While spiders may strike fear among many humans, dogs don’t feel the same way. It’s up to us as owner to look out for any threatening spiders or signs of a venomous bite that need veterinary care.

Thankfully, of the 3,500 species of spiders in the United States, only two are dangerous to humans and dogs.¹ Spiders would much rather catch and eat insects than use their energy (and potential venom) biting you or your dog. Though rare they do bite when in danger.

Which Spiders Are Dangerous to Dogs and Where Do They Live?

The two venomous spiders in the United States are the brown recluse (also known as the fiddleback and brown spider) and the black widow.

Brown recluse spiders are found in the south-central and western regions. They’re tan to brown with a darker brown fiddle or violin-shaped mark on their back. They live in undisturbed areas of homes, barns, garages, and basements or outdoors in rock crevices, under bark, and in woodpiles.

Black widow spiders are found throughout the United States. They’re shiny black or dark brown with a red hourglass shape on their belly. Only female black widow spiders have toxic venom. They tend to attach their webs to sheltered places outdoors, such as woodpiles, rock piles, rodent burrows, inside debris, and under houses.

Spider Bite Signs and Treatment for Dogs

The two venomous spiders are most active at night in late summer or early fall, which is when dogs may be more likely to be bitten. They may also get bitten when rooting around in spiders’ habitats. You likely won’t see a spider in the act of biting your dog, making it a challenge to know it happened. If you see a spider on your dog or suspect a bite, take them to the nearest veterinary clinic.

Non-Venomous Spiders

Most spider bites on dogs have no clinical signs because the bites are tiny and carry no venom. But if any bite may result in an infection and signs may include:

• A red spot similar to a tick or mosquito bite.
• Mild swelling and redness.
• Mild itching, similar to a mosquito bite.
• Temporary discomfort

Don’t hesitate to call your veterinarian if you have any concerns or suspect a spider bite. This is especially important if: 

• There’s an increase in pain, swelling, or redness, the area is warm to the touch, or develops a discharge (any oozing fluid) following the bite. 
• Your dog has any vomiting or diarrhea or facial swelling.
• Your dog has behavior changes like lethargy, whining, restlessness, inability to get comfortable, or trembling.

Brown Recluse Spiders

Signs your dog was bitten by a brown recluse include possible pain and itching that may progress to a blister or ulcer. Some dogs may experience lethargy, vomiting, trembling, fever, a rash or, in severe cases, suffer damage to their red blood cells resulting in anemia. 

If your dog has any of the signs above or you know that a brown recluse bit your dog, take them to your veterinarian immediately. Don’t try at-home remedies since they may delay important treatment and may worsen the effects of the bite. 

Don’t take a chance with brown recluse bites. Infection from untreated wounds, clotting abnormalities, and potential hemolytic anemia from brown recluse venom can be life-threatening for dogs.

There is no anti-venin for a brown recluse bite. Veterinary treatment is supportive and may include:

• Medications for pain, inflammation, and to treat infection, 
• Wound care
• Intravenous fluids and blood products to help dogs who develop anemia or blood clotting abnormalities.

Black Widow Spider

Signs your dog was bitten by a black widow may be difficult to detect, but can include:

• Immediate pain:

  • Redness and mild swelling at the bite site
• Within 30 minutes to 8 hours:

  • Severe pain with whining, panting, and restlessness
  • Muscle tremors or spasms
  • Reluctance to move
  • Drooling
  • Difficult or fast breathing
  • Facial swelling

If you know that a black widow spider bit your dog or they have any of the signs above, immediately take your dog to a veterinarian or emergency veterinary clinic. Black widow bites can be life-threatening. Do not delay life-saving treatment by attempting treatment at home.

Veterinary treatment is supportive and may include:

• Fluids and medications to manage your dog’s pain and muscle spasms
• Anti-venin may be administered, but it’s not universally available and has risks associated with its use. Your veterinarian best suited to determine if anti-venin is an option for your dog.

Seek veterinary guidance immediately if you know or suspect a spider has bitten your dog. Take comfort in knowing that most spider bites are treatable with proper care.