Spider Bite or Skin Infection? How Doctors Actually Tell
By IdentifyThese Editorial · · 3 min read
A large proportion of wounds that arrive at clinics described as spider bites are not spider bites. In several published series of suspected necrotic arachnidism, the majority turned out to be bacterial skin infections — most often community-acquired MRSA.
This is not a trivia point. It is the reason the distinction matters urgently: an infection gets worse while it is being treated as a bite.
This describes how the question is approached. It is not medical advice, and a spreading or painful skin lesion is a reason to be seen.
The single most useful question
Did you see a spider bite you?
Not "was there a spider in the room". Not "I found one later". Actually seeing the animal make contact with the skin.
If the answer is no, a spider bite is a guess, and it is usually the wrong one. Spiders bite defensively when trapped against skin — the event is normally noticed, even if it is not immediately painful.
What tends to point where
| Feature | More consistent with infection | More consistent with a bite |
|---|---|---|
| Onset | Over days, worsening steadily | Sudden, at a known moment |
| Number of lesions | Sometimes several, or recurring | One |
| Fever, feeling unwell | Common | Uncommon except in systemic envenomation |
| Pus, a central point that drains | Common | Not typical |
| Household contacts with similar sores | Strongly suggestive | No |
| Recent gym, contact sport, shared towels | Relevant history | Irrelevant |
| Spider seen in the act | No | Yes |
Why "necrotic bite" is over-diagnosed
Recluse bites can cause tissue damage. That is real. But the reasoning frequently runs backwards: a wound looks necrotic, therefore a recluse, therefore there are recluses here.
That chain breaks at the first link. Many things produce a similar-looking lesion — bacterial infections, fungal infections, vasculitis, pyoderma gangrenosum, diabetic ulcers, chemical burns, even certain drug reactions. A great many of them respond to specific treatment, and a spider bite mostly does not, so getting this wrong costs time in one direction only.
The problem is worst outside the recluse range, where the spider being blamed does not live. See do brown recluses live where you do.
The two-lesion rule of thumb
Spiders bite once. They are not mosquitoes, they gain nothing from repeating it, and a defensive bite is a single event.
So two or more sores appearing around the same time, or a sore that resolves and is followed by another somewhere else, points away from a spider and towards something in the body or something being passed between people. Recurring lesions in more than one household member is close to conclusive, and it is worth mentioning unprompted — clinicians ask about it for a reason.
When to get seen quickly
- The area is spreading, especially with a visible advancing edge
- Increasing pain out of proportion to how it looks
- Fever, chills, or feeling generally unwell
- Red streaking away from the site
- Any lesion on the face or near a joint
- You have diabetes or a weakened immune system
What is worth doing
If you genuinely saw the spider and it is safely contained, keep it — identification can be useful, and it costs nothing. If you did not see one, resist the temptation to construct the story backwards from the wound.
Either way the wound gets treated on its own merits, and identification never delays that.
For the one bite type that presents systemically rather than locally, see what actually happens after a widow bite.
If you think you were bitten
- Most suspected spider bites are not spider bites. Studies of patients presenting with a “spider bite” repeatedly find that the large majority are bacterial skin infections — frequently MRSA — which need antibiotics rather than reassurance. A spreading, painful, pus-filled sore is far more likely to be an infection than a spider.
- Wash it, ice it, and watch it. Clean the area with soap and water, apply a cold pack, and keep an eye on it.
- Get medical care if the wound spreads or darkens, if you develop cramping, sweating, fever or severe abdominal pain, or if you are confident it was a widow or a recluse.
- Bring the spider only if it is already dead and to hand. Do not go looking for it. Doctors treat symptoms, not species.
In the US, the Poison Help line is 1-800-222-1222. This site is a reference, not medical advice.
Keep reading
- A Black Widow Bit Me. What Actually Happens Next?The realistic timeline, why the pain is muscular rather than local, when antivenom is actually used, and how rare a fatal outcome really is in the United States.
- How to Know If It Really Is a Brown RecluseSix eyes in three pairs, uniform tan legs, and no pattern anywhere. The violin is the feature everyone checks first and the least dependable one on the animal.
- Do Brown Recluses Come Out During the Day?They are nocturnal, but that is not the useful answer. What matters is when and why they end up on an open floor, and which encounters actually lead to a bite.