(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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Model explains bubonic plague's persistence - research speculates that the bubonic plague never truly goes away, but attacks humans only when the rat population
From Science News, 10/21/00 by J. Travis

After bubonic plague reached Europe in the 14th century, major cities such as Venice enacted strict quarantines intended to keep out infected, shipborne rats. The cities would remain free of the disease for a time, but every decade or so, the plague returned. People have presumed that infected rats had sneaked back into town.

Challenging that assumption, computer modeling of how bubonic plague spreads from rats to people now suggests that the disease never went away. It probably persisted in the rat populations living in the cities, say Matthew J. Keeling and Chris A. Gilligan, both of Cambridge University in England.

Beyond suggesting that bubonic plague can lurk within rat populations for years without causing human epidemics, Keeling and Gilligan's work published in the Oct. 19 NATURE comes to the counterintuitive conclusion that killing rats may sometimes exacerbate plague outbreaks among people.

"There's a lot here for public health officials to think about," says Ben Bolker of the University of Florida in Gainesville, who also models infectious diseases.

Before looking into the plague for this new work, says Keeling, he felt the disease was of only historical interest. Yet bubonic plague has made a comeback, he notes. In the 1990s, it struck Africa and India, and scientists found that the bacterium responsible has developed antibiotic-resistant strains.

Past efforts to model bubonic plague treated it as primarily a human disease, which is inappropriate, says Keeling. "It's a rodent disease that occasionally infects humans," he explains. The fleas that transmit the plague feed upon people only when the disease wipes out so many rats that the insects turn to another food source.

By showing human outbreaks about every decade, the computer analysis by Keeling and Gilligan mirrors historical records of plague epidemics in medieval Europe. At the same time, the model reveals that in its hypothetical city, bubonic plague never disappears completely from the rat population.

A key feature of the new model, says Keeling, is that it treats the rat population as a collection of many subpopulations having limited interactions with each other. Many scientists today consider such an approach more realistic than modeling a single, large rodent population.

In their model, Keeling and Gilligan found that bubonic plague can persist in subpopulations of rats in which many of the animals are immune to the disease. In subpopulations in which almost all the rats are vulnerable, an epidemic can wipe out all the animals and then spread to people.

The new model suggests that the best way to prevent bubonic plague in people is to stop its spread in rats, a strategy that public health officials have begun to embrace, says Keeling. But once human cases appear, it's too late to simply kill rats, the model indicates.

"If you ever got an outbreak in a large city in the developed world, there would probably be a strong public opinion wanting to kill the rodents, which isn't an effective strategy," says Keeling. "This will just release many infected fleas."

That could worsen a human epidemic. So, Keeling suggests that insecticides are a more appropriate response once bubonic plague appears in people.

Bolker suggests that the investigators could make their model more realistic with better data on the ecology of rats. Keeling agrees but adds that there's very little relevant data available.

Keeling and Gilligan note that today rat populations infected with bubonic plague still exist in rural regions of the United States. The researchers hope to extend their model to determine the circumstances under which the disease could spread to rats in U.S. cities.

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