Examining the West's ‘Plague Line’ After Colorado's Second Death This Year

An adult in Colorado is dead after contracting the plague from an unknown source, officials announced Wednesday. (Yes, the plague.) This marks the second death from the plague in Colorado this summer.

The Pueblo City-County Health Department has not revealed the identity of the most recent victim, but said the person may have developed the disease after coming into contact with fleas on a dead rodent or other animal. The department also noted that a dead prairie dog in the western part of the county tested positive for the disease.

In June, a Colorado teenager died just days after coming down with flulike symptoms caused by the plague.

Taylor Thomas Gaes was a promising athlete who was killed by septicemic plague, a rare form of the bacterial infection that he was thought to have contracted from a flea bite, the Denver Post reports.

This isn’t the first time the plague has been reported in Colorado recently. The Centers for Disease Control and Prevention (CDC) released information in May about four people in the state who contracted the plague last year from a dog. (All four survived.)

Why is Colorado seemingly such a hot-bed for the disease?

Board-certified infectious disease specialist Amesh A. Adalja, MD, an assistant professor at the University of Pittsburgh Medical Center, tells Yahoo Health that Colorado’s plague cases could be linked to a plague outbreak that happened in San Francisco around 1900, which “seeded the American rodent populations.”

An estimation of the “plague line” in the American West. To the west of the plague line, outbreaks happen. To the east, they usually don’t. It corresponds with the 100th meridian longitudinally, along with prairie dog habitats. (Graph: Yahoo Health)

The last urban plague epidemic in the United States occurred in Los Angeles from 1924 through 1925, according to the Centers for Disease Control and Prevention. Plague then spread from urban rats to rural rodent species, and became entrenched in many areas of the western United States.

“They have dispersed throughout the western U.S. and there is a rough ‘plague line’ where cases occur, based on rodent migration patterns,” he says.

The concentration of plague cases in the U.S. West. The outbreak in Illinois was due to a lab error. (Map: Centers for Disease Control and Prevention)

Plague now occurs as scattered cases in rural areas, and most human cases are in two regions: Northern New Mexico, northern Arizona, and southern Colorado; and California, southern Oregon, and far western Nevada

Those rodents may carry infected fleas, which can then bite—and infect—a human.

There are three common forms of the plague: bubonic (an infection of the lymph nodes), pneumonic (an infection of the lungs), and septicemic (an infection of the blood). Gaes died of septicemic plague, while last year’s outbreak was pneumonic.

All forms of the plague are caused by the same bacteria,Yersinia pestis — the same bacteria that caused the Black Death, which wiped out an estimated 200 million people in the 14th century.

But … didn’t this die out in the Middle Ages?

Nope, says David Hooper, MD, chief of the Infection Control Unit at Massachusetts General Hospital. The plague has been carried by rodents ever since and “hangs around in low amounts” among some of them, he tells Yahoo Health.

Colorado, New Mexico, and Arizona have occasionally seen cases of the plague due to bites by infected fleas. However, last year’s Colorado outbreak had some cases that were thought to have spread to other people from a person who had been infected by his dog. “It doesn’t occur very often, but several cases of the plague are seen each year,” Hooper says.

(Map: Centers for Disease Control and Prevention)

Indeed, an average of seven cases of the plague occur in the U.S. each year, the CDC reports. They’re typically seasonal as well, with most cases occurring between late spring and early fall.

According to Hooper, symptoms can vary depending on the type of plague a person contracts. But a high fever is present in nearly all cases, as well as flulike symptoms. People with pneumonic plague may also develop a bloody cough, and those who contract bubonic plague usually experience painful, swollen lymph nodes.

(Graphic: Centers for Disease Control and Prevention)

While many symptoms of the plague are similar to those of the flu, Hooper says, the big indicator is exposure to an infected animal. If you have symptoms of the plague and have been exposed to rodents or fleas recently, call your doctor immediately.

The plague is serious — if left untreated, it can have a death rate of 50 percent or higher, Hooper says.

But there are ways to treat it if it’s caught early enough.

The FDA approved the drug Levaquin in 2012 to treat the plague, joining other antibacterial drugs, such as streptomycin, doxycycline, and tetracycline, that are approved for the treatment of the infection. A vaccine for the plague is also in the works, according to the CDC, but nothing is expected to be available to the public in the near future.

Should you be overly concerned about contracting the plague? Probably not. “It’s pretty rare and is a very extremely low risk to the overall population,” says Hooper. “All things considered, there are many other common diseases you are much more likely to encounter than the plague.”