The symptoms of spider bites in canines

07-12-07 © Mark Kostich

Spider bites are difficult to detect or diagnose accurately. Insect bites, bee stings and spider bites can all raise itchy red welts on a dog's skin.

In the United States, only three types of spider pose a threat to a canine's health---the black widow (and related widow spider species), brown recluse and the hobo spider. Symptoms exhibited by human spider bite victims will be the same in canines. It is through prompt and accurate reading of these symptoms that you can receive proper medical attention to a spider bite in your canine.

The Facts

All spiders are venomous but only a few species in North America have fangs long enough or venom potent enough to do any damage to canines. The widow spiders, such as the black widow, and the brown spiders, such as the brown recluse, are the most likely culprits of a dangerous spider bite. The unrelated hobo spider has a similar venom to the brown spiders, and a bite from the hobo results in the same symptoms. By the time symptoms arise, the spider will be long-gone; the bite itself from spiders is painless.


Spiders we think of as "non-venomous" will leave a red welt on the skin much like an overlarge mosquito bite. This may itch. Widow spiders and brown spiders have different venoms from each other, which affect different systems in dogs.

Black widow venom is a neurotoxin, meaning the venom affects nerve synapses. Muscles will cramp involuntarily (feeling solid to the touch), sometimes for days, including the diaphragm needed for breathing. Respiratory distress is common; listen for rapid, shallow breathing.

Brown spider and hobo spider venom is necrotic, meaning the venom kills tissue by breaking down cellular structure. Brown spider and hobo spider bites leave red, itchy skin lesions that bleed irregularly. A cyst will develop and a white ring around the bite, forming a "bullseye," will appear. The centre may become bluish and bruised-looking. These first signs of tissue death may go unnoticed until the venom has spread to deeper tissues.

Time Frame

Symptoms of spider bite venom arise 30 minutes to 6 hours after the bite occurs. Within 4 to 8 hours of a brown spider or hobo spider bite, a white ring will develop around the reddened bite site. Deeper tissue damage may take 3 to 4 days to develop; symptoms of this include fever, vomiting, internal bleeding, kidney failure and shock.


Widow spiders are found worldwide. Four species are common within the United States: the common black widow, the western black widow (western states), the brown widow (southern states) and the red widow (Florida). Ten species of brown spider live in the United States, but the brown recluse is the most common. The brown recluse is found in the south central Midwest, while other species of brown spider occupy the desert southwest. Hobo spiders live in the western United States.


Restlessness combined with partial paralysis, muscle rigidity and rapid, shallow breathing are the only outward signs an owner can use to guess black widow bite. Itching may be the only sign of a bite from a brown spider or hobo spider. Dog owners should check their dog's skin carefully if they notice the dog scratching or showing other signs of itch. This common symptom could mean many things that need attention. Catching spider bite symptoms early could alleviate deeper tissue damage.


If the bite site is located, you may see two tiny puncture wounds less than 1/16-inch apart. Symptoms from widow spiders, brown spiders and hobo spiders are systemic and not limited to the bite location.


Widow spiders and brown spiders prefer dark quiet places to hunt their food, such as woodpiles, under houses and even inside doghouses. Exterminators can help you rid your property of these spiders, or you can block the dog's access to such areas. Widow spiders do not move quickly and are very easy to see---they resemble dark grapes with long legs. You can remove individual spiders yourself if you identify them but be careful---their bites are dangerous to humans for the same reasons.

Veterinarians have pain medications and muscle relaxants to ease the dog's symptoms while the widow spider venom runs its course, but expect recovery to take several days. Antivenin is reserved for at-risk individuals such as the elderly or the very young. Ice packs at the bite site can help alleviate and slow symptoms of brown spider and hobo spider bites, and veterinarians can give corticosteroids to help stop tissue damage if caught early. Leprosy drugs have been found effective in combating brown spider bites. Antibiotics may also be prescribed.