How to Tell if a Pet has a Spider Bite

Spider bites can be difficult to diagnose with certainty, as they often resemble insect bites, bacterial infections or injuries. Accordingly, pet owners must monitor their pets for symptoms, such as severe muscle cramps, pain, disorientation or partial paralysis, which are commonly associated with dangerous spider bites. Most spiders are harmless, but consult your veterinarian anytime you suspect your pet has sustained a bite, as the venom of some species can cause serious illness or death.

Black Widow Spider
Close-up of black widow spider.
credit: jdornoff/iStock/Getty Images

Widow Spiders

Five different widow spiders inhabit to the United States, including three different black widow species, one brown widow species and the red widow. The three black widow species are the most medically significant; although small differences between the three species exist, they are all inky-black spiders with bright red abdominal markings, sometimes resembling an hourglass. Black widows are common in woodpiles and outbuildings, and under logs, rocks or debris.

Brown Recluses

The United States is home to six to 10 different recluse species, all of which belong to the genus Loxosceles. (Reference 1 and Reference 4) Brown recluse spiders are most common in the Midwestern states, as far north as southern Illinois, Indiana, Iowa and Ohio, but they also inhabit the Southern states, from Florida to California. True to their name, brown recluses are secretive, unaggressive spiders who often live in basements, tunnels and woodpiles. Laypersons regularly misidentify small, brown spiders as brown recluses, based on the observation of a violin-shape marking on their back. To accurately identify a brown recluse, you must note that is has unmarked legs with no hairs or spines, an unmarked abdomen and -- most importantly -- six eyes, arranged in three pairs.

Other Spiders

Scientists have described approximately 40,000 spider species, and the vast majority of these are venomous. However, very few species possess both medically significant venom and fangs that are massive enough to penetrate the skin of a dog or cat. Some authorities worry that hobo spiders (Tegenaria agrestis), European natives who began colonizing parts of the Pacific Northwest in the 1980s, may be dangerous. However, the virulence of their venom remains unclear, as very few bites have been documented. Hobo spiders spin funnel-shaped webs -- although they are not the only species to do so -- near vegetation or rock piles.

Initial Symptoms

The symptoms caused by spider bites vary widely from one bite to the next, and individual pets respond differently to their venoms. Dogs and cats commonly experience redness, inflammation, swelling, hair loss or discoloration at the bite site -- but many times, bites go unnoticed until systemic symptoms arise. Some of these systemic symptoms include anxiety, muscle weakness, partial paralysis, disorientation and seizures. Dogs and cats often suffer from debilitating muscle cramps after black widow envenomation. Cats are particularly susceptible to black widow venom, and they often begin to drool and exhibit paralysis after suffering a bite. Symptoms progress rapidly.

Veterinary Measures

Veterinarians largely treat minor spider bites symptomatically and by providing pain medication and supportive care. They will usually collect blood samples and monitor your pet’s progress. However, serious envenomation may require medications to halt muscle contractions, to counteract the necrotizing action of the venom or to reduce blood pressure. Antivenin exists for black widow bites, but it is rarely used in the treatment of bitten pets, largely due to its short shelf life, its high cost and the relative rarity of bites.

Avoiding Bites

Hollywood arachnids notwithstanding, spiders are retiring creatures, uninclined to bite pets or people. Most bites occur from some type of accidental contact, such as when your dog or cat lies down upon the spider, who -- fearing for its life -- bites in defense. Prevent your pet from spending time in places where spiders may lurk, such as under porches, in unfinished basements and near woodpiles, to reduce the chances of spider bites.