With over 3,000 types of spiders in the U.S. and more than 45,000 around the world, including the brown recluse, you may be concerned about one of those spiders potentially biting your favorite furry friend. Most spider bites on dogs only cause localized redness and swelling and do not require veterinary care. However, there are a few species of spiders whose bites can be life-threatening in dogs. Learn more about which spiders pose a greater threat, how to identify a spider bite on a dog and what to expect during a visit to your veterinarian.

Which Spiders Are Dangerous?

There are 11 species of spiders indigenous to the United States, and out of these, two main species of venomous spiders to be aware of are:

• Brown Recluse: The most common venomous spider that bites dogs is Loxosceles reclusa, the brown recluse. They have a violin-shaped pattern on their body, and they're mostly active at night. Dogs are usually bitten by recluse spiders that are hiding in bedding, but these spiders also hide in closets, attics and dry basements. These spiders live mostly in the Midwest; however, some may be found in Southern California, southern New Mexico and western Arizona.
• Black Widow: Black widow spiders can be identified by their shiny black bodies and a red or orange hourglass mark on their underside. Young spiders are brown, and have red or orange stripes that gradually morph into the hourglass mark as they grow older. Black widows are found in every state in the U.S. except Alaska. They like to make their homes around buildings. Only the female spiders are toxic.

Identifying a Spider Bite on a Dog

Spider bites may vary depending on the type of spider. For example, brown recluse spiders contain a venom that causes a local skin reaction in humans. There is no consensus on what these bites look like for dogs, but known clinical signs in humans include:

• Initial pain around the bite, followed by itchiness and soreness
• Development of a classic target lesion, which is an area of skin that loses its blood supply and turns dark, and is surrounded by redness
• Fever, chills, rash, nausea or joint pain
• A week to five weeks from the first bite, the dark area usually blisters and fall off, leaving a deep ulcer that doesn't heal. The extent of damage is dependent on how much venom was injected in the bite.
• On occasion, brown recluse bites can cause anemia and kidney problems

On the other hand, black widow spiders have a venom that contains a compound called alpha-latrotoxin which is a strong neurotoxin. But according to the sixth edition of The 5-Minute Veterinary Consult, 15% of black widow bites do not contain venom and don't cause any signs other than some redness around the bite. If there is only mild envenomation, signs may not show up for weeks. If a dog is heavily envenomated by a black widow, clinical signs may include:

• Tremors and cramping
• Pain
• Hard belly
• Restlessness
• Fast heart rate
• Excessively salivating
• Facial swelling if bitten on the face