Catching an illness from a pet is rare, and people don’t usually have to worry about their own health just because their furry friends are feeling under the weather. But a truly terrifying case of cat-to-human transmission described in this month’s New England Journal of Medicine is enough to give even the most fervent feline fanatics reason to be cautious: According to his doctors, a 68-year-old man caught a life-threatening infection from his sick cat—and developed large, swollen bulges on his face and neck as a result.
The patient in question visited his primary care doctor after experiencing a week-long fever followed by two months of “progressive, painful swelling on the right side of his neck,” his doctors wrote. That swelling turned out to be his lymph nodes; further tests revealed that the man had been infected with Francisella tularensis, a highly contagious, toxic bacterium.
The patient told his doctors that two days before his symptoms began, his outdoor cat had died of what a veterinarian had diagnosed as feline leukemia. But that diagnosis had never been confirmed with lab tests, and doctors now suspect that the cat was sick from Francisella tularensis as well.
So what exactly is this bacterium, and how common are infections? And what’s up with those “giant boils,” as the Daily Mail called them? We spoke with Andrej Spec, MD, assistant professor of medicine at Washington University School of Medicine and co-author of the report, to get the whole scoop.
What is tularemia?
Doctors diagnosed the unnamed patient in the case report with tularemia, the term used for the disease caused by Francisella tularensis. Tularemia is rare in humans: In 2016, the most recent year with accurate data available, there were 230 cases diagnosed in the entire U.S.
The disease is much more common in animals, says Dr. Spec, mostly in wild rabbits and mice. (Cats can become infected if they attack a sick mouse.) The disease can also be carried by ticks and deer flies.
Tularemia has been diagnosed in every state except for Hawaii, according to the U.S. Centers for Disease Control and Prevention (CDC). It’s most common in the south central United States, the Pacific Northwest, and parts of Massachusetts, including Martha’s Vineyard.
Missouri is also a “hot spot” for tularemia, says Dr. Spec, although he admits that term is relative. “I’ve seen maybe three cases in my whole life,” he says. Still, when he saw the patient’s symptoms and heard his story, he suspected right away that he’d been infected with Francisella tularensis.
How do humans become infected?
Humans can be infected with Francisella tularensis through a tick or deer fly bite, by handling sick or dead animals, or by breathing in bacteria particles from these animals. “Sometimes, a person will be mowing their lawn and will mow over a baby rabbit, and that can aerosolize the infection,” says Dr. Spec.
The infection can also be passed from an animal to a human through a scratch or bite, which is what he suspects happened with his patient. The patient had been giving the cat medicine for what he thought was feline leukemia. “If you’ve ever tried to give a cat medicine, you know that they hiss, they spit, they bite and scratch, and that’s what happened here,” he says.
Because the bacterium that causes tularemia can be spread through the air or the water, there’s also concern that it can be used as a bioterrorism weapon. The good news, though, is that it can’t spread from animal to human without some type of bite, scratch, or other contact with blood or saliva. “You won’t get sick just by being around a sick animal,” says Dr. Spec.
What are the symptoms of tularemia?
There are several types of tularemia. The type that this patient had, called glandular tularemia, is defined by swollen lymph nodes and a high fever. Another type, called ulceroglandular tularemia, is similar but also causes skin ulcers, or open wounds, at the site of the infection.
Other types of tularemia can involve swollen and irritated eyes, sore throat, mouth ulcers, and swollen tonsils. Pneumonic tularemia is the most serious form of the disease and is most likely to occur when the bacterium is inhaled. This type can cause cough, chest pain, and difficulty breathing.
Any type of tularemia can become life-threatening if left untreated, says Dr. Spec, but the pneumonic type is the most dangerous. Luckily, most cases can be treated with antibiotics; Dr. Spec’s patient, for example, was given four weeks of doxycycline and his lesions went away within three.
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How can you protect yourself?
Wearing insect repellant and avoiding sick or dead animals are two easy ways you can protect yourself against tularemia (and other dangerous bacteria that could be carried by these critters). But animal lovers should also be aware that pets can also carry disease, says Dr. Spec—especially cats.
“I have nothing against cats and I enjoy hanging out with them, but there tend to be a lot more human infections from cats than from dogs,” he says. (Cat scratch disease, also known as cat scratch fever, is another bacterial infection that can be passed from felines to people. In 2016, the CDC said that human infections were on the rise.)
“If you just get scratched by a cat and you don’t develop symptoms, you’re OK—but if you start to develop a fever or these nodules, come see us right away,” says Dr. Spec. Anyone who gets bitten by a cat should see a doctor regardless of symptoms, he adds, because they should be treated with prophylactic antibiotics to ward off any potential infection.
That being said, dog owners aren’t totally in the clear, either. Two recent cases of serious infections—one fatal—caused by dog saliva show that toxic bacteria can be transmitted by man’s best friend as well. Dogs, cats, and other animals can also pass on rabies, salmonella, ringworm, and other diseases, too.
The bottom line, says Dr. Spec, is to get to the doctor as soon as possible if you develop strange nodules on your neck or any other area of the body. The armpits and groin, which also have lymph nodes, are other common areas for swelling from this type of infection.
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