Because of the steady increase in awareness of the varied and generally unwanted effects of natural pathogens, greater efforts have been made to exclude pathogens from research animals. Before 1950, original stocks of mice and rats used as research animals harbored a variety of natural, or indigenous, pathogens. Improvements in sanitation, nutrition, environmental control, and other aspects of animal husbandry led to a significant reduction in the range and prevalence of pathogens found in laboratory rodents. The period from 1950–1980 was a period of gnotobiotic derivation, when cesarean rederivation was used to replace infected stock with uninfected offspring. In this procedure, full-term fetuses are removed from an infected dam and transferred to a germ-free environment and foster care. This procedure was successful in eliminating pathogens not transmitted in utero. Since 1980, eradication of indigenous murine viruses has been undertaken. Reduction in infectious agents has been accomplished through serologic testing of animals for antibodies to specific pathogens and subsequent elimination or cesarean rederivation of antibody-positive colonies.
Most modern animal facilities incorporate some form of health monitoring into their animal care program (see Infectious Agents Commonly Tested for in Laboratory Rodents ). The well-being of the colony is more important than the well-being of an individual animal; laboratory animal medicine is effectively a type of herd medicine. While health monitoring is costly, it results in significant longterm savings in time, effort, and money. Through these programs, research veterinarians can monitor the health status of the colony, inform researchers of the pathogen status of their animals, prevent entry of most pathogens into the facility, and promptly detect and manage pathogens that do enter.