Are House Spiders Dangerous? Bites, Venom and Real Risk

Do house spiders bite?

House spiders are shy animals that live in quiet corners and run or freeze when disturbed. They have no interest in people as food, and they do not attack. Nearly all bites happen when a spider is pressed against the skin, for example:

• In clothing, shoes or gloves that have been lying on the floor
• In bedding, when someone rolls onto a spider
• When someone tries to pick a spider up with bare hands

Even then, many house spiders cannot bite effectively. Small species such as cobweb spiders and cellar spiders have fangs that are often too short or weak to break human skin.

What happens if a house spider bites you?

The giant house spider, one of the largest spiders in European homes, has fangs that can pierce skin. A bite typically causes a sharp pinprick, followed by local pain, redness and slight swelling that fade within hours to a day or two, much like a bee sting.

Allergic reactions to spider bites are possible but rare. Anyone who has severe symptoms after a bite should seek emergency medical help immediately: call 911 in the US, 112 in the EU or 999 in the UK.

Which spiders found in homes can be dangerous?

A small number of spiders that sometimes live in or near buildings have venom of medical significance.

Widow spiders (Latrodectus) live across much of North America and in southern Europe. They favor sheds, garages, woodpiles and outdoor furniture more than living areas. Their neurotoxic venom can cause severe pain, muscle cramps and sweating. Deaths are extremely rare today, but bites need medical care. See Are all spiders venomous? for how widow venom compares with other spiders.

Brown recluse (Loxosceles reclusa) lives in the south-central and midwestern United States, where it can be common inside houses. Most bites are minor, but a minority cause a slow-healing skin wound. It does not live in most of North America or in Northern Europe, despite frequent claims. Read Where do brown recluse spiders live?.

Noble false widow (Steatoda nobilis) has spread widely in Britain and Ireland and is often found on and in houses. Its bite can be painful, with swelling, but it is not considered dangerous to healthy people. See spiders in the UK.

Why are spider bites often misdiagnosed?

Spiders get blamed for far more than they do. Doctors and arachnologists, including the American researcher Richard Vetter, have shown repeatedly that most skin sores attributed to spider bites are caused by something else. Common culprits include:

• Bacterial skin infections, especially those caused by MRSA (methicillin-resistant Staphylococcus aureus)
• Bites from insects such as fleas, bedbugs, mosquitoes and ticks
• Allergic reactions, shingles, Lyme disease rashes and other skin conditions

A large share of reported “brown recluse bites” come from regions where the spider does not live at all. Studies of patients who arrived at emergency departments with suspected spider bites found that most actually had skin infections. Mistaking an infection for a spider bite can delay the right treatment, which is one reason doctors are cautious about the diagnosis.

A bite can only be confirmed when someone sees the spider biting and the spider is identified by an expert. Without that, the diagnosis is a guess.

How to tell a harmless house spider from a risky one

Most large brown spiders running across a floor in Europe or Canada are giant house spiders or their relatives: long-legged, hairy, banded and fast. Widow spiders look very different, with a shiny, round, usually black abdomen and, in many species, a red or orange marking, and they hang in messy, tangled webs rather than running in the open. The brown recluse is a plain, medium-sized brown spider with six eyes instead of eight and a dark violin-shaped mark behind the eyes, though this mark is hard to see without magnification.

When in doubt, the safest approach is simply not to handle the spider. Using a glass and a sheet of card to move it, or leaving it alone, avoids any chance of a bite.

Why house spiders are more help than harm

House spiders eat flies, moths, mosquitoes, woodlice and other small invertebrates, including other spiders. They live quietly for years in the same corner and play a small part in keeping indoor pests down.

They also have good reasons to avoid us. To a spider, a human is a huge, dangerous animal, and running away is a far safer strategy than biting. That is why even big, fast species like the giant house spider so rarely bite despite sharing homes with millions of people. For the reason these spiders become so visible each autumn, see Why do spiders come indoors in the fall?.