WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases - Plague
Background of the disease
Plague is primarily a disease of rodents and their fleas, which can infect humans. It is transmitted between rodents by rodent fleas, and can be transmitted to people when infected rodent fleas bite them. As with many primarily zoonotic diseases, plague is a very severe disease in people, with case fatality rates of 50-60% if left untreated.
Plague has been responsible for widespread pandemics with high mortality. It was known as the "Black Death" during the fourteenth century, causing an estimated 50 million deaths, approximately half of them in Asia and Africa and the other half in Europe, where a quarter of the population succumbed.
There are three main forms of plague in humans, namely bubonic, septicaemic and pneumonic. Bubonic plague is the result of an insect bite in which the plague bacillus travels through the lymphatic system to the nearest lymph node where it forms a swelling. The most usual place for this swelling is the groin, but it also occurs in the armpits and the neck. These swellings are known as buboes (derived from the Greek word for groin). The septicaemic form of plague occurs when the insect deposits the bacillus directly into the bloodstream. This form is almost always fatal. Pneumonic plague is an infection of the lungs with the plague bacillus. Pneumonic plague can be transmitted directly from person to person via infected air droplets or through infected clothing and other contaminated articles.
Plague has declined dramatically since the early part of the twentieth century, when outbreaks could cause tens of millions of deaths. This is due primarily to improvements in living standards and health services. However, a substantial number of countries continue to be affected by plague, case fatality rates remain high and antimicrobial resistance has begun. Therefore, continued vigilance is required, particularly in human populations living near natural plague foci. Plague foci are not fixed, and can change in response to shifts in factors such as climate, landscape, and rodent population migration. Natural foci of plague are situated in all continents except Australia, within a broad belt in tropical, subtropical and warmer temperate climates, between the parallels 55° N and 40° S. These foci are presented in Map 3.1.
Map 3.1 Natural plague foci (in rodent populations)