If I had a dollar for every time a patient told me that they were bit by a spider, I could retire (or at least buy a condo in San Diego).
Poisonous spider bites are extraordinarily rare; wounds blamed on spiders are extraordinarily common. I have seen dozens of patients who thought they had been bitten by a spider, and I have never made a diagnosis of an actual spider bite. That is until now.
It started off as a phone call from a primary care physician. He described a man that he felt had a spider bite on his hand. I was a little disappointed at first; usually the primary care docs are pretty good at dispelling the blame-it-on-a-spider myth, but this guy was convinced.
“Well, I don’t know,” I said, “send him over and I’ll take a look.” In dermatology, the skin always tells the best story.
When my patient arrived I took one look and thought: Wait a minute, this might be the real thing. It didn’t look like a typical staph or MRSA infection (which is almost always the true diagnosis).
The patient had been cleaning his garage in a rural, desert part of the county and had pulled a gas stove out of his attic. He thought nothing of it until several hours later when his finger began to throb. The next day he had pain and swelling of the middle finger on his right hand. This progressed to redness and tenderness spreading up his hand and arm. When I saw him, it was 3 days later, and the redness had subsided, but he had this purple-red wound on the back of his finger.
Now in truth, I cannot definitively diagnose him with a spider bite unless he either saw a spider bite him or knew for a fact that his garage was infested with spiders. Poisonous spiders are rare in southern California and spiders that cause local necrosis even more rare.
Despite the widespread belief that skin wounds are caused by bites from various, ferocious spiders, only a handful of spiders can give necrotic (deep, dead skin) wounds. The most likely culprit of such wounds, the brown recluse spider, Loxsoceles reclusa, does not live in southern California.
There is another plausible explanation; there is a close cousin of the brown recluse called Loxsoceles deserta that does live in the desert regions of southern California. It is possible he was bitten by one of those spiders.
If you think you have been bitten by a spider, remember these facts:
- Spider bites are exceedingly rare. Studies have shown that the number of spider bites attributed to spiders far exceeds the number of poisonous spiders living in that area. For example, in South Carolina, there were 478 diagnoses of brown recluse spider bites one year, but entomologists were only able to confirm brown recluse spider in the whole state (it must have been the “Jaws” of spiders).
- If you did not see a spider bite you, then it is very unlikely that you were bit by a spider.
- Spiders do not come out at night to bite you. They are, in fact, reluctant to bite, even when provoked.
- Very few spiders in the US (some would say only one spider, the brown recluse) are likely to cause a necrotic wound.
- The black widow spider is also a poisonous spider, but it releases a neurotoxin that causes abdominal pain and paralysis. It does not cause a necrotic skin wound.
Brown recluse spider with characteristic violin clearly visible on its back.
My final diagnosis: Spider bite. I think.
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