Epidemiology
Yersinia pestis exists worldwide as an infection of wild rodents transmitted through flea bites. It is thought to have been responsible for 3 major pandemics in the past 2000 years.[6] It is endemic in rural areas of central and southern Africa (especially Madagascar, Democratic Republic of Congo, and Uganda), central Asia and the Indian subcontinent, northeastern parts of South America, and western parts of the US. Sporadic human cases may occur when people come into contact with these natural foci. Infection of urban rats potentially leads to epidemics, as large numbers of rats may live in proximity to large numbers of people. The rat flea Xenopsylla cheopis is an extremely efficient vector. Pneumonic plague is the only clinical form that is transmissible from person to person; however, this is uncommon.[7]
The World Health Organization (WHO) reported 3248 cases between 2010 and 2015, resulting in 584 deaths. Most cases were reported from Africa, mainly Madagascar.[8] Difficulties in diagnosis and a reluctance to report cases make this an underestimate. The last major outbreak was in Madagascar in late 2017, with nearly 2500 cases and over 200 deaths reported.[9] A smaller outbreak was reported in Madagascar in 2021.[10] Currently, the most endemic countries are Madagascar, the Democratic Republic of the Congo, and Peru.[11]
The natural foci of plague in North America occur in 15 western states of the US and its bordering regions, including southwestern Canada and northern Mexico.[12] Between 2000 and 2019 a total of 107 cases and 12 deaths were reported in the US (an average of 5 cases each year; range 1-17 cases per year). CDC: map of plague in the US Opens in new window Three cases were reported in the US in 2020, two cases in 2021, and no cases in 2022 (as of 13 August 2022).[13][14] Incidence in the US may increase as residential areas spread onto rural land. Plague incidence is seasonal and is maximal in warmer months when the animal reservoirs and their associated fleas are most active and people are most likely to be outside.[15] Squirrels seem to be the most important reservoir in the US, although transmission from domestic cats is described, particularly in veterinary workers.[16] CDC: map of plague in the US Opens in new window
From antiquity through to the 20th century, plague has been used as a biologic weapon. Aerosolized plague bacilli released in a confined space would cause significant mortality. An outbreak of pneumonic plague in a nonendemic area should raise suspicion of deliberate release.
Yersinia enterocolitica and Yersinia pseudotuberculosis are distributed worldwide and may be isolated from food, water, and a wide range of wild and domestic animals. Infection usually follows oral ingestion, but transmission through contaminated blood products has been described.[17] In 2017, the incidence rate of laboratory-confirmed yersinia infection transmitted via food was 1 in 100,000 people (489 cases) in the US, 116% higher than the 2014 to 2016 rate. The increase is likely to be due, in part, to more laboratories testing for Yersinia and more infections being diagnosed.[18]
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