Plague Surveillance -- 1999-2005

The word "plague" may conjure up images of the Black Death, the epidemic that scourged Europe in the Middle Ages. While plague in the United States has rarely reached such epidemic proportions, it has maintained a constant and continued presence since its introduction in the 1920's through California ports. Since then clusters of human cases numbering about a dozen usually occur every year primarily in the Southwest.

Identifying high rates of plague in wild animal populations can be a risk factor for predicting disease in humans and domestic animals such as cats. Plague is also a potential agent of bioterrorism; therefore it is important to know background levels of plague in wild animals to assist in determining whether an outbreak is a possible bioterrorism event.

The Vector-Borne Disease Prevention Program (VBDPP) in cooperation with United States Department of Agriculture (USDA)-Wildlife Services conducts annual surveillance of plague activity statewide. This effort utilizes blood samples from coyotes and other carnivores as indicators for plague.

Coyotes (Canis latrans) and many other carnivores routinely consume rodents as a part of their diet. In nature the bacteria that cause plague, Yersinia pestis, cycles between several rodent species and their fleas. When coyotes capture and eat these rodents they can become infected with plague. While the coyote will not develop a serious disease, an antibody response is triggered that is detectable through blood tests. By sampling coyote blood, one can obtain data that provides a picture of plague activity in a given area. This method is a particularly effective tool because each coyote has "sampled" hundreds of rodent prey and their fleas.

Wildlife Services staff collects specimens during the course of their regular duties. The Plague Branch of the Centers for Disease Control and Prevention in Ft. Collins, Colorado, performs laboratory testing on the specimens. The VBDPP staff use the information gathered for focused control efforts (flea control) and timely public health alerts to health care providers and local residents to prevent human cases of plague.

As illustrated in the accompanying map, there were significant pockets of plague activity in 2005 scattered throughout the State primarily in Washoe, Lincoln, and White Pine Counties. The table below lists those counties with positive results by year from 1999 through 2005: