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, a disease that was inadvertently introduced from Asia into the North American prairie ecosystem around 1900. Today sylvatic plague has been identified in at least 76 species of mammals in the United States including bobcats, antelopes, domestic cats, rock squirrels, ground squirrels, wood rats, chipmunks, prairie dogs, deer mice, voles, tree squirrels, and rabbits. Plague has spread from the point of introduction in California in 1900 to include most states west of the Mississippi River today, according to the World Health Organization.

Below is the World Health Organization’s map showing where plague is found in the world today.

In the United States, most of the human plague cases occur in two regions:

• Northern New Mexico, northern Arizona, and southern Colorado.

• California, southern Oregon, and far western Nevada.

The distribution of the number of cases of human plague for these areas have changed in the last ten years as shown in the chart below, where N = the number of reported cases of human plague.

Plague is transmitted from animal to animal and from animal to human by the bites of infective fleas. Less frequently, the organism enters through a break in the skin by direct contact with tissue or body fluids of a plague-infected animal, for instance, in the process of skinning a rabbit or other infected animal. Also, plague can be transmitted by inhaling infected droplets expelled by coughing, from a person or animal. This is especially true of domestic cats, with pneumonic plague. Transmission of plague from person to person is uncommon and has not been observed in the United States since 1924 but does occur as an important factor in plague epidemics in some developing countries.

Human plague cases in the U.S. have been sporadic cases acquired from wild rodents or their fleas. Rock squirrels and their fleas are the most frequent sources of human infection in the southwestern states. For the Pacific states, the California ground squirrel and its fleas are the most common source. Many other rodent species, for instance, prairie dogs, wood rats, chipmunks, and other ground squirrels and their fleas, suffer plague outbreaks and some of these occasionally serve as sources of human infection. Deer mice and voles are thought to maintain the disease in animal populations but are less important as sources of human infection. Other less frequent sources of infection include wild rabbits, wild carnivores, and even antelopes, which pick up their infections from wild rodent when certain environmental conditions are such that plague reaches an epizootic scale (affecting many animals in any region at the same time). Domestic cats are readily infected by fleas or from eating infected wild rodents. Cats may serve as a source of infection to persons exposed to them. Pets may also bring plague-infected fleas into the home. Between outbreaks, plague is normally enzootic (present in an animal community but occurring in only a small number of cases), within populations of certain species of rodents without causing excessive mortality. Such groups of infected animals serve as silent, long-term reservoirs of infection.

The onset of plague is usually 2 to 6 days after a person is exposed. Initial symptoms include fever, headache, and general illness, followed by the development of painful, swollen regional lymph nodes. This type of plague is known as bubonic plague and is the most common form. People usually get it from a flea bite. The disease progresses rapidly and the bacteria can invade the bloodstream, producing severe illness. Plague in the bloodstream is known as septicemic plague. People could also get septicemic plague by getting the bacteria in through an open cut on the skin while skinning an infected animal. Once a human is infected, a progressive illness generally results unless specific antibiotic therapy is given. Progression leads to blood infection and, finally, to lung infection. The infection of the lungs is termed as pneumonic plague and is the least common, but most dangerous, form of plague. In this form plague can be easily passed from one person to another through coughing and sneezing. Cats can get this form and pass it to people, also. If pneumonic plague is the primary way you contacted the disease the incubation period is 1 to 3 days and is characterized by development of an overwhelming pneumonia with high fever, cough, bloody sputum, and chills. For plague pneumonia patients, the death rate is over 50%.

Plague will probably continue to exist in its many localized geographic areas in the southwest since attempts to eliminate wild rodent plague are impractical and futile. Therefore, primary preventive measures are directed toward reducing the threat of infection in humans in high-risk areas through four techniques:

• Environmental management
• Public health education
• Preventive drug therapy
• Vaccines (very rarely)

Effective environmental management of plague reduces the risk of persons being bitten by infectious fleas of rodents and other animals. Preventing plague outbreaks requires sanitation measures to reduce rat and other rodent populations in places where people live, work, and play. It is important to reduce food sources used by rodents and to make homes, buildings, warehouses, or feed sheds rodent-proof. Eliminate shelter for rodents around homes, work places, and recreation areas by removing brush, rock piles, and junk piles from the site.

Public health education should include the following preventive recommendations:

• Watch for plague activity in rodent populations where plague is known to occur.
• Eliminate sources of food and nesting places for rodents around homes, work places, and recreation areas. Make your home and outbuildings rodent-proof.
• If you anticipate being exposed to rodent fleas, apply insect repellents to clothing and skin, according to label instructions, to prevent flea bites. Wear gloves when handling potentially infected animals.
• If you live in areas where rodent plague occurs, treat pet dogs and cats for flea control regularly and do not allow these animals to roam freely.
• Health authorities may use appropriate chemicals to kill fleas at selected sites during animal plague outbreaks.

Medical experts advise that antibiotics be given for a brief period to people who have been exposed to the bites of rodent fleas during a plague outbreak or have handled an animal infected with the plague bacterium. Such experts also recommend that antibiotics be given if a person has been exposed to another person or to an animal (for example, a house cat) with suspected plague pneumonia.

Plague vaccine has very limited use. The following persons should consider vaccination:

• Scientists who work with the plague bacterium in the laboratory.
• People in plague-infected areas who handle or have close contact with potentially infected animals as part of their routine work (such as rodent biologists).

If you hunt, fish, camp, or otherwise visit an area where there is plague you should:

• Use insect repellent. Follow the directions on the container carefully.
• Protect your pets with a safe flea control product or leave your pets at home.
• Avoid all contact with chipmunks, prairie dogs, squirrels, mice, rats, or other wild animals. DO NOT FEED THEM!
• Do not camp, rest, or sleep near animal burrows. AVOID ANIMAL FLEAS.
• Wear gloves if you are hunting and must handle wildlife.
• Do NOT touch sick or dead animals.
• If you see a large number of dead or dying animals, call the Colorado Department of Health.
• Call a doctor if you get sick within a week of visiting an area with plague. If you have plague, a doctor can treat you for the disease.