In July 1968 a case of plague occurred in a child in Denver, Colorado. A dead fox squirrel (Sciurus niger) was found near the home of the patient and Yersinia (Pasteurella) pestis was detected in its tissues. This prompted a survey of these squirrels, and other animals, that defined an epizootic of plague in the fox squirrels. Fluorescent antibody (FA), passive hemagglutination, and bacteriologic techniques were used. Of 768 animals examined, 81 (all S. niger) were positive for plague by one or more methods. The FA test was most useful because it produced reliable and rapid results. This is the first documented epizootic of plague in urban-dwelling tree squirrels, a species not previously recognized to be naturally involved in the epidemiology of plague.
Zoonoses Section, Ecological Investigations Program, P. O. Box 551, Fort Collins, Colorado 80521.
Environmental Health Service, Department of Health and Hospitals, City and County of Denver, Colorado 80202.
Division of Laboratories, State Department of Health, Denver, Colorado 80202.