Bubonic plague

Medical quality assurance by Dr. Albrecht Nonnenmacher, MD at January 28, 2016
StartDiseasesBubonic plague

Bubonic plague is one of three types of infections caused by the Yersinia pestis bacteria. Plague symptoms develop within a week of exposure and include flu-like symptoms that develop into swollen, painful lymph nodes and gangrene of the extremities.

Once responsible for killing more than half of the European population in the 14th century, bubonic plague can now be treated effectively with antibiotics. The disease causes death in 10-11% of cases.


Definition & Facts

Bubonic plague is a bacterial infection of the lymphatic system caused by the Yersinia pestis bacteria. Spread by infected fleas, bubonic plague is one of three types of infections caused by Yersinia pestis along with pneumonic plague and septicemic plague. Bubonic plague was the cause of the Black Death in Europe, Asia, and Africa that killed about 50 million people in the 14th century, or up to 60% of the population of Europe.

Today, bubonic plague infection can be treated effectively with antibiotics and there are less than 1,000 documented cases per year which result in less than 150 deaths. While rare in the United States, bubonic plague can be acquired through contact with infected mice, rats, squirrels, and chipmunks.

Symptoms & Complaints

It can be difficult to distinguish bubonic plague from more benign problems during the early stage of the infection when it is most treatable. Symptoms of bubonic plague usually develop within 2 to 6 days of exposure. Bubonic plague first presents with a high fever, malaise, chills, muscle cramps, and pain in the lymph nodes.

The classic symptom of bubonic plague is infected, painful, and enlarged lymph nodes also called buboes. After a human is infected with Y. pestis, the bacteria localizes in inflamed lymph nodes so it can colonize and spread. Lymph nodes usually associated with plague are those in the armpits, groin, neck, and upper femoral area. These buboes will swell to about the size of a chicken egg and they are often warm and tender to the touch.

Some people also develop acral necrosis of the extremities like the nose, toes, and lips. Some people also develop laborious breathing, vomiting blood, sore limbs, coughing blood, coughing in general, extreme pain as tissue decomposes from gangrene, gastrointestinal problems, delirium, and coma.


Bubonic plague is caused by a bite from an infected rat flea that has fed on infected rodents like mice, rats, squirrels, or chipmunks. Rarely, bubonic plague can be transmitted by contact with infected tissue from a dead animal or infected human. The rat flea is a parasite that infects rodents and seeks out other prey. While the Y. pestis bacteria is not harmful to the flea, it aggregates in the flea's gut and causes the flea to regurgitate infected blood into the bite of a rodent or human.

In the United States, there are usually about one dozen or fewer cases of plague each year and infections usually occur in rural and semi-rural areas in the Western U.S., especially Colorado, Arizona, Mexico, and Northern California. The plague bacteria is usually present from early spring to early fall, but it is naturally found throughout the year. Many people who catch bubonic plague are hikers or campers who come in contact with rodents or are bitten by fleas while sleeping in a tent.

Diagnostics & Tests

Anyone who suspects they have been infected with Y. pestis should seek medical attention immediately. A doctor may suspect bubonic plague if there is a known flea bite, swollen lymph nodes, or the patient was recently in the rural Western U.S. If a doctor suspects bubonic plague, a fluid sample will be taken from the swollen buboes with a needle.

A blood test will be used to test for septicemic plague in which the Y. pestis bacteria entered the bloodstream. A fluid sample from the airways is used to check for pneumonic plague, which is the most serious form of plague. Laboratory testing is necessary to confirm the presence of bubonic plague, usually by identifying a Y. pestis culture from one of these samples. A rapid diagnostic test may be used to quickly screen for the Y. pestis antigen. Once plague is identified as a possible cause of symptoms, treatment should begin right away.

Treatment & Therapy

Antibiotics are the most effective treatment against a bacterial infection of Y. pestis. There are a few classes of antibiotics used to treat bubonic plague such as aminoglycosides like streptomycin and gentamicin, tetracyclines like doxycycline, and fluoroquinolones such as ciprofloxacin. Prior to the widespread use of antibiotics, the mortality rate of bubonic plague in the U.S. was 66% to 90%. The overall mortality rate is now about 11%.

People who are believed to have been infected with plague by a flea bite should seek immediate treatment as antibiotic treatment within 24 hours of the first symptoms can dramatically reduce the mortality rate. Supplemental treatments such as IV fluids, respiratory support, and oxygen may also be prescribed. When left untreated, bubonic plague can spread to the bloodstream and cause septicemic plague or to the lungs where it can cause pneumonic plague. Septicemic and pneumonic plague both have a mortality rate of 100% if untreated, but the mortality rate drops to about 40% if treated.

Prevention & Prophylaxis

People who live in areas in which the plague bacteria is found can guard against infection by wearing pants and using insect repellent while outdoors. Plague is often caught by people hiking or camping in the woods or rural areas. It's also important to protect pets from fleas. At home, remove food sources for rodents like garbage kept outside.

Ground and rock squirrels are the main carriers of the plague bacteria in the United States along with rats and even domestic cats. Exercise caution when participating in outdoor activities like camping and sleeping on the ground, working as a veterinarian, and coming into contact with sick animals, especially small rodents.