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Is That A Brown Recluse Spider Bite Or Skin Cancer?

A recluse spider bite (left) can be confused with skin cancer.

The best way to diagnose a strange skin bump is often to decide what it's not. So say the researchers who have devised a mnemonic device useful for determining that the lesion or lump isn't a bite from a brown recluse spider — a diagnosis that can be very common and very false, they say.

Around 40 conditions have been or could be misdiagnosed as a nibble from the brown recluse, including Lyme disease, herpes, antibiotic-resistant staph infection, diabetic ulcers and skin cancer. You wouldn't want to miss out on treatment for any of those things because the doctor said you had a spider bite. And the brown recluse's venom can cause serious wounds or kidney failure, so isn't to be trifled with, either.

The aptly named mnemonic NOT RECLUSE could help limit misdiagnoses and make sure people get the right treatment. It stands for things that would not be a factor if a patient was bitten by a brown recluse, like numerous bites (N), the circumstances where the wound occurred (O), or the time of year it came about (T). Tally two or more NOT RECLUSE symptoms, and the brown recluse explanation should be thrown out, the researchers suggest in their JAMA Dermatology paper.

Brown recluse bites have a reputation for causing pus and rotting flesh, so many people assume they've been bitten if they have a nasty wound. But only 10 percent of the bites ever need medical attention. In the most serious cases, skin turns black and starts to die in what's called skin necrosis. If you know you've been bitten for sure, the Centers for Disease Control and Prevention recommends seeking immediate medical attention and to bring the spider with you for identification.

Most typically, though, a bite from the spider looks like a pimple or a small white blister and heals on its own over a month or two. And the spiders, just larger than a penny, won't bite unless threatened. One woman even trapped over 2,000 of the arachnids in her own house over a six-month span, and no one in her family was ever known to have been bitten.

Many people self-diagnose another skin condition as a brown recluse bite because "it's a sexy diagnosis," says Rick Vetter, a retired entomologist of the University of California, Riverside, a brown recluse expert and co-author on the study. "It's like saying you were bitten by a rattlesnake. It's dramatic. Bacterial infections? Not so much." These self-diagnoses even happen outside the brown recluse's range of Southern and central Midwestern states in the U.S.

Physicians tend to falsely diagnose brown recluse bites, Vetter says, because "it's a really comfortable scapegoat. People want a diagnosis. Doctors want to give them a diagnosis." So if physicians can't figure out the identity of an ambiguous bump, they jump to spider bite.

Vetter worked with two dermatologists on the paper; they hope that physicians will make use of NOT RECLUSE to ensure that patients get the right treatments.

"When we see people with cancer or MRSA who get this false diagnosis," Vetter says, "it really drives home that you can't just call everything a recluse bite."

Eager for the whole mnemonic? Here you go. More than two of these would typically exclude a brown recluse spider bite:

Numerous: More than one injury

Occurrence: Not in a secluded area like a box, closet or attic

Timing: The wound arose sometime between November and March

Red Center: The center of the wound is red

Elevated: The middle of the wound is elevated, not sunken

Chronic: The wound has persisted for more than three months

Large: The injury is more than 10 centimeters wide

Ulcerates Too Early: The injury gets crusty within the first week

Swollen: The wound swells up if it's below the neck

Exudative: The wound is "wet," oozing pus or clear fluid

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