Three pandemics of plague have been recorded. In the Justinian pandemic (circa 542-767 ad), it is thought that plague moved from upper Egypt to the Mediterranean and spread from there to Europe and Asia Minor, ultimately causing an estimated 40 million deaths. The second plague pandemic began in Central Asia early in the 14th century, caused epidemics in China and India, and moved along caravan routes to the Near and Middle East. Entering Messina by ship in 1347, plague swept swiftly through Europe and the British Isles, receiving the epithet "Black Death." Medieval plague killed a quarter or more of the affected populations at its height and was followed by successive epidemics in the European region over the next several centuries. The last global surge of plague, the third (modern) pandemic, arose in the latter half of the 19th century in Yunnan Province, China, struck Hong Kong in 1894, and spread from there by rat-infested steamships to port cities throughout the world, including several in the United States.3-5 Within 30 years of its appearance in Hong Kong, the third plague pandemic had resulted in 26 million human cases and more than 12 million deaths, most of them in India.3
The plague bacillus was first identified in Hong Kong in 1894 by Alexandre Yersin, who isolated the organism from enlarged lymph nodes (buboes) of plague victims.6 In 1898, Paul-Louis Simond, a French scientist sent to investigate epidemic plague in Bombay, identified the plague bacillus in the tissues of dead rats and subsequently proposed that the organism was transmitted from rat to rat, and from rats to humans by rat fleas.
After 1900, the global spread of plague was limited by regulations that controlled rats in ports and imposed inspection and rat-proofing of ships. Y. pestis did, however, become newly established among urban and rural rodent populations in many previously unaffected areas of the Americas, Europe, Africa, and Asia, resulting in scattered zoonotic foci that still exist throughout the world today.7,8 In San Francisco, between 1900 and 1908, major outbreaks of rat-borne plague killed more than 200 persons. By 1908, plague was epizootic in ground squirrels in counties surrounding the city,5 and in subsequent years spread to wild rodent populations throughout California and other states in the western third of the country. The last outbreak of urban plague and of person-to-person pneumonic transmission in the United States occurred in a slum area of Los Angeles in 1924-1925.
By the middle of the 20th century, cities in the United States and elsewhere had enforced higher sanitary standards and building codes, effective insecticides and rodenticides had become widely available, and several classes of antibiotics had been shown to be efficacious in treating plague. Most human plague since then has been sporadic and rural in distribution. Outbreaks have been relatively slow to develop and readily controlled by a combination of surveillance, early diagnosis and treatment, and flea and rat suppression.9 The major exceptions to this generalization were the large ratborne plague epidemics that occurred from 1962 to 1975 in war-torn Vietnam and in the 1990s in Madagascar.6,10
Plague is one of the three diseases (plague, cholera, yellow fever) subject to quarantine under World Health Organization (WHO) international health regulations.11 In 1994, articles of these regulations were used to prevent potential spread of plague from India after a reported outbreak of pneumonic plague there evoked local panic and an exaggerated international response to the perceived health emergency.12,13
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