Merck Manual

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Pet Owner Version

Spider Bites


Charles M. Hendrix

, DVM, PhD, Department of Pathobiology, College of Veterinary Medicine, Auburn University

Reviewed/Revised Oct 2020 | Modified Nov 2022

Poisonous spider bites are relatively uncommon and difficult to recognize. The bite of spiders of medical importance in the US is not particularly painful, so it is unusual for a spider bite to be suspected until signs appear. It is also unlikely that the offending spider will remain nearby the victim while signs develop, which may take 30 minutes to 6 hours. Poisonous spiders in the US belong to 2 groups—widow spiders (Latrodectus species) and brown spiders (mostly Loxosceles species).

Widow Spiders

Widow spiders usually bite only when accidentally touched. The most common species is the black widow spider, which has a red hourglass shape on the lower abdomen. Widow spider venom is very potent and causes severe, painful cramping of all large muscle groups.

Unless a widow spider bite has been witnessed, diagnosis must be based on signs, which include restlessness, anxiety, rapid and irregular breathing, shock, a rigid or tender abdomen, and painful muscles. The muscles may be rigid and will relax now and then.

Symptomatic treatment includes pain relievers and muscle relaxants. Tetanus antitoxin should also be given. Recovery may take some time, and weakness and partial paralysis may remain for several days.

Brown Spiders

There are at least 10 species of brown spiders in the US, but the brown recluse spider (Loxosceles reclusa) is the most common. Most of these spiders have a violin-shaped marking on the upper surface of the body, although it may be indistinct or even absent in some species. Brown recluse spider venom causes the blood vessels to narrow, which increases blood pressure and decreases blood flow. It also causes blood clots and destruction of red blood cells.

Unless a brown spider bite has been witnessed, a presumptive diagnosis may be based on the presence of a discrete, red, itchy spot on the skin that may bleed into surrounding tissue. Within 4 to 8 hours, a blister develops at the bite wound, and sometimes a pale area surrounds the redness of the lesion, so that it appears like a “bull’s-eye.” The central area sometimes appears pale or has a bluish tinge. The blister may turn into a slow-healing sore that may enlarge and extend to underlying tissues, including muscle. Sometimes, the blister may become raised, fill with pus, and then break down leaving a black scab. The affected area may take months to heal.

Additional signs may not appear for 3 to 4 days after the bite. They include destruction of red blood cells, abnormal blood clotting, fever, vomiting, blood in the urine, kidney failure, and shock.

Early treatment can be successful, but unfortunately, many cases are not recognized until skin damage has become extensive. Treatment at that stage is less successful but can still help. Surgically removing the wound is of questionable value. In addition to cold packs and medication to prevent infection, tetanus vaccination may also be recommended by your veterinarian.

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