Plague

As has been noted previously, plague caused by Yersinia pestis first appeared in North America in San Francisco, in 1900. Plague is now endemic in North America from the Pacific Coast eastward to the western Great Plains and from British Columbia to Alberta, Canada and southward to Mexico. Most of the human cases occur in two regions; one in northern New Mexico, northern Arizona, and southern Colorado, another in California, southern Oregon and far western Nevada. Transmission of plague from person to person is uncommon and has not been reported in the USA since 1924. The last rat-borne epidemic in the USA occurred in Los Angeles in 1924–1925. Since then, all human plague in the country has been sporadic cases acquired from wild rodents or their fleas or from direct contact with plague-infected animals. Historically the plague bacillus has been demonstrated in western Canada during field studies involving rodents and fleas. More recent studies have resulted in the demonstration of antibodies in feral cats and dogs in southeastern Alberta and southwest Saskatchewan suggestive of ongoing plague activity in Canada. However, there have been no human cases identified in these areas.

In the USA during the 1980s plague cases averaged about 18 per year. During 1988–2002, a total of 112 human cases of plague were reported from 11 western states. The majority (87%) were from four states – New Mexico (48), Colorado (22), Arizona (16) and California (11).