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Plague killed a cat in S.W. Idaho, sickened a child. What is the plague? Why is it here?

How infectious diseases like the plague are spread

Dr. Pritish Tosh, an infectious diseases specialist at Mayo Clinic, discusses the ways in which infectious diseases like the plague are spread rapidly.
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Dr. Pritish Tosh, an infectious diseases specialist at Mayo Clinic, discusses the ways in which infectious diseases like the plague are spread rapidly.

A cat died July 8 of the plague, Southwest Idaho health officials say, bringing to three the cases of plague identified this year in Elmore County.

Central District Health Department said Monday that a feral cat that an Elmore County family cared for as an outdoor pet died after contact with ground squirrels, which are known to carry the plague. Tests indicate plague, and family members are being treated with antibiotics as a precaution.

In May, a child in Elmore County contracted the plague, the first confirmed human case of plague in Idaho since 1992. In June, a pet cat was also identified as having plague. Both recovered after treatment.

For more than a century, the Western U.S. has had to deal with periodic outbreaks of the plague. Luckily, the number of cases are few (an average of seven annually in the U.S.) and modern medicine greatly reduces the risk of death.

What is the plague? Where did it come from? Why is it prevalent in the Western U.S.? For answers to these questions and more, read on.


According to Trisha Hebdon, lab manager of Idaho Department of Fish and Game’s Wildlife Health Lab, plague “season” corresponds to the life cycles of plague-carrying animals. In Idaho, the ground squirrel is dormant in the hottest months and is active primarily between March and July, though can be active as early as January and as late as August.

By far the most widespread and numerous carriers are rodents, especially rats, ground squirrels, prairie dogs, marmots and voles. Rabbits are particularly susceptible as well. These species are primarily responsible for passing the plague to humans.

An increase in plague levels in these animals is often accompanied by a die-off in the population, though Idaho wildlife officials have not detected one this year.

Wild and domestic cats are highly susceptible and quickly exhibit symptoms like humans. They have a high likelihood of encountering plague during an outbreak in the wild because they hunt rodents. Transmission of plague from affected cats is a serious health risk for human.


What we refer to as the plague is a disease caused by an extremely virulent bacterium named Yersina pestis. It was the hidden agent behind the Black Death of the 13th to 15th centuries, responsible for killing one-fourth to one-third of the population of Europe. The disease is far older, likely even referenced in the Bible.

The first recorded pandemic of the plague, known as the Justinian plague, happened in the 5th century and ravaged the Mediterranean and surrounding regions. It is estimated that this plague killed more than half the population and is considered by some to be one of the key factors in the fall of the Roman empire.

The oldest confirmed plague victims from around 3,800 years ago suggest that the plague as we know it originated in the Bronze Age somewhere in Central Asia.


The modern pandemic of the plague emerged in China in 1855 and was spread around the world by ships. It was introduced to America by an infected ship from Hong Kong that docked in Hawaii and then San Francisco in 1899.

Genetic analysis and historical records shown that all strains of plague in the U.S. can be traced back to that single import of the disease. It quickly spread from urban rats to native species through fleas and was then carried east to Arizona and New Mexico by the 1930s, seemingly contained to the U.S. west of the 100th meridian.

Since then, the plague has slowly moved northwards, making it to Idaho in the 1940s. There have been just five reported cases of the plague in humans in the state’s history.


According to the CDC, the primary delivery agent is the bite of an infected flea. The fleas can transmit the disease between infected wild animals or pets and humans.

Direct contact with a sick or dead animal can spread the disease as well.

A third method of transmission is through inhaling infected droplets exhaled by sick people or animals.

People should avoid feral animals and protect their pets with flea control according to a statement from Sarah Correll, epidemiologist with Central District Health Department. “People can be exposed to plague when pets bring infected fleas back into the home, by caring for a sick pet or feral animal without proper precautions, or by contact with rodents carrying fleas.”


There are many different forms of the plague, all caused by Yersina pestis, but three forms are the most common, as described by the CDC.

Bubonic plague accounts for more than 80 percent of the cases seen in the U.S. It is usually transmitted by flea bites. Swollen lymph nodes, called buboes, appearing in the groin or armpit are the most visible symptom and give this form its name.

Septicemic plague occurs when the bacteria get into the bloodstream. This can happen through flea bites, from handling an infected animal with an open wound, or as a complication from untreated bubonic plague.

Pneumonic plague develops when the bacteria get into the lungs, either by breathing them in or from advanced untreated plague. This is the only form of the plague that can be spread from person to person.


Bubonic plague has flu-like symptoms such as fever, headache, chills and weakness. It’s accompanied by swollen lymph nodes, usually in the groin or armpit, making it the most obvious form.

Septicemic plague symptoms can fever, headache, chills and extreme weakness. More serious symptoms include abdominal pain and hemorrhaging. Skin, especially on the fingers, toes, and nose, can turn black and die. That’s what gave the Black Death its name.

Pneumonic plague is the most serious form. Symptoms include fever, headache and weakness that rapidly develops into pneumonia with shortness of breath, chest pain and cough with clear or bloody mucous. Without swift treatment, it is almost certainly fatal. Luckily, it is rare with less than 2 percent of cases developing in the U.S. since 1970.


Bottom line: If you have flu-like symptoms as described above, have visited or live near a region known for plague, or have recently encountered a sick or dead animal, seek medical attention.

Plague is treated using swiftly administered antibiotics, preferably within 24 hours of the first symptoms in case of pneumonic plague.

The mortality rate in cases treated with antibiotics is 8 to 10 percent, according to the World Health Organization. Untreated, the situation is much worse, with a 66 percent mortality for bubonic plague and a near 100 percent rate for septicemic and pneumonic plagues.

Fast diagnosis and treatment is the key. There is no vaccine against the plague.


The chances of getting the plague are, luckily, quite low. But there are many things you can do to prevent it effectively.

Because rodents are the primary carriers, anything you can do to reduce rodent habitat around your home or elsewhere is good. Avoid clutter, piles and wood stacks in your yard and remove food supplies rodents can get to.

If you must handle animals that may be infected, wear gloves. Contact the health department if you have questions about disposal of dead animals and especially if you find multiple carcasses.

Do what you can to avoid fleas on your person and your animals. Use repellant if you think you could be exposed while hiking or camping. Wear long-sleeved shirts and long pants when in areas identified to have plague. Talk to your veterinarian about using appropriate flea control on your pets.

If your pet becomes sick, take it to the vet immediately. Do not let animals that roam freely in known plague areas sleep in your bed.


You can get detailed information about plague and its history from the Center of Disease Control and Prevention:

To learn more about outbreaks of plague in Idaho and to report dead rodents, visit the plague information page from the Idaho Department of Fish and Game:

Kevin Davenport is an experimental physicist at the University of Utah and a 2018 AAAS Mass Media Science and Engineering Fellow with the Idaho Statesman: 208-377-6411, @tropnevaDniveK.