Why Plague Still Exists in the United States

By Ross Pomeroy - RCP Staff
May 05, 2020
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Western Europe hosted the most fatal plague pandemic in history – the Black Death killed over 50 million in the mid-14th century. Today, however, plague is essentially extinct in that part of the world. Across the Atlantic, the United States still sees between one and seventeen cases of the infamous bacterial disease each year. At least 106 people have been infected since 2000, with twelve deaths.

Plague is caused by the bacterium Yersinia pestis, which most often spreads from the bite of infected fleas. When the bacterium infects humans or other mammals, it usually multiplies quickly, causing fever, weakness, headache, and a variety of other symptoms depending upon the part of the body the pathogen attacks. Lymph nodes swollen to characteristic buboes hint that the bacterium has entered the lymphatic system, while blackened skin indicates that plague has reached the blood.

Thankfully, the disease is readily treatable today with early administration of antibiotics. Its rarity in the U.S. can result in misdiagnosis, however, which contributes to the roughly ten percent death rate.

In the U.S., plague became a harsh reality rather than a distant piece of history relegated to Europe in 1900, when steamships bearing immigrants, goods, and infected rodents arrived on the west coast. California experienced almost all of the resulting 280 cases and 172 deaths over the next eight years. Many politicians, officials, and newspapers initially covered up the outbreak, worried that it would devastate the state's lucrative and growing agriculture industry.

Plague's initial burst into the United States concluded in 1908. It was followed by a brief flare-up in Los Angeles in 1924. We have not witnessed an outbreak for almost a hundred years since, but Y. pestis is still out there. Concentrated almost entirely in sparsely populated regions of the western states, it persists in rural rodents like ground squirrels, deer mice, and voles, which show little to no symptoms of the disease. Larger mammals like cougars, cats, and dogs contract the disease from these wild rodents and pass it on to humans, most frequently via infected fleas. A recent survey of cougars in Yellowstone National Park found that nearly half had been exposed to plague.

A case report detailed one of the few plague deaths in the United States.

"A 37-year-old male wildlife biologist working at Grand Canyon National Park in Arizona performed an autopsy on a mountain lion that he had tracked by a radio collar and found dead in 2007. Three days later he developed fever and cough productive of blood-tinged sputum. Four days after the onset of symptoms, he was found dead at his home... The patient had thought the animal died of chest trauma inflicted by another lion, explaining why he took no precautions while handling the carcass and not wearing gloves or face mask during the autopsy carried out in his garage."

Reported Cases of Human Plague - United States, 1970-2018. Since the mid–20th century, plague in the United States has typically occurred in the rural West.

Domestic cats are far more of a risk to humans than their larger cousins, however. Felines permitted to frolic outdoors eat infected rodents, contract the disease, and pass it on to owners or veterinarians. This has occurred dozens of times in the past forty years, almost entirely in Arizona, California, Colorado, and New Mexico.

Plague poses little threat of returning to prominence today, primarily owing to highly effective antibiotic treatments. It's also difficult to spread from person to person in its most common, bubonic form, requiring contact with infected bodily fluids. In its far rarer and deadlier pneumonic form, plague infects the lungs and can be spread by airborne droplets.

Guarding against plague infection is simple: If you live in the western United States, keep yourself and your pets away from wild rodents. If you do come into close contact, check yourself for fleas.

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