How is a black widow bite diagnosed?

There is No Test for Latrodectism

Black Widow Spider
GK Hart/Vikki Hart/Stone/Getty Images


How is a black widow bite diagnosed?

From a reader:

My 20-year-old son was bitten by a black widow the other day. He had all the symptoms, went to ER and was given Morphine via IV for pain, ALL day long! Doctor's did not find anything in his blood. Why not?


There isn't a blood test for black widow bites. The best way to diagnose a black widow bite is to feel the sting and look down in time to watch the spider rub its belly in satisfaction.

Other than catching the spider in the act of biting, black widow bites are diagnosed through a bit of detective work.

Latrodectism is the medical term for black widow spider envenomation, and it's not easy to identify. Doctors have to diagnose black widow bites by asking patients when they started to feel the spider bite symptoms, how they discovered their bites, and whether they saw the spider.


Black widow (and brown widow) spider bites are known to cause severe pain and muscle cramping, especially in the abdomen and back. In many cases, the patient never feels the bite—just the cramps. If you do see the spider, it will be shiny and black (in the United States that is, but brown in most other parts of the world). Most black widows have an hourglass-shaped red mark on their abdomens.

Black widows make webs, which not all spiders do. They're called black widows because the female is known to kill males after mating.

Common Mistakes

Folks often think any skin lesion is a spider bite (see our spider bite pictures for examples of spider bites and skin infections). Most of the time, these lesions are bacterial infections like MRSA or streptococcus. Black widow bites don't look the same as these skin lesions. Sometimes, a black widow bite will look like two small holes.

There will probably be some swelling and redness at the area of the bite.


Black widow spider bites are rarely fatal. The pain can be severe, even in relatively benign envenomations.

The bites can be treated in the emergency room with IV infusions of calcium and narcotic pain relievers. There is also an antivenin for severe reactions.

It sounds like this reader's experience was pretty typical. I'm glad he's all right.


Monte AA, Bucher-Bartelson B, Heard KJ. A US perspective of symptomatic Latrodectus spp. envenomation and treatment: a National Poison Data System review. Ann Pharmacother. 2011 Dec;45(12):1491-8. doi: 10.1345/aph.1Q424. Epub 2011 Nov 24. Review.

Timms, P.K., and R.B. Gibbons. "Latrodectism - Effects of the Black Widow Spider Bite." West J Med. Mar 1986.

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