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Conservation > Fish & Wildlife Diseases Plague

For questions/concerns about this disease in humans, please call your doctor or the Montana Department of Public Health and Human Services  (DPHHS).

For questions about this disease/parasite in wildlife, please call the FWP Wildlife Health Lab at (406) 577-7882.

Cause

Plague is a bacterial disease caused by infection with Yersinia pestis. Yersinia pestis is the same bacteria that caused the “Black Death” in 14th century Europe, which claimed approximately 25 million lives.

Distribution

Plague is found worldwide. In the United States, the disease is primarily found in western states and is most common in rural or semi-rural areas. Although detailed data on the distribution of plague in wildlife in Montana is not available, the disease is believed to be present throughout the state. The most recent human case of plague in Montana was in 1992, when a hunter contracted the disease from a bobcat. In 1987, a hunter was infected after skinning an infected antelope.

A map showing human cases of plague in the United States from 1970-2019 can be found here: https://www.cdc.gov/plague/maps/index.html

Species affected

Plague mostly affects rodent populations such as chipmunks, ground squirrels and prairie dogs, but can also affect other mammals, such as carnivores and scavengers that feed on rodents. Plague represents a health and safety threat to humans, especially in places where humans and rodents interface (USDA APHIS website).

 

In addition to rodents, other mammalian species that have been naturally infected with Y. pestis include lagomorphs, felids, canids, mustelids, and some ungulates.  Domestic cats and dogs have been known to develop plague from oral mucous membrane exposure to infected rodent tissues, typically when they are allowed to roam and hunt in enzootic areas. Birds and other nonmammalian vertebrates appear to be resistant to plague (Merck Veterinary Manual).

Transmission

Animals and humans most often become infected with the Y. pestis following the bite(s) of one or more infective fleas (Adrian, 1981). Bacteria from the blood of an infected animal remain in the gastrointestinal tract of the flea and are transferred when the flea begins its blood meal from its next uninfected host. A predator or scavenger may become infected upon ingestion of tissues of infected animal. Sharp objects, such as bones, may puncture tissues of the mouth and throat thereby enabling entry of the bacteria. Humans may be infected while skinning or field dressing an infected animal. Rarely, the disease may be transmitted through inhalation of aerosolized bacteria (Miller et al, 2003)

Most cases in the U.S. occur during the warmer months of the year but cases can occur during any month of the year. Winter cases in the U.S. typically occur among hunters, trappers, and cat owners handling infected animals. Warm season cases are most often attributed to flea bite (Diseases, 2009).

Signs in Wildlife

  1. Observation of clinically affected wild mammals is unlikely. The discovery of dead animals is more common. Lesions vary according to the route of transmission and susceptibility of the host. Accordingly, symptoms of animals that have contracted plague will vary but may include swollen lymph nodes near the site of inoculation (flea bites, oral punctures), abscess formation near site of inoculation, muscle soreness, loss of appetite, fever, depression, necrosis of lymph tissue, or edema in the lungs. Death may occur rapidly before the appearance of clinical symptoms (Miller et al, 2003). At necropsy, wild animals found dead of acute plague may show lesions such as hyperemia (increased blood flow to tissues) and congestion of spleen, liver, lungs and other organs. However, gross macroscopic lesions are uncommon. Small, whitish gray nodules sometimes may be seen on the surface and on cross section of these organs in animals which survived plague infection for 5 or more days before death. The spleen in these animals is often grossly enlarged (3-4x) (Adrian, 1981).

Public Health Concerns

Plague is a potentially deadly zoonotic disease and precautions should be taken when handling an animal suspected of having plague. Wild rodents are the natural reservoir of plague, but any infected mammal may be a source of infection to humans. People can become infected by being bitten by an infected flea, by contacting tissues or fluids of infected animals, through bites and scratches, or by inhaling infectious airborne droplets (rare). There are precautions that can be taken to reduce the risk of exposure and infection. Avoid rodents if possible, and take measures such as removing attractants and habitat for rodents to keep them and their fleas away from areas shared by humans. Prevent flea bites by wearing long sleeves and using repellants while working or recreating outdoors, wear gloves when handling wildlife or when skinning or field dressing any wild animal, always wash hands and instruments with warm soapy water after handling any wildlife. Treatment of pets for fleas should also help to reduce transmission to humans and other wildlife (Miller et al, 2003).

Members of the cat family, Felidae, such as bobcats, mountain lions and domestic cats are particularly susceptible to plague while the Canidae (dog) family, including coyotes, wolves and domestic dogs, tend to be fairly resistant to plague. Because cats are susceptible to plague and can develop the highly-contagious form of the disease (pneumonic plague), they can represent a health threat to people who come in contact with them (ie, sportsmen, wildlife personnel & pet owners).

Symptoms of plague in humans depend on the route of transmission. Transmission by flea-bite or by direct contact with tissues or fluids from an infected animal typically lead to bubonic or septicemic plague (see section of signs of disease). Transmission by inhalation leads to pneumonic plague.

1. Bubonic plague is initially characterized by sudden onset of fever, headaches and chills, and by swollen, painful lymph nodes called buboes. Transmission by flea bite or by contact with an infected animal may cause bubonic plague in humans. If left untreated, bubonic plague can lead to more severe and life threatening forms of the disease.

2. Septicemic plague typically presents as fever, chills, weakness, abdominal pain, shock, and bleeding into the skin or other organs. Skin, often of fingers, toes, and nose may turn black and die, hence the term “Black Death”. If not treated early in the course of disease, bacteria in the blood may spread to lungs leading to the pneumonic form of disease. Transmission by flea bite or by contact with an infected animal may cause septicemic plague in humans.

3. Pneumonic plague is the most serious form of the disease. Patients suffer from fever, headaches, weakness, and rapidly develop symptoms of pneumonia which include chest pain, cough, and difficulty breathing. Pneumonic plague may result in respiratory failure and shock (CDC). Pneumonic plague is caused by inhalation of aerosolized droplets (mist) containing bacteria, or is the result of untreated bubonic or septicemic plague. This form of the disease is often fatal (Miller et al, 2003).

 

 Plague is a very serious and potentially fatal disease; however, the disease is treatable with commonly available antibiotics (CDC). The earlier in the course of disease that treatment is started, the greater the chance it will be successful. Bubonic plague is the most common and most treatable form of disease, but if it is left untreated, plague bacteria invade the bloodstream. When plague bacteria escape the person’s immune defenses and freely multiply in the bloodstream, they can spread rapidly throughout the body and cause a severe and often fatal condition called septicemic plague. Infection of the lungs with the plague bacterium causes the pneumonic form of plague, a severe respiratory illness (Diseases, 2009).

Plague has caused three human epidemics in recorded history. Today, wildlife act as reservoirs for the bacteria throughout the world in semi-arid areas on every continent except Australia and Antarctica. Human plague in the United States has occurred as mostly scattered cases in rural areas (an average of five to 15 persons each year). Although detailed data on the distribution of plague in wildlife in Montana is not available, the disease is believed to be present throughout the state. The most recent human case of plague in Montana was in 1992, when a hunter contracted the disease from a bobcat. In 1987, a hunter was infected after skinning an infected antelope. Globally, the World Health Organization reports 1,000 to 3,000 cases of plague every year. Most of these non-U.S. cases occur in impoverished rural environments that are heavily rat-infested. About 14% (1 in 7) of all plague cases in the United States are fatal. Most cases in the U.S. receive some antibiotic treatment during their course of illness and deaths typically result from delays in seeking treatment or misdiagnosis. Reportedly, about 50-60% of bubonic plague patients who fail to receive any antibiotic treatment die. Untreated septicemic or pneumonic plague is almost always fatal (Diseases, 2009).

 

 

Is it safe to eat the meat?

Human infection has been reported from contact with recently dead animals (e.g., dressing/skinning). Hunters should not harvest animals that appear sick and should wear gloves and use good hygiene when skinning or field dressing any wild animal. If you suspect an animal has been infected with plague, do not consume the meat and contact the nearest Wildlife or health authority. Plague is a potentially deadly zoonotic disease and precautions should be taken when an animal suspected of having plague is encountered (Miller et al, 2003). If you harvest an animal and are unsure whether it is safe to eat, contact your local FWP staff for guidance soon after the animal is harvested.

Citations

Adrian, W.J., editor. 1981. Manual of Common Wildlife Diseases in Colorado. Denver: Colorado Division of Wildlife.

Diseases From Wildlife. Center for Disease Control. November 4, 2009. Website

Miller, M.J.R., R.D. Dawson, and H. Schwantje. 2003. Manual of Common Diseases and Parasites of Wildlife in Northern British Columbia. University of Northern British Columbia.