Microbiological Assessment of Laboratory Rats and Mice

Participants in the September 1997 Workshop on Opportunistic Infections of Rats/Mice, held in Washington, DC, attempted to deal with the issue of defining "opportunistic" agents. Although they were unable to reach consensus regarding universally applicable guidelines to usefully distinguish between "primary pathogens," "opportunists," and "commensals," they did agree that the issue hinges largely on host constitutional factors of relative resistance and susceptibility to given microbial forms. Impaired immunocompetency was first recognized in the 1950s and 1960s as a physiological consequence of animals undergoing protocols in radiation biology and pharmaceutical corticosteroid and antimetabolite development. Latent, usually clinically silent microbial forms that include Pneumocystis carinii, Corynebacterium kutscheri, Pseudomonas aeruginosa, and Clostridium piliforme became recognized as concomitants of such research (Weisbroth 1995) and were important in prompting development of health surveillance programs. Since then, in addition to chemical modes of impairment of normal rodents, numerous inherited models of immunodeficiency have been developed and used commonly. Examples include athymic (nude) mice and rats and mice with severe combined immunodeficiency (SCID)Confounding the issue is the proliferating use of mouse and rat inbred strains and transgenics with bioengineered genomes that may have impaired or fragile immunocompetency as a deliberate (or accidental) inherited character. Since the early 1980s, a substantial literature base has accumulated to document disease episodes in immunodeficient rodents with infectious agents that are usually clinically silent in their immunocompetent counterparts. Workshop participants, for example, considered both "primary pathogen" and "opportunistic" as a definition for