Plague is a systemic zoonosis caused by Yersinia pestis. It predominantly affects small rodents in rural areas of Africa, Asia, and the Americas and is usually transmitted to humans by an arthropod vector (the flea). Less often, infection follows contact with animal tissues or respiratory droplets. Plague is an acute febrile illness that is treatable with antimicrobial agents, but mortality rates among untreated patients are high. Patients can present with the bubonic, septicemic, or pneumonic form of the disease. Although there is concern among the general public about epidemic spread of plague by the respiratory route, this is not the usual route of plague transmission, and established infection-control measures for respiratory plague exist. However, the fatalities associated with plague and the capacity for infection via the respiratory tract mean that Y. pestis fits the profile of a potential agent of bioterrorism. Consequently, measures have been taken to restrict access to the organism, including legislation affecting diagnostic and research procedures in some countries (e.g., the United States).
The genus Yersinia comprises gram-negative bacteria of the family Enterobacteriaceae (gamma proteobacteria). Overwhelming taxonomic evidence showing Y. pestis strains as a clonal group within Y. pseudotuberculosis suggests recent evolution from the latter organism—an enteric pathogen of mammals that is spread by the fecal-oral route and thus has a phenotype distinctly different from that of Y. pestis. When grown in vivo or at 37°C, Y. pestis forms an amorphous capsule made from a plasmid-specified fimbrial protein, Caf or fraction 1 (F1) antigen, which is an immunodiagnostic marker of infection.
Human plague generally follows an outbreak in a host rodent population (epizootic). Mass deaths among the rodent primary hosts lead to a search by fleas for new hosts, with consequent incidental infection of other mammals. The precipitating cause for an epizootic may ultimately be related to climate or other environmental factors. The reservoir for Y. pestis causing enzootic plague in natural endemic foci between epizootics (i.e., when the organism may be difficult to detect in rodents or fleas) is a topic of ongoing research and may not be the same in all regions. The enzootic/epizootic pattern may be the result of complex dynamic interactions of host rodents that have different plague susceptibilities and different flea vectors; alternatively, an environmental reservoir may be important.
In general, the enzootic areas for plague are lightly populated regions of Asia, Africa, and the Americas (Fig. 159-1). Between 1989 and 2003, 38,359 cases of plague were reported to the World Health Organization (WHO) under the International Health Regulations then in force, which required national authorities to report all cases of plague in their jurisdiction and according to which plague was one of just three infectious diseases to be so reported. More than 80% of these cases were in Africa, and the percentage of all cases in Africa increased over this period; the majority of cases were reported from East Africa and the island of Madagascar. In 2007, the second edition of the International Health Regulations came into force, widening reporting to any disease event that could have a serious public health impact or could rapidly spread internationally. For plague, this requirement entails specific reporting of pneumonic plague or any suspected case of plague in an area not known to be endemic for plague. Recent outbreaks of pneumonic plague have been recorded in Uganda, the Democratic Republic of the Congo, and China.
Approximate global distribution of Yersinia pestis. (Compiled from WHO, CDC, and country sources. Reprinted with permission from DT Dennis, GL Campbell: Plague and other Yersinia infections, in Harrison's Principles of Internal Medicine, 17th ed, AS Fauci et al (eds). New York, McGraw-Hill, Chap. 152, 2008.)
Plague was introduced into North America via the port of San Francisco in 1900 as part of the Third Pandemic, which spread around the world from Hong Kong. The disease is presently enzootic on the western side of the continent from southwestern Canada to Mexico. Most human cases in the United States occur in two regions: “Four Corners” (the junction point of New Mexico, Arizona, Colorado, and Utah), especially northern New Mexico, northern Arizona, and southern Colorado; and further west in California, southern Oregon, and western Nevada (http://www.cdc.gov/ncidod/dvbid/plague/epi.htm). From 1990 to 2005, 107 cases of plague were reported in the United States, with a median of seven cases per year. Most cases occurred from May to October—the time of year when people are outdoors and rodents and their fleas are most plentiful. Infection is most often acquired by fleabite in peridomestic environments. Infection can also occur through the handling of living or dead small mammals (e.g., rabbits, hares, and prairie dogs) or wild carnivores (e.g., wildcats, coyotes, or mountain lions). Dogs and cats may bring plague-infected fleas into the home, and infected cats may transmit plague directly to humans by the respiratory route. The last recorded case of person-to-person transmission in the United States occurred in 1925.
Plague most often develops in areas with poor sanitary conditions and infestations of rats—in particular, the widely distributed roof rat Rattus rattus and the brown rat R. norvegicus (which serves as a laboratory model of plague). Rat control in warehouses and shipping facilities has been recognized as important in preventing the spread of plague since the early twentieth century and features in the current WHO International Health Regulations. Urban rodents acquire infection from wild rodents, and the proximity of the former to humans increases the risk of transmission. The oriental rat flea Xenopsylla cheopis is the most efficient vector for transmission of plague among rats and onward to humans in Asia, Africa, and South America.
Worldwide, bubonic plague is the ...