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Cat bites to the hand can cause serious infections, Mayo study finds

The small joints and tendons make great hiding places for bacteria.

Cats' sharp teeth make relatively deep puncture wounds that are prone to infection.
MinnPost photo by Corey Anderson

Cat bites should be taken seriously, for they can lead to difficult-to-treat bacterial infections — in fact, much more so than dog bites — according to a new study by researchers at the Mayo Clinic.

The study, published in the February issue of the Journal of Hand Surgery, found that almost a third of the people who sought treatment at the Mayo Clinic for a cat bite to the hand during a recent three-year period had to be hospitalized. And of the patients who were hospitalized, two-thirds ended up needing surgery to flush out the bacteria and remove infected tissue.

The Mayo Clinic researchers undertook the study to see if they could identify any new risk factors that could help predict which patients who seek treatment for a cat bite to the hand will eventually require hospitalization. Most of what they found, however, simply confirmed what they already knew: Such bites are more likely to lead to hospitalization when they occur over a joint or tendon and are accompanied by swelling, redness and pain. The risk increases for people with existing immune-deficiency disorders.

“The hand surgery community isn’t so shocked by our paper, but I do think the public may be surprised,” said Dr. Brian Carlsen, a hand surgeon at the Mayo Clinic and one of the study’s authors, in a phone interview Monday.

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After all, when most people think of dangerous animal bites, they tend to think of dogs, not cats.

Sharper teeth

What is it about cats that make their bite wounds so prone to infection? Their teeth are sharp and make relatively deep puncture wounds. When the bite is on the hand (and other research has shown that up to 85 percent of cat bites occur on the hand or wrist), the puncture may easily pierce a joint or the membrane sheath around a tendon. Joints and tendons have closed spaces, and are thus great growing places for bacteria. And the mouths of cats (like those of dogs — and of humans, for that matter) are home to many types of bacteria.

Dr. Brian Carlsen
Dr. Brian Carlsen

“When bacteria gets into the joints or into the tendon sheaths, that’s a surgical emergency, no matter what the source,” said Carlsen. That’s because antibiotics tend to be unable to reach those infections. Only through surgery can the bacteria be washed out and removed.

If the surgery is delayed too long, patients can develop permanent damage to the hand, including a loss of joint mobility. Those patients may then require reconstructive surgery to replace the damaged tendons or joints.

“I have seen some really bad infections [from cat bites] that have required multiple operations,” said Carlsen. “I had one patient, a farmer, whose tendons were destroyed on the back of his hand by the infection. He couldn’t straighten his fingers out. We had to reconstruct the tendons.”

Study details

For the current study, Carlsen and his colleagues looked at the medical records of people who had shown up at the Mayo Clinic’s emergency room in Rochester with a cat-bite wound to the hand between January 2009 and December 2011. They were interested only in people who had been bitten on the hand because those are the wounds that tend to cause the most problems.

The Mayo Clinic researchers also were interested only in people who had been bitten by house cats, and therefore eliminated three patients from the study who said their wounds had been caused by either a lynx or a bobcat. (Unfortunately, information about how or where those bites occurred wasn’t part of the study. “I was surprised by them, too,” said Carlsen.)

In the end, the researchers identified 193 patients who fit their criteria. They then dug down further into those patient’s medical data. They found that 57 of the patients had been hospitalized. Thirty-six had been hospitalized immediately after showing up in the emergency room so that they could be given antibiotics intravenously. The other 21 had been initially sent home from the emergency room with oral antibiotics, but were subsequently hospitalized when that treatment failed. The average length of hospital stay for all of these patients was three days.

Twenty-six of the 36 people (72 percent) who had been immediately hospitalized and 12 of the 21 (57 percent) who were hospitalized after first trying oral antibiotics ended up requiring surgery. Eight patients required more than one surgery.

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The data also revealed that people took an average of 27 hours to seek medical care after their hand was bitten. In addition, the mean age of the patients who showed up at the Mayo Clinic with a cat bite to the hand was 49 years. And most of the patients (69 percent) were women.

Don’t ignore symptoms

Up to 2 percent of all emergency room visits in the United States are for animal bites, according to background information provided in the Mayo Clinic study. Most of those bites (60 percent to 90 percent) involve dogs, but about 10 percent to 15 percent are from cats.

Serious injuries from cat bites are, therefore, relatively rare. Still, if you receive a cat bite, even if it’s a “playful” bite, you should keep alert for signs of infection. Seek medical care if you develop any swelling, redness and pain (particularly pain that worsens), or if you’re having any difficulty moving the hand.

“It takes only a very superficial bite,” said Carlsen. “So, if you have symptoms, don’t ignore them.”