Spider Bites

Which spiders are associated with problematic spider bites?

There are over 30,000 species of spiders worldwide, and most of these spiders are venomous. Most spiders are unable to produce medically-significant envenomations (the process by which venom is injected into an animal) however, because their small mouthparts are unable to penetrate the skin.

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


Widow spiders are found in tropical and temperate areas worldwide. There are five Latrodectus species found within the United States and the most common of these is the black widow spider (Latrodectus mactans). Female black widows 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, being red, brown, or beige in color, however, they are also capable of producing 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 in color, with a violin-shaped marking on their back. These spiders are reclusive and not aggressive, but can bite when they feel cornered or trapped (if accidentally laid on, or trapped between an animal and its bedding).

What are the clinical signs of a spider bite?

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

Patients who are bitten by a black widow spider may develop clinical signs within 8 hours. Approximately 15% of black widow bites are 'dry bites,' in which envenomation does not occur, and 85% of bites involve some degree of envenomation. Although the majority of bites result in envenomation, the number of patients who go on to develop significant clinical signs is low. Initially, a black widow bite is moderately painful and accompanied by mild redness or swelling. In the following hours, more severe signs may occur. These signs include muscle pain and cramping, vomiting, diarrhea, tremors, and agitation. Affected dogs and cats may also develop muscle rigidity and may seem painful in their abdomen (belly).

Brown recluse bites, in contrast, are often not painful at the time of the bite. A small amount of stinging or redness may be noted, and a blister may develop at the site within 2-8 hours. This blister often develops into a 'bulls-eye' lesion, with a red circle surrounding a pale area that will progress to a dark area of tissue death. Fever and lethargy may be noted in the days following the bite, and in rare cases kidney and clotting abnormalities may occur; however, brown recluse bites primarily cause a local reaction and localized tissue damage.

How does spider venom produce these effects?

The effects of black widow and brown recluse spider bites vary significant due to differences in the venom of these two types of spiders.

Black widow spider venom contains a neurotoxin. This substance causes massive excitation of neurons (the cells within the nervous system), leading to the muscle tremors and other effects seen in black widow envenomation.

Brown recluse spider venom, in contrast, contains a number of 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, all of which contribute to local inflammation and tissue necrosis (death).


"Black widow envenomation affects the whole body, causing muscle tremors and other neurologic effects. Brown recluse envenomation only causes local tissue destruction."


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 be used to guide a diagnosis, although this is not always the case.

The only method to definitively diagnose a spider bite is collection and identification of the spider responsible for causing the bite, however, this is rarely performed.


How is a spider bite treated?

"Complete recovery may take up to several weeks, especially if antivenin is not available."

There is an antivenin available for black widow spider bites. This medication is given intravenously (IV) and signs of envenomation typically resolve within 30 minutes. Allergic reactions to the antivenin may occur; however, your veterinarian will likely give a medication to decrease the likelihood of allergic reaction. In the absence of antivenin, pain medications and muscle relaxers are used to control clinical signs and 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 three weeks and systemic complications or death are very rare. In very severe cases, wounds may require surgical treatment or leave permanent scarring, but this is uncommon.  


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