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3/23/99

PLAGUE AND BLACK-TAILED PRAIRIE DOGS

 

BASIC FACTS

Plague is an infectious disease caused by the bacterium, Yersinia pestis. All forms of plague in wild animals are generally referred to as sylvatic plague. Seventy-six species of mammals carry plague, but it is primarily a disease of wild rodents other than black-tailed prairie dogs. In cases where people have contracted the disease, it is usually referred to as bubonic plague. When people contract the disease, it is usually from coming in contact with an infected rodent (such as a rat, a squirrel, or a prairie dog) or their fleas.

Before the advent of modern medicine, bubonic plague struck the human population in epidemic proportions every few centuries. Today, improved sanitation practices and modern insecticides and antibiotics have reduced the threat of plague epidemics in developed countries like the United States.

PLAGUE AND PEOPLE

Plague can also be transmitted between humans or animals by direct contact (coughing, sneezing, etc.). Dogs are mostly immune to the plague. Cats are extremely susceptible to plague, although the occurrence of plague in cats is very rare. Although extremely rare, cats can pass the disease directly to people. Awareness of the disease and avoidance of close contact with wild rodents and other potential carriers or their fleas is the key to protection.

Plague is curable in humans if diagnosed and treated in its early stages. At first, plague feels like a bad case of the flu; symptoms include chills and fever. The bacteria invade the lymph nodes (bubonic or lymphatic plague), which become swollen and tender; then the bloodstream (septicemic plague); and, sometimes the lungs (pneumonic plague). Pneumonic plague is much more virulent than the initial, bubonic form, because the disease can be spread by the coughing of the infected person. Without antibiotics, the mortality rate for pneumonic plague is nearly 100 percent.

Though most prairie dogs have fleas, few fleas are infected, and most public health officials believe the chance of humans contracting plague from prairie dog fleas is very low. Apparently, prairie dog fleas do not like human hosts, preferring instead to bite other animal species. The black-tailed prairie dog is unlikely to contribute to the spread of plague in the U. S. because plague kills nearly all infected prairie dogs within a very short period of time.

Since 1959, the Fort Collins, Colorado branch of the U.S. Centers for Disease Control and Prevention (CDC) has recorded 393 cases of plague in humans. Of the 240 cases (61 percent) for which a source of infection was identified, 31 (13 percent) were attributed to contact with prairie dogs or their fleas, from actually handling the animals. Only two (both in Colorado) of the 31 cases were in areas where the black-tailed prairie dog is the only species of prairie dog that could have been the source of infection

(Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Vector Borne Infectious Diseases, Bacterial Zoonosis Branch, Plague Section, unpublished data, 1998).

During the last five years (1994 through December 7, 1998) there were 40 human plague cases in the United States. Of the 17 cases for which a source of infection was identified, 9 were attributed to prairie dogs. Improved sanitation practices, such as rodent control in homes, garages, and other human dwellings; flea removal from pets; and avoidance of sick or dead animals, along with antibiotics, help prevent plague in people.

 

PLAGUE AND BLACK-TAILED PRAIRIE DOGS

The inadvertent introduction of sylvatic plague into the North American prairie ecosystem around 1899 has had a direct negative effect on the black-tailed prairie dog. This disease may be the most important factor in the recent reduction of black-tailed prairie dog populations across their range. In black-tailed prairie dogs, the plague was first documented in a colony near Lubbock, Texas, in 1946.

Plague has been active in black-tailed prairie dog populations in the northern Great Plains only within the last decade although it was present 40-50 years ago. The disease appears to be spreading to encompass the entire range of the species. Plague is not widespread throughout the west because of the prairie dog. The disease is maintained in other wildlife species and periodically devastates black-tailed prairie dog populations; recent population losses due to plague have been observed in Montana, Wyoming, Colorado, Kansas, and Oklahoma.

At present, sylvatic plague is widespread throughout the western United States, except in South Dakota. It is likely no coincidence that 4 of the largest 7 remaining black-tailed prairie dog complexes are in South Dakota or that approximately 32 percent of all remaining individuals of the species are located in this state. Black-tailed prairie dog populations are extinct in Arizona and are much reduced in the southwestern states (New Mexico, Colorado and Utah) near the epicenter for more recent sylvatic plague outbreaks.

A plague outbreak in a black-tailed prairie dog colony results in near 100 percent mortality. Black-tailed prairie dogs show neither effective antibodies nor immunity. If there are any survivors, they do not exhibit resistance to plague; surviving animals appear to have avoided death only by the remote chance of avoiding exposure.

Because most black-tailed prairie dog colonies are already fragmented and reduced in size, they are more vulnerable to further reduction or elimination due to the continued effects of plague, poisoning, and shooting. Colonies that have been poisoned or shot may recover more slowly from additional plague outbreaks. Available information suggests that colonies recovering from plague recover to a consistently smaller percentage of their original size, and plague will probably at sometime reoccur.

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