What is plague?

Causes and Symptoms

Plague is caused by infection with a bacterium called Yersinia pestis (formerly Pasteurella pestis).
Yersinia
is a gram-negative, bipolar-staining bacillus that predominantly infects rodents, with humans being accidental hosts for three species, including the plague-causing Yersinia pestis. The disease is transmitted by the bite of a flea that has become infected after a blood meal from another animal with the bacterium in its bloodstream or by the ingestion of contaminated animal tissues.

There are three cycles that perpetuate plague in animals and humans. Sylvatic or wild plague is maintained in the wild rodent population, such as ground squirrels, rock squirrels, and prairie dogs. Urban rat plague occurs in developing countries, especially seaports, and is maintained by the infection of urban and domestic rats, which during epidemics may be as high as 10 percent of the rat population. Finally, there is pneumonic plague, a form of the disease limited to humans, in which transmission occurs directly between hosts via infected aerosol droplets from a person with a lung infection.

The most common plague illness in humans is called bubonic plague. After an incubation period of two to eight days following the bite of an infected flea, there is a sudden onset of fever, chills, weakness, and headache. Within hours, extremely tender oval swellings one to ten centimeters in length appear in one anatomic area of lymph nodes, usually the groin, axilla (armpit), or neck. These buboes presumably result when the bacteria inoculated into the skin by the infected flea migrate to the regional lymph nodes. Many of these patients will have bacteria intermittently present in the bloodstream during the acute stage of the illness.

Less common, but more severe, forms of the illness include septicemic and pneumonic plague. Septicemic plague occurs when the inoculated bacteria proliferate rapidly in the blood, overwhelming the patient before producing a bubo. Pneumonic plague may occur as a secondary pneumonia in bubonic plague, when the lung becomes infected by bacteria carried in the bloodstream, or as a primary pneumonia, through direct inhalation following exposure to a coughing plague patient. Septicemic and pneumonic plague are often fatal, especially if antibiotic therapy is delayed. Rarely, plague can be manifested as a pharyngitis resembling acute tonsillitis or meningitis.

Treatment and Therapy

A diagnosis of plague is suspected in febrile patients who have been exposed to rodents in areas of the world known to harbor the disease. The causative bacteria are usually identified by microscopic examination and culture of material obtained from aspiration of a bubo. Blood, sputum, throat swabs, or spinal fluid can be processed in a similar manner.

Quarantine, rat control, and insecticides to kill fleas have successfully controlled urban plague in many cities around the globe. Sylvatic plague has been more difficult to control because of the range and diversity of the world rodent reservoirs. A vaccine is available for selected high-risk individuals. Untreated plague has a mortality of 30–60 percent, but this high risk of death can be reduced by the early institution of antibiotic treatment with either streptomycin or tetracycline and the use of modern medical supportive care.

Perspective and Prospects

Plague has been recorded in human history dating back to antiquity. The first known pandemic of plague is thought to have began in Egypt or Ethiopia in 540 CE. and continued for sixty years, killing about one hundred million people. The second pandemic, called the Black Death, began in the fourteenth century in central Asia and then spread to Europe, where a quarter of the total population died. The third pandemic began in China during the 1890s and spread worldwide along trade routes when infected rats were inadvertently transported on ships to ports in the Americas, Asia, Australia, and Africa. By the time the pandemic was declared ended in 1959, the disease had reached as far as the western United States, and an estimated 15 million people had died, including a high concentration in India. A 2010 study found that all three pandemics could be traced back to strains of Yersinia pestis originating in China.

Smaller outbreaks of plague were recorded in China in 1983, Zaire in 1992, India, Zimbabwe and Mozambique in 1994, and in Madagascar from 1995 to 1998. Another rapidly spreading outbreak in Madagascar involving insecticide-resistant fleas was reported in November 2014, causing at least seventy-one known deaths, and a task force was established to prevent the incident from becoming an epidemic.

Typically one to two thousand cases of plague are reported to the World Health Organization per year, with African countries accounting for the majority of incidents, though actual incidence is likely higher due to unreported cases in developing countries. The United States reports an average of one to seventeen cases per year, mostly in the areas of New Mexico, Arizona, and Colorado or California and southern Oregon. Though overall risk of infection is low, activities that may bring one into contact with infected animals, including domestic cats that may have preyed on infect rodents, can increase risk.

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