Spider Bites

Which spiders cause problematic spider bites?

There are over 53,000 species of spiders worldwide, most of which are venomous. However, most spiders cannot produce a medically significant envenomation (the process by which venom is injected into an animal) because their small mouthparts cannot penetrate the skin.

The two groups of spiders responsible for most medically significant spider bites are the widow spiders (Latrodectus spp., including the black widow) and the recluse spiders (Loxosceles spp., including the brown recluse).

Widow spiders are found in tropical and temperate areas worldwide. Five Latrodectus species are found in the United States, and the most common is the black widow spider (Latrodectus mactans). 

Female black widow spiders are significantly larger and more venomous than males. They are dark grey to black, with a red or orange hourglass marking on their round abdomen. Immature female black widows lack this coloration, and are red, brown, or beige, but they can also produce envenomation.

Recluse spiders are found worldwide in dark, warm places. There are six Loxosceles species in the United States, with the most notable being the brown recluse spider (Loxosceles reclusa). Female brown recluse spiders are larger and more venomous than males. 

Brown recluse spiders are yellow-brown and have a violin-shaped marking on their backs. These spiders are reclusive and not aggressive, but they may bite when they feel cornered or trapped (if accidentally laid on or trapped between an animal and its bedding).

How is a spider bite diagnosed?

Spider bites are primarily diagnosed based on clinical signs. Black widow spider bites may produce bloodwork abnormalities that can guide a diagnosis, although not always. The only way to definitively diagnose a spider bite is to collect and identify the spider responsible for causing the bite.

What are the clinical signs of a spider bite?

The clinical signs of a spider bite vary depending on which species delivered the bite.

Black widow bites: Pets bitten by a black widow spider may develop clinical signs within eight hours. Although most bites result in envenomation, few pets develop significant clinical signs. 

• Initially, a black widow bite is moderately painful and accompanied by mild redness or swelling. 
• In the following hours, more severe signs may occur, including muscle pain and cramping, vomiting, diarrhea, tremors, and agitation. 
• Affected pets may also develop muscle rigidity, have rapid heart and respiratory rates, and may have pain in their abdomen.

Brown recluse bites: Brown recluse spider bites primarily cause a local reaction and localized tissue damage. 

• Initially, a brown recluse bite is often not painful. A small amount of stinging or redness may be noted.
• A blister may develop at the site within two to eight hours. This blister often develops into a “bulls-eye” lesion, with a red circle surrounding a pale area that progresses to a dark area of tissue necrosis (death). 
• Fever and lethargy may develop in the days after the bite. 
• In rare cases, kidney and clotting abnormalities may occur. 

How does spider venom produce these effects?

Black widow and brown recluse spiders have differences in their venom, which results in significantly different effects.

Black widow spider venom contains a neurotoxin that overstimulates the nervous system, leading to muscle tremors and other effects. Disease caused by black widow spider venom is called latrodectism. Cats appear more susceptible to the effects of latrodectism. 

Brown recluse spider venom contains several substances that trigger local tissue effects. The first part of brown recluse venom contains a substance known as sphingomyelinase D, which interferes with blood clotting at the site of the bite. Additional toxins are then injected into the bite victim, contributing to local inflammation and tissue necrosis.

How is a spider bite treated?

There is an antivenin available for black widow spider bites. This medication is given intravenously (in the vein), and signs of envenomation typically resolve within 30 minutes. Allergic reactions to the antivenin may occur, but your veterinarian will likely give medication to decrease the likelihood of an allergic reaction. In the absence of antivenin, pain medications and muscle relaxers are used to control clinical signs, and intravenous (IV) fluids are used for supportive care. Complete recovery may take up to several weeks, especially if antivenin is not available.

Brown recluse spider bites are treated with supportive care, as there is no available antivenin in the United States. Wounds are treated with regular wound cleaning, cool compresses, and immobilization using bandages. Antibiotics may also be needed for infected bites. Most bites heal within one to eight weeks, and systemic complications or death are rare. In very severe cases, wounds may require surgical treatment or leave permanent scarring, but this is uncommon.