Distribution and Incidence

With the historically ironic exception of western Europe, Y. pestis today occurs naturally throughout the world among the wide variety of rodents and lagomorphs (i.e., rabbits and related species). Some of the more than 300 rodent species affected are relatively resistant to disease from Y. pestis and can survive and reproduce while technically infected by the organism. Y. pestis infects new animals either because fleas transmit it or because the microbe is shed and survives in the protective microclimate of warm rodent burrows. Some literature refers to this part of the plague cycle as "sylvatic" plague, or "enzootic" plague.

The "disease," then, is not always a disease, and it is ecologically very complex. Indeed, it is occasional ecological change or disturbance that brings susceptible rodents into contact with Y. pestis. Historically the most important of these rodents is considered to be Rattus rattus, the common, commensal black or brown house rat, that literally "shares man's table." When infected by Y. pestis these susceptible animals die quickly of an overwhelming infection, with blood levels of the microbe so high that their rat fleas imbibe large numbers of organisms.

The oriental rat flea Xenopsylla cheopis and the human flea Pulex irritans are thought to be historically important arthropod vectors transmitting "epizootic" plague to humans. X. cheopis is an effi cient vector because a bend in its feeding tube, or proventriculus, creates a location for growth of Y. pestis, such that the flea becomes "blocked," unable to swallow a full blood meal. Attempting to dislodge this bolus or wad, the flea infects new mammalian hosts. Other fleas clear Y. pestis more quickly, so that only the excreta of the flea, or the crushing of its body, will infect. P. irritans had an important historical role because it is a flea that feeds indifferently upon both humans and the common house rat.

Human plague usually arises after an epizootic plague has produced high mortality among susceptible rodents, when infected fleas, deprived of rodent hosts, begin to feed on humans. Although some historians speak of "endemic plague," no such phenomenon can exist. Humans do not normally carry the Y. pestis organism, and thus cannot infect fleas or otherwise pass the disease to new hosts. For human communities, plague is an acute infection ultimately derived from infected rodents.

In areas of the world today where plague routinely infects resistant rodents, ecologists and public health officials try to monitor the passage of the disease to susceptible species. In these regions - the American southwest, south central Eurasia, and Southeast Asia - humans who are likely to encounter infected animals or their fleas must be revacci-nated often, for human immunity to Y. pestis is short-lived.

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