Plague (Bubonic plague; Yersinia pestis)

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What is Plague?

Plague is an infection caused by the organism Yersinia pestis. It is carried by wild rodents and transmitted to humans. It has a high mortality rate.

Who gets Plague?

Yersinia pestis has been one of the most devastating diseases in history, although currently it is rare in developed countries.

Epidemics of the plague were recorded in the Old Testament. There have been three major pandemics. The first took place between the sixth and ninth century AD, causing 40 million deaths, he second pandemic started in 1320s and caused around 25 million deaths in Europe alone (30% of the population). The third pandemic began in China in the 1860s, and spred to Africa, Europe, and the Americas.

Nowadays the disease occurs mostly in sub-Saharan Africa and South East Asia, and epidemics do occur. In developed countries it is seen in people undertaking outdoor pursuits.

Predisposing Factors

Woodland rodents are the main reservoir of the disease. They transmit the infection to domestic rats (Rattus rattus) by the rat flea (Xenopsylla cheopis). The fleas then transmit the infection to humans through biting.

Occasionally the infection is also transmitted to humans by infected faeces being rubbed into skin wounds or though inhalation of droplets.

Risk factors for plague include a recent flea bite; and occupational or environmental exposure to rodents (especially rabbits, squirrels, or prairie dogs; or scratches or bites from infected domestic cats).

Progression

There are 4 major clinical forms of the plague:

(1) The bubonic plague: this is the most common form (90%). It has an incubation period of 1 week with acute onset of illness including high fever, chills, headache, myalgia, nausea, vomiting and in severe cases, prostration. This is follwed by the development of matter, tender buboes which suppurate in 1-2 weeks. Petechiae, ecchymoses and bleeding from the gastrointestinal tract may also occur. Mental confusion results from the development of toxaemia;

(2) The pneumonic plague: there is abrupt onset of the features of a fulminant pneumonia with bloody sputum, marked respiratory distress, cyanosis and almost invariably death;

(3) The septicaemic plague: there is an acute fulminant infection with shock and disseminated intravascular coagulation. Death results in 2-5 days if left untreated; and;

(4) The cutaneous plague- there is a pustule, eschar, papule or extensive purpura which can be become necrotic and gangrenous.

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