, federal funding should be made available to explore the potential role of humans as reservoirs of microbial infections.

To increase the value of publications involving laboratory animals, investigators should be encouraged to describe the health monitoring program used by their institution and to demonstrate the results of its use. This information should be included in the Materials and Methods section of research reports.

CONCLUSION

The extent to which meaningful information is generated through biomedical research will depend in large part on our understanding and control of the microbes of laboratory rodents. Future research surely will reveal new microorganisms and additional effects of established microorganisms on their rodent hosts. To maximally serve the biomedical research community, laboratory animal professionals should take a proactive role in understanding, documenting, and publicizing the effects of these agents. Such efforts surely will be intellectually rewarding and of great benefit to the biomedical research community and, by extension, to society.

REFERENCES

Agersborg S, Garza KM, Baker DG, Tung SK. 1998. Syphacia obvelata (pinworm) infection: A potent environmental modifier in the neonatal induction of autoimmune disease and pathogenic Th2 memory response. Keystone (Colorado) Symposium, January 26-February 1, 1998.

Baker DG. 1998. Natural pathogens of laboratory mice, rats, and rabbits and their effects on research. Clin Micro Rev 11:231-266.

Carthew P, Sparrow S, Verstraete AP. 1978. Incidence of natural virus infections of laboratory animals. Lab Anim 12:245-246.

Einarsson O, Geba GP, Zhu Z, Landry M, Elias JA. 1996. Interleukin-11: Stimulation in vivo and in vitro by respiratory viruses and induction of airways hyperresponsiveness. J Clin Invest 97:915-924.

Heidt PJ, Vossen JM. 1992. Experimental and clinical gnotobiotics: Influence of the microflora on graft-versus-host disease after allogeneic bone marrow transplantation. J Med 23:161-173.

Henderson KS, White WJ, Cail SP, Perkins CL. 1998. Environmental monitoring for the presence of rodent parvoviruses on barrier room air intake filters via the polymerase chain reaction (PCR). Contemp Top 37:88.

Lussier G. 1988. Potential detrimental effects of rodent viral infections on long-term experiments. Vet Res Commun 12:199-217.

NRC [National Research Council]. 1991. Infectious Diseases of Mice and Rats. Washington DC: National Academy Press.

Weckmann AL, Alcocer-Varela J. 1996. Cytokine inhibitors in autoimmune disease. Semin Arthritis Rheum 26:539-557.

Weisbroth SH. 1996. Post-indigenous disease: Changing concepts of disease in laboratory rodents. Lab Anim 25:25-33.