The Basics
The plague is a disease caused by the bacterium Yersinia pestis. Means by which the disease is transmitted between vertebrates include flea bites, ingestion or inhalation of the bacteria, or direct transmission into the blood through an open wound.

The transmission is cyclic and two primary types have been associated with Y. pestis. One cycle is called the ‘urban cycle’ in which plague is transmitted among rodents (e.g. rats), their fleas, and people. A second cycle, called ‘sylvatic’ circulates within wild rodents and their fleas but the disease can also ‘amplify’ within a host species, spreading the bacterium over a relatively wide area, often resulting in high mortality rates in an ‘epizootic’ or outbreak of disease in wild animals. Humans and other species not usually associated with the plague are then placed at greater risk for contracting the disease.

The plague infection manifests itself in three ways: bubonic, septicemic, and pneumonic. While all three can be found in humans and non-human animals, they are usually most easily identified in humans.

Bubonic plague is characterized by ‘buboes’ (hence the name). Buboes are swollen lymph nodes resulting from the infection and inflammation caused by Y. pestis. They are usually found in the groin, armpit, or neck area. In addition to the swollen lymph nodes, infected people may experience flu-like symptoms such as fever, headache, vomiting, nausea, chills, and exhaustion.

Septicemic plague occurs when Y. pestis enters the bloodstream. This is considered a primary infection when the bacterium enters the bloodstream directly, or secondary when the infection is a consequence of an earlier lymph node invasion. While symptoms of septicemic plague are similar to bubonic plague, they may become more severe, and lead to shock, internal bleeding, and even mortality.

Pneumonic plague is a severe, life-threatening respiratory and systemic illness. Primary pneumonic plague is acquired through inhalation. Secondary pneumonic plague is a consequence of untreated bubonic or septicemic plague. Symptoms of pneumonic plague include high fever, chills, cough with bloody sputum, and breathing difficulties. Symptoms usually occur within 2-3 days of exposure.

Treatment and Prevention
NPS personnel who work in plague endemic areas (see map) should become aware of the symptoms of plague. They should notify their health care provider if they believe they have been exposed to the plague and become ill.

A person with suspected plague will undergo laboratory tests, for example, blood culture for plague bacteria, microscopic examination of lymph glands, blood, and sputum samples. Plague can be effectively treated with antibiotics if diagnosed early.

Prophylactic antibiotics also may be given when exposure is suspected. Prophylactic antibiotics—effective for 2-3 weeks—can be provided to people who will be in areas where plague exposure may take place. Currently, a plague vaccine is not available in the U.S.

Ecological Impact
While rodents and lagamorphs are the species most likely infected by plague, carnivores and ungulates are also susceptible. In the U.S., ground squirrels, prairie dogs, and wood rats are most commonly associated with epizootics. During an epizootic, 90% or more of a susceptible population can succumb to plague. In areas where susceptible species (e.g. prairie dogs) serve as keystone species (????) , large-scale die-offs may have a significant effect on ecosystem form and function. Extirpation of black-footed ferrets from most of their historic range was very likely due to plague. In addition, other top predators (e.g. wild felids) are susceptible to plague; the larger-scale effects of such mortality are under investigation.  As a result, management of plague is necessary to protect native wildlife species, ecosystem function, and human health.

NPS Management
Wildlife health and public health can be jeopardized by outbreaks of this non-native disease. Although management to protect humans from plague has historically been conducted in NPS units, actions to protect wildlife have been less common. Currently, the most common method of managing plague in and near parks is to apply insecticide powder (deltamethrin) to prairie dog colonies, and to decrease flea populations and hence plague transmission. (NOTE: Use of pesticides at NPS units must be approved through the NPS Integrated Pest Management Pesticide Use Proposal System.) Currently, wild black-footed ferrets and ferrets released as part of reintroduction efforts are vaccinated against plague when handled. Other experimental methods of plague control include the use of oral flea control products fed to prairie dogs, (e.g. imidacloprid) and an oral vaccine for use in prairie dogs.