Diagnosing Black Widow Bites

We occasionally get a question from a reader that leads to an article. This question has to do with the way that spider bites are diagnosed in the emergency department. The reality is that most lesions diagnosed as spider bites aren't really spider bites at all. Most of them are skin infections from bacteria like streptococcus or staphylococcus aureus

A black widow spider
Wikimedia Commons/Shenrich91 ( CC SA license)

From a reader:

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

Diagnosing Black Widow Bites Is Not Simple

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 (you should check out 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 in the area of the bite.


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

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

2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. 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

  2. Poison Control. Black Widow Spider Bites.

Additional Reading

By Rod Brouhard, EMT-P
Rod Brouhard is an emergency medical technician paramedic (EMT-P), journalist, educator, and advocate for emergency medical service providers and patients.