(A20.0) Bubonic plague is an infectious disease that is believed to have caused several epidemics or pandemics throughout history. The disease is caused by the bacterium Yersinia pestis. Bubonic plague is the most common form of plague, and is characterized by swollen, tender, inflamed lymph glands (called buboes).
(A20.7) Septicemic plague occurs when plague bacteria multiply in the blood.
(A20.2) Pneumonic plague occurs when the lungs are infected.

Infection/transportation

Bubonic plague is primarily a disease of rodents, particularly marmots (in which the most virulent strains of plague are primarily found), but also black rats, prairie dogs, chipmunks, squirrels and other similar large rodents. Human infection most often occurs when a person is bitten by a rat flea (Xenopsylla cheopsis) that has fed on an infected rodent. The bacillus multiplies in the stomach of the flea, blocking it. When the flea next bites a mammal, blood consumed by the flea is regurgitated along with the bacillus into the bloodstream of the bitten animal. Any serious outbreak of plague is started by other disease outbreaks in the rodent population. During these outbreaks, infected fleas that have lost their normal hosts seek other sources of blood.

In 1894, bacteriologists Alexandre Yersin and Shibasaburo Kitasato independently isolated the responsible bacterium and Yersin further determined that rodents were the likely common mode of transmission. The disease is caused by the bacterium Yersinia pestis.

Symptoms and treatment

The disease becomes evident 2–7 days after infection. Initial symptoms are chills, fever, headaches, and the formation of buboes. The buboes are formed by the infection of the lymph nodes, which swell and become prominent. If unchecked, the bacteria infects the bloodstream (septicemic plague), and can progress to the lungs (pneumonic plague).

In septicemic plague there is bleeding into the skin and other organs, which creates black patches on the skin. Untreated septicemic plague is universally fatal, but early treatment with antibiotics (usually streptomycin or gentamicin) is effective, reducing the mortality rate to around 15% (USA 1980s). People who die from this form of plague often die on the same day symptoms first appear.

With pneumonic plague infecting lungs comes the possibility of person-to-person transmission through respiratory droplets. The incubation period for pneumonic plague is usually between two and four days, but can be as little as a few hours. The initial symptoms of headache, weakness, and coughing with hemoptysis are indistinguishable from other respiratory illnesses. Without diagnosis and treatment, the infection can be fatal in one to six days; mortality in untreated cases may be as high as 95%. The disease can be effectively treated with antibiotics.

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Side Lights - history of the bubonic plague in Europe between 1347 and 1351
From Current Events, 1/29/99

* One of the worst plagues in history was the Black Death. Between 1347 and 1351, this form of bubonic plague killed an estimated 75 million people--about one-third of Europe's entire population at the time, devastating regions from Italy to Norway.

* The plague was brought from the Middle East in ships docked at Italian ports. The bacterium that caused (and still causes) the disease was carried by fleas that lived on infected rats. Once ashore, the fleas spread to other rats and to humans. The disease spread rapidly. According to one account, victims died a horrible death: "Those infected felt a pain throughout their whole bodies ... then developed in their thighs and upper arms a boil ... This infected the body so far that the patient vomited blood. This vomiting continued for three days, then the patient died. Men and women carried their dead children on their shoulders and threw them into a common pit."

* People at the time knew nothing about viruses or bacteria. Many physicians wore plague attire that consisted of a gown and a mask in the shape of a bird's head. (Drawing above). Some historians believe the bird mask stemmed from a desire to scare away disease-carrying spirits.

* Doctors tried to cure the Black Death by lancing the boils. This, however, actually helped spread the disease by releasing germs into the environment. The Black Death left towns empty and farmers' fields untended. In once-prosperous villages, wolves wandered corpse-strewn streets. It took Europe more than 100 years to fully recover.

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