Evidence for the extinction of plague in Hawaii.

Plague (infection by Yersinia pestis) was present in Hawaii for the period 1899-1957 and caused at least 370 fatalities. The first infections came from immigrant commensal rats, probably Rattus rattus and Rattus norvegicus, on ships from the Orient. Both species were already established in Hawaii and became the widespread local carriers of plague, supplemented by Rattus exulans which had colonized the islands in ancient Polynesian times. The flea Xenopsylla vexabilis arrived with R. exulans, and its near relative Xenopsylla cheopis accompanied the ship rats. Following each introduction to port cities, plague subsided after a few years but remained active in rural areas of two islands for nearly 50 years. In Hamakua District on Hawaii, the demise of plague was characterized by its repeated terminal migrations from persistent foci, a rapid decline in expected numbers of infected rats and fleas, and negative serologic evidence from rodent reservoirs and mongooses in massive surveys. The simple biotic system--bacterium, three rodents, and two fleas--appeared unable to maintain the disease over time. Improved sanitation, mechanized agriculture, gradients in rainfall and temperature, and the collapse of reservoir and vector populations during drought are cited as probable factors in the extinction of plague.