Spider bite or skin infection?
Most wounds diagnosed as spider bites are bacterial skin infections. Unless a spider was seen biting, treat a hot, spreading, painful lesion as a possible infection — that assumption is right far more often and it changes the treatment.
Side by side
| Wolf spider bite | Brown recluse spider bite | |
|---|---|---|
| Looks like | a red swollen area with two faint puncture points | a red bump that may develop a pale or blue-grey centre and, in a minority of cases, an ulcer |
| Pattern | single site | single site |
| Size | 5–30 mm | 5–50 mm |
| Appears | immediate sharp pain | often painless at first |
| Lasts | usually resolves in a few days | most heal in weeks; a true necrotic lesion can take months |
| Itch | ●●○○○ 2/5 | ●●○○○ 2/5 |
| Pain | ●●●○○ 3/5 | ●●●●○ 4/5 |
| Usually on | hands, feet, lower legs | torso under clothing, upper arms, thighs — often where clothing was put on with the spider inside |
| Active | night — this spider hunts on the ground rather than building a web | night |
| Where from | lawns, gardens, garages and basements across the whole country | undisturbed storage — boxes, wardrobes, behind furniture — in the south-central states |
| Disease risk | none known in the US | none known in the US |
What actually settles it
Did anyone see a spider? If not, the odds strongly favour infection. Spiders bite defensively, when pressed against skin — usually while dressing, gardening or moving stored boxes.
Direction of travel. Infection expands and worsens over days, with heat and increasing pain. A genuine bite reaction is worst early and then settles.
Fever points to infection.
Multiple lesions are almost never spider bites — spiders bite once.
Why it matters: infection needs antibiotics. Wound care aimed at a suspected recluse bite while a staph infection spreads is a bad trade. If it is hot, spreading and painful, be seen.
When either one needs a doctor
- spreading redness, warmth or pus over the following days
- fever
- a wound that darkens, blisters or breaks down over days
- fever, chills or widespread rash
- rapidly spreading redness and pain — more often a bacterial infection than a spider
Questions
Can you have both at once?
Yes, and it happens more often than people expect — which is why the pattern of the bites matters more than any single mark.
Which one is more serious?
Neither carries a significant disease risk in the US. For both, the common complication is infection from scratching.
What if I still cannot tell?
Photograph the bites over several days and note where on your body they appear and when. Progression and position identify a bite far more reliably than a single photograph.
Get a photo assessment in the app
Everything above is free to read. The app adds what a page cannot: point your camera at the bite and get an assessment of the most likely cause, a severity read, and a day-by-day care plan — including the specific signs that mean you should be seen.
iPhone and iPad. Free to download.