From Poor to Rich: The Migration of the Plague in the United States

The plague, once a disease affecting mostly lower socioeconomic communities in the United States, has made a shift to middle and upper class neighborhoods over the past decade. Today, cases of the plague are few and far between in the United States (an average of 11 cases per year since 1976), but researchers at the Center for Disease Control who noticed the shift are still closely monitoring this once pandemic infectious disease.

Plague is caused by Yersinia pestis, a bacterium carried by rodents and transmitted to humans via fleas. Symptoms can range from fever and chills to vomiting and death depending on the severity of the case. Transmission has anecdotally been thought to be more likely in poorer areas where less than ideal hygienic conditions allow its hosts to persist. However, specific socioeconomic risk factors have not been fully investigated, which prompted the CDC study, published in the July edition of Emerging Infectious Diseases.

Plague epidemics date back to biblical times and have killed thousands at a time. Nicknamed the “black death,” the plague is believed to have killed up to one-third of the Western European population during the 14th century. Still endemic in some areas of the world, the plague can be found in Africa, Asia, and South America. In the United States, most cases have occurred in New Mexico, where this study was conducted.

In their study, researchers analyzed 123 case reports of the plague in New Mexico and cross-referenced these with socioeconomic data of various neighborhoods from the U.S. Census bureau.

In the 1980s the plague was almost twice as likely to be found in census block groups with high rates of poverty. When examining today’s records, those living in high-poverty communities were no more likely than those in other income brackets to be at risk for the plague. In fact, by the 2000s those most at risk were groups living in high-value homes and homes built within the previous five years. A shift in risk was also reflected geographically. Plague cases had previously been more common in rural northwestern New Mexico. They have now migrated to the north central area, to suburbs of Santa Fe and Albuquerque (see map). The wealth of New Mexico’s Native American culture, typically a lower income population, is in the northwestern part of the state where plague cases are declining.

Plague, researchers learned, is not dependent solely upon socioeconomic status, but rather, environmental factors. The bacterium is now found in what are called transitional or ecotone habitats, like those between forests and cleared land. "Migration of persons into suitable plague habitat would potentially increase the likelihood of human exposure to infected rodents and their fleas," they explained. Ultimately the bacterium exists in complimentary physical environments, which are not necessarily poor socioeconomic ones.

Why exactly is the shift occurring? Using wood as fuel could be an underlying cause. Burning wood, perhaps once a necessity of the poor in rural areas, is now a recreation of the middle class in suburban neighborhoods. Storing wood outside in piles provides favorable nesting ground for rodents the researchers noted. The use of wood as fuel was found to be associated with plague cases in the study during all time periods.

Further research about the bacterium, the rodents, and the individuals affected could help us understand the association in the future. The NIH recommends those feeling symptoms that are traveling or living in areas affected by plague seek immediate medical attention after exposure to fleas or rodents.

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