Opportunistic Infections in Research Rodents: The Challenges Are Great and the Hour is Late.

The series of papers in this issue reflects the present state of knowledge on opportunistic infections of laboratory rodents and provides speculation into where the field is headed. What has not been covered is the phenomenal growth in knowledge of murine infectious diseases over the last 50 yr. Remarkably, more than 60 genera of overtly or opportunistically pathogenic viruses, bacteria, fungi, protozoa, helminths, and arthropods are known to infect laboratory mice (Percy and Barthold 1993). Wild mice and rats are host to an even longer list of agents that can potentially reenter domestic rodent colonies. It is equally remarkable that although laboratory rodent populations are often free of these agents, many of these pathogens are still around. Coronaviruses (mouse hepatitis viruses in the mouse and sialodacryoadenitis viruses in the rat) are regularly monitored agents, but their sheer contagiousness and mutability allow them continued and repeated access to rodent colonies. Pinworms, whose ova are resistant to desiccation and tend to drift in the air, are often the first pathogens to gain access to well-maintained barriers and the last to be eliminated (NRC 1991). Even rarely encountered agents continue to reappear in rodent populations. Polyoma virus of mice has been readily eliminated by simple husbandry practices for years (Barthold 1985), but it remains a virus of scientific interest that has been iatrogenically introduced into mouse colonies and has produced disease in immunodeficient mice (Sebesteny and others 1980). Ectromelia virus in mice has recently been incriminated as a contaminant of commercially available mouse serum, resulting in an outbreak of disease (Dick and others 1996). Freezers, transplantable tumors and biological products of murine origin continue to harbor murine pathogens. Entry of feral or wild rodents into laboratory rodent colonies is a constant threat. A case in point was infection of a "pathogen-free" mouse colony with lymphocytic choriomeningitis virus resulting from entry of wild mice into the animal room (Skinner and others 1977).