The brown recluse (Loxosceles reclusa) belongs to a group of spiders called recluse or brown spiders. In North America they are also called violin spiders, because many have a dark mark shaped like a violin on the front part of the body [4]. The brown recluse is the most widespread of these spiders in the United States [3].
If your dog or cat has a red, swollen patch, a blister or a sore that will not heal, see your vet.
Where recluse spiders live
Recluse spiders are found in many parts of the world, with more than 140 species described [4]. In the United States, brown recluse bites are mostly seen in the Midwest and the South [3][4]. As the name suggests, these spiders tend to stay hidden [4].
Why a bite can go unnoticed
A recluse bite is often hard to notice when it happens [3], and the spider is rarely seen. A pet cannot tell you it was bitten, so the first thing you may see is the skin change that follows.
That also means a sore can be blamed on a spider when something else is the cause. In one reported case, a cat’s swollen, draining leg wound was first treated as a brown recluse bite; tests a few weeks later showed it was a tumour [2]. There is no simple test that proves a pet was bitten, and vets work through a list of criteria before calling a sore a spider bite [1].
Skin signs
Most of what is known about these bites comes from people, and skin changes are by far the most common result [3][4]. They can include redness, swelling, pain and itching at or near the bite [4]. In one reported case the bite site was a swollen red patch with a blood-filled blister in the middle [3].
In some bites the skin at the centre dies and forms a dark scab, which happened in about half of the human cases in a large review [3]. The damaged area can also spread downward from the bite over time [4]. A sore like this can be slow to heal.
Signs beyond the skin
Whole-body illness after a recluse bite is much less common than skin damage, but it can be serious [3][4]. In people it can include fever, breakdown of red blood cells (which can cause pale gums, yellowing of the skin or dark urine) and kidney damage [4]. In human reports, red blood cell problems have shown up anywhere from about 2 to 7 days after the bite [4].
Treatment
Your vet will advise on treatment for your pet. A review of brown recluse bites in small animals found no agreed treatment beyond supportive care, and the outlook depends on how severe the signs are and how the pet responds to that care [1].
At home, as directed by a vet:
- do not open, squeeze or drain a blister or sore yourself;
- do not give your pet human or leftover medicines unless your vet says so;
- do not stop or change any medicine without talking to your vet;
- give any treatment exactly as your vet prescribes, for as long as your vet says;
- go back for any recheck your vet books.
When to see your vet
See your vet if your dog or cat has:
- a red, swollen or itchy patch on the skin [4];
- a blister, especially one with blood in it [3];
- a sore that grows, spreads or heals slowly [4];
- a dark scab where the skin has died [3];
- fever, pale or yellow gums, or dark urine [4];
- any spot, blister or sore you are unsure about.
See your vet now if your dog or cat seems weak, collapses or is breathing fast.
The bottom line
A brown recluse spider bite can start as a red, swollen patch and turn into a blister and a slow-healing sore. The bite itself often goes unnoticed, and other problems can look the same, so only a vet can work out the cause of a sore like this.
This article is for general information only and does not replace advice from your veterinarian. If your dog or cat has a blister or a sore that will not heal, see your vet.
Sources
- Pace LB, Vetter RS. Brown recluse spider (Loxosceles reclusa) envenomation in small animals. Journal of Veterinary Emergency and Critical Care. 2009;19(4):329-336. PubMed abstract. https://pubmed.ncbi.nlm.nih.gov/25164631/
- Iqbal W. A peculiar case of soft tissue sarcoma in a cat. Canadian Veterinary Journal. 2020;61(1):79-81. PubMed abstract. https://pubmed.ncbi.nlm.nih.gov/31892760/
- Hart SA, Gailani D, Bibb LA, Zwerner JP, Booth GS, Jacobs JW. Coagulation abnormalities following brown recluse spider (Loxosceles reclusa) envenomation: a description of 2 cases and review of the literature. American Journal of Clinical Pathology. 2025;163(6):822-836. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC12137049/
- Gremski LH, da Justa HC, Polli NLC, et al. Systemic loxoscelism, less frequent but more deadly: the involvement of phospholipases D in the pathophysiology of envenomation. Toxins. 2023;15(1):17. Open access, CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC9864854/