'' seen at 2000x magnification. This bacterium, carried and spread by fleas, is the cause of the various forms of the disease plague.]]
is the most well-known variant of the deadly
Infectious Disease '''plague''', which is caused by the
Enterobacteria ''
Yersinia Pestis .''
Plague has impacted human society for millennia. Most scientists believe that it was responsible for the
Black Death , which killed perhaps a third of Europe's population during the Middle Ages, with additional large numbers of casualties in Asia and the Middle East.
Plague is
Endemic in many countries in Africa, in the former Soviet Union, the Americas and Asia. In 2003, nine countries reported 2,118 cases to the
WHO , of which 182 ended in death. All were isolated cases, except for an outbreak in a village in Algeria (the first in 50 years), which caused 11 infections and one death. Plague is most common in
Madagascar and the
Democratic Republic Of The Congo . These two countries have on average 600-800 cases each per year. They accounted for 2,025 of the 2,118 cases and 177 of the 182 deaths in 2003. Other countries with annual but much fewer cases are
Tanzania ,
Peru ,
United States ,
China ,
Mongolia and
Vietnam . According to the WHO, the actual number of cases in the world is probably much higher than reported, due to the reluctance of certain countries to declare cases, the lack of diagnosis because the clinical picture of cases is not very specific, and the absence of laboratory confirmation.
The most recent
Outbreak of plague happened in Zobia, in the northern part of the
Democratic Republic Of The Congo in December 2004. The outbreak, which only appeared as the variant pneumonic plague, began among workers in a diamond mine. By mid March 2005 the
WHO regarded the outbreak as over. 130 people had been infected, of whom 57 died.
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There has not been a plague
Epidemic (i.e an outbreak affecting a larger area) for many years.
Plague is primarily a disease of
Rodent s, particularly
Marmot s (in which the most virulent strains of plague are primarily found), but also
Black Rat s,
Prairie Dog s,
Chipmunk s,
Squirrel s 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 bacteria multiply inside a flea, blocking its stomach and causing it to become very hungry. The flea then voraciously bites a host and continues to feed because it is unable to satisfy its hunger. During the feeding process, infected blood carrying the plague bacteria flows into the wound. The plague bacteria then has a new host, and the flea eventually dies from starvation. 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 , two bacteriologists, the Swiss
Alexandre Yersin and the Japanese
Shibasaburo Kitasato , independently isolated the responsible bacterium in
Hong Kong during the
Third Pandemic . Though both investigators reported their findings, there was a series of confusing and contradictory statements by Kitasato that eventually led to the acceptance of Yersin as the primary discoverer of the organism. Yersin named it ''Pasteurella pestis'' in honour of the
Pasteur Institute , where he worked, but in 1967 it was renamed ''
Yersinia Pestis '' in honour of Yersin. Yersin also noted that rats were affected by plague, not only during plague epidemics but also often preceding such epidemics in humans, and that plague was regarded by many locals as a disease of the rats: villagers in China and India described that when large numbers of rats were found dead, plague outbreaks in people soon followed.
In
1898 , the French scientist
Paul-Louis Simond (who had also come to China to battle the Third Pandemic) established the rat-flea mechanism that drives the disease. He had noted that persons who became ill did not have to be in close contact with each other to acquire the disease. In Yunnan, China, inhabitants would run away from their homes as soon as they saw dead rats, and on the island of Formosa (Taiwan), residents considered handling dead rats a risk for developing plague. These observations led him to suspect that the flea might be an intermediary factor in the transmission of plague, since people acquired plague only if they were in contact with recently dead rats, but not affected if they touched rats that had been dead for more than 24 hours. In a now classic experiment, Simond demonstrated how a healthy rat died of plague after infected fleas had jumped to it from a plague-dead rat.
Depending on the symptoms and the route of infection, plague appears in several forms, classified by the WHO with different
ICD-10 codes:
:Main disease:
:(A20) (''Pestis''). Infections caused by ''
Yersinia Pestis ''.
:Forms:
: (A20.0) (''Pestis bubonica'') occurs when ''Yersinia pestis'' causes an inflammation of the
Lymph Node s, making them tender and swollen (from lat. ''bubo'' = bump). This is the most common form of plague.
:(A20.1) (''Pestis cellulocutanea'') is a very unusual form, with ''Yersinia pestis'' causing a skin infection.
:(A20.2) or '''pulmonic plague''' (''Pestis pneumonica'') occurs when the
Lung s are infected by ''Yersinia pestis''. The second most common form of plague. It may be a secondary infection, caused by bacteria spreading from the lymph nodes and reaching the lungs, but can also exist on its own, caused by inhalation of airborne bacteria.
: (A20.3) or '''plague meningitis''' (''Pestis meningealis'') looks like
Meningitis at the outset. It is most common in children and is usually the end result of ineffective treatment for other forms of plague. Unusual.
: (A20.?) occurs when ''Yersinia pestis'' is consumed, often through food. It can resemble
Tonsillitis . Very rare form.
: (A20.7) (''Pestis septic(h)aemica'') occurs when ''Yersinia pestis'' multiply in the
Blood . The third most common form. It is usually associated with hunting and skinning of animals, but can also occur secondary to bubonic and pneumonic plague.
:(A20.8) (''Aliae formae pestis'') include the milder forms '''abortive plague''', '''asymptomatic plague''' and '''pestis minor''', all three often resulting only in a mild fever and light swelling of the lymph glands, usually resolved in approximately a week if appropriate treatment is given.
Bubonic plague becomes evident 3–7 days after the infection. Initial symptoms are chills, fever, diarrhea, headaches, and the swelling of the infected lymph nodes, as the bacteria replicate there. If untreated, the rate of mortality for bubonic plague is 30–75%.
