Plague-riddled prairie dogs are forcing officials to close off areas in Colorado state, including sections of the Rocky Mountain Arsenal National Wildlife Refuge.

Areas in the Denver suburb Commerce City and the First Creek at DEN Open Space will stay closed until the first weekend of September (or Labor Day weekend) to give the authorities time to treat the prairie dogs' holes with insecticide. The chemicals should kill any fleas that may be infected with the plague-causing bacterium Yersinia pestis. 

Although the Rocky Mountain Arsenal National Wildlife Refuge re-opened after a temporary closure on August 17, some areas (including certain trails and parking lots) will remain shut, the Tri-County Health Department announced in a statement.

"The prairie dog colonies are being monitored and burrows are being treated with insecticide, but there is still evidence of fleas in the hiking and camping areas, which could put people and pets at risk, so those areas will remain closed," said John M. Douglas, executive director of Tri-County Health Department.

The plague may dredge up images of beak masks, open graves, and abandoned Medieval villages. It might make you think of history textbooks and the Justinian Plague or the Black Death (aka the Great Plague), which wiped out up to 60 percent of the European population.

So, it might be hard to get your head around the fact that Yersinia pestis has been present in the western US since it first arrived in San Francisco at the turn of the 20th century, at the time of the third pandemic (or Modern Plague).

The Modern Plague began in China in the 1860s, spreading to Hong Kong by 1894. The bustling port city provided the ideal springboard to send the disease right across the world – and over the next 20 years, stowaway rats aboard steamships did exactly that. The resulting pandemic led to approximately 10 million deaths.

The largest epidemic to affect urban America took place in Los Angeles from 1924 to 1925. But while health officials were able to bring the rat-associated plague under control, the bacteria-carrying fleas had already moved on to new hosts. According to research wildlife biologist Dean Biggins, in an interview with NPR, Yersinia pestis is relatively common within colonial squirrel populations – a group of animals that includes prairie dogs.

There are roughly seven human plague cases in the States each year. Fortunately, few are fatal. The availability of antibiotics has drastically cut mortality over the 20th century so that between 1990 and 2010, death rates shrank to 11 percent.

 

The plague can take different forms depending on the way it is transmitted but the three most common are: bubonic (which accounts for more than 80 percent of cases in the US), pneumonic, and septicemic.

Bubonic plague presents a set of symptoms that includes sudden-onset fever, headache, chills, and – most distinctively – swollen lymph nodes called buboes. This tends to happen if the patient is bitten by an infected flea.

Septicemic plague can also develop after a bite from an infected flea or after the handling of an infected animal. It can lead to fever, chills, fatigue, abdominal pain, shock, internal bleeding, and parts of the flesh turning black.

Pneumonic plague is spread through droplets in the atmosphere (untreated bubonic or septicemic plague can also spread to the lungs) and is the most serious form of the disease. Symptoms include fever, headache, weakness, and rapidly developing pneumonia complete with shortness of breath, chest pain, cough, and (sometimes) mucus with a bloody or watery consistency.

The US Centers for Disease Control and Prevention (CDC) recommend anyone who suspects they may be infected seek medical attention immediately – antibiotics should be administered within 24 hours after symptoms first start to develop to prevent high risk of death.