The Plague in the USA. Wait what?

To view this post in german click here/ Für die deutsche Version hier klicken:

On my recent trip to the USA, I was hiking in the Grand Canyon and came across the sign pictured above. While I agree with the notion of not interacting, e.g. feeding or trying to pet, wild-life, I laughed at the reason. Squirrels that contract the Plague. Surely the “Black Death”, the disease that killed one-third of Europe’s inhabitants in the 1300s, wasn’t still raging in the USA. I was wrong.


I looked up Plague and USA the same night and learned that the disease is still occasionally occurring in rural southwestern parts of the USA, mainly in Arizona, Colorado and New Mexico. On average seven cases are reported per year. Although in 2015, 15 cases and 4 deaths were confirmed by the CDC. Overall these numbers are very low – but still, how come the plague, which is extinct in Europe, is still found in the USA.

 What is the Plague?

  • An infectious disease caused by bacteria called Yersinia pestis.
  • The disease is endemic in Madagascar, Peru and the Dem Rep of Congo.
  • 1000-2000 cases are reported worldwide per year.
  • Three forms of the disease exist: bubonic, septicemic and pneumonic.
  • Bubonic is the most common form of the plague, accounting for 80% of cases. It infects the lymph system and causes swollen, sometimes black lymph nodes called buboes (hence the name bubonic). Symptoms arise after 2-6 days and are flu-like with fever, headache, chills, and weakness.
  • If the bacteria reach the blood stream, the infection turns septicemic.
  • Pneumonic plague is the most serious form of plague infecting the lung, causing pneumonia.
  • Antibiotics can treat the disease, if recognised early, lowering the mortality rate from 60-90% to 10%.
  • No vaccine is currently available for humans.

The bacteria causing the plague are spread by fleas (Xenopsylla cheopis). In the widely excepted infection model: rodent → flea → human, rodents are infected with the disease and the fleas, for which the bacteria are harmless, are the vectors “carrying” the bacteria from rodent to human.

The natural occurrence of the disease in rodents such as rats, squirrels, chipmunks and prairie dogs is one of problems when trying to eradicate this disease. We do not “just” have to extinct the bacteria in humans but also in all rodents. The only disease ever eradicated by men – smallpox – could only infect humans. As eradicating all rodents is impossible and dangerous for our ecosystem, vaccinations for ferrets and prairie dogs are being investigated. Injection of a vaccine has been shown to successfully protect ferrets in a recovery program, where animals are born captive but released into the wild to increase population size. In order to also protect wild animals, oral vaccination strategies are currently tested. Apparently, the animals prefer peanut butter baits over other flavours. Oral vaccination of wild animals has shown huge success in keeping rabies at bay in Europe. Baits containing the vaccine are dropped out of airplanes into areas where red foxes live. First trials were conducted in Switzerland in 1978, and many countries such as Finland, Switzerland, France and Germany have been declared rabies free since then.

Until the vaccination programs are set up, National parks in the USA will continue to monitor rodent colonies. When an outbreak is observed, warning signs will be posted and rodent colonies are treated with insecticides to kill fleas and limit spread of the plague.

While chances of transmitting the disease are low, remember these easy precautions. Do not engage wild-life (dead or alive). Wear long trousers. Use insect repellent to keep of any fleas. Some is true for pets – as some of the recent cases have been spread by pets for example cats bringing back dead squirrels.

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