How Dangerous Is a Black Widow? Venom Facts and Real Risk

How strong is black widow venom?

Widow spiders, genus Latrodectus, produce venom containing alpha-latrotoxin, a nerve toxin that acts on vertebrates. It makes nerve endings release a flood of chemical messengers at once, which causes the pain, cramps and sweating of a widow bite. Doctors call this condition latrodectism.

Measured by the dose needed to kill laboratory mice, widow venom is very potent, more so than the venom of many rattlesnakes. Potency is only half the story, though. A widow is a small spider that injects a tiny droplet of venom, and some bites inject little or none at all, known as “dry” bites.

Only adult females matter. A female southern black widow has a body about 8–13 mm (0.3–0.5 in) long and fangs about 1 mm (0.04 in) long, enough to pierce skin. Males are a fraction of her size and pose no real risk.

What happens after a black widow bite?

The bite itself often feels like a pinprick and may leave little or no visible mark. If venom was injected, pain usually builds at the bite within about an hour, sometimes with local sweating and goosebumps.

In more significant cases, the pain spreads, for example from a hand up the arm into the chest, or from a foot into the abdomen and back. Typical effects include:

• Painful muscle cramps and stiffness, often worst in the abdomen, back or chest
• Heavy sweating, sometimes only in one area of the body
• Nausea, vomiting and headache
• A raised heart rate and blood pressure, restlessness and a general feeling of being unwell

Symptoms usually ease within one to a few days. Some people feel ill for up to a week, and in rare cases aches or weakness linger for weeks. Abdominal cramping can be severe enough to be confused with other medical emergencies.

How often do black widows bite people?

A large review of US poison center records counted 23,409 widow spider exposures in 47 states from 2000 to 2008, an average of about 2,600 a year. Only 9,872 of these people had any symptoms. Among them, about 65% had minor effects, 33.5% moderate effects and 1.4% major effects, the most serious category. No one died. Reported bites peaked in September and were lowest in January and February.

Most bites happen when a spider is pressed against skin: reaching into a woodpile, putting on a glove or boot left in a shed, or moving stored boxes or outdoor furniture. Black widows are not aggressive. In experiments with western black widows, the spiders seldom bit when they were simply poked and bit mainly when squeezed. Their first choices are to retreat, drop out of the web, play dead or flick sticky silk at the threat.

Who is most at risk?

Healthy adults almost always recover fully. Serious complications are reported most often in very young children, older adults, pregnant women and people with heart disease or high blood pressure.

It helps to put the risk in context. Bees, wasps and hornets cause an average of about 60 deaths a year in the United States, mostly through severe allergic reactions, while confirmed black widow deaths have become exceedingly rare.

Have black widow bites ever killed anyone?

Yes, in the past. Reports from the early 20th century gave death rates from well under 1% to several percent of bites, mostly from small case series, often without modern hospital care. In Europe, bites from the Mediterranean black widow came in waves during hand harvests of grain, and deaths were reported. In 1933 a University of Alabama medical professor, Allan Blair, let a black widow bite him to prove its venom affects people; he spent about two days in hospital in severe pain.

Today the picture is very different. Intensive care, effective pain relief and, for some patients, antivenom mean that deaths are almost unknown in the US. In Australia, no deaths have been directly linked to the closely related redback spider since an antivenom was introduced in 1956.

Black widows compared with other dangerous spiders

Black widows and recluses are the only US spiders of real medical importance. The recluse is compared in how dangerous is a brown recluse?, and the world’s most dangerous species are ranked in most venomous spiders.

When to call emergency services

Anyone who develops spreading pain, muscle cramps, chest pain, difficulty breathing or heavy sweating after a suspected widow bite should seek emergency medical help immediately: call 911 in the US, 112 in the EU or 999 in the UK. The same applies if a young child, a pregnant woman, an older person or someone with heart disease may have been bitten. If it can be done safely, a photo of the spider helps doctors.

The best protection is simple respect. Wear gloves when moving firewood, rocks, bricks or stored items, shake out shoes, boots and gloves left in garages or sheds, and look before reaching into dark corners. Knowing where black widows live makes them easy to avoid.