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Black Widow Bites in Dogs

A white dog lays on green grass

Dr. Erica Irish author of Black Widow Bites in Dogs

When thinking about insects that can cause health problems in dogs and cats, common ectoparasites like flea or ticks usually come to mind. These are often easily identified because fleas like to live on their furry hosts, and ticks remain attached to their hosts for several days while feeding.

Flea and ticks are parasitic insects that require hosts to complete their life cycle. In contrast, spiders are not parasitic, meaning that they don’t require a dog or cat host to life out their lives. Most spiders would very much like to hide away from other animals, but there may be situations when a spider bite occurs.

Almost all species of spider inject venom into their bites, but the amount of venom that is injected is very small. For black widow spiders, however, even the tiniest amount of venom can cause serious health problems. What happens when a dog gets bitten by a black widow spider, and what can you do about it?

Where can dogs get bitten by a black widow spider?

Black widow spiders (Latrodectus mactans) are tiny insects that are only three to ten millimeters long. They can be dark gray to black in color, and mature spiders have the classic red hourglass shape on their abdomens. Younger spiders, however, may not develop this pattern until later in life.

Black widow spiders favor warm, temperate climates like those in the southern United States. They can be found in outdoor locations such as tree stumps, rodent holes, and piles of leaves. They can also set up webs inside of garages, basements, and open buildings such as barns.

Bites can occur when dogs go for walks outside or if the spider becomes trapped or accidentally stepped on. Black widows are not an aggressive spider species and prefer to hide away from other animals, but in situations where they are stressed, they may bite. Sometimes, this bite is considered “dry,” meaning that it did not break skin or no venom was injected. Unfortunately, envenomation is successful more than 80% of the time.   

Symptoms of a black widow spider bite

Black widow spider venom contains a neurotoxin that can cause clinical signs related to the nervous system. It excites the neurons, aka nervous system cells, and can lead to muscle twitching or tremors. Bites can also cause muscle cramping and very stiff or tense abdominal muscles. Excitation of the nervous system can lead to an elevated heart rate and respiratory rate. In severe cases, seizures, paralysis, and respiratory arrest can occur.

Vomiting and diarrhea are among the most common clinical presentations for spider bites. Drooling, which can indicate nausea, is also common. Pain and swelling usually occur at the bite site. Because of this pain and discomfort, spider bite victims may appear restless and will whimper or cry often. The good news is that these signs happen much more frequently than the severe or life-threatening signs.

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What to do if your dog has been bitten by a black widow spider

Spider bites are rarely witnessed, but it is very helpful if you can identify a spider near your dog when the bite has taken place. If your pup has acute swelling in one part of his body, and if he is lethargic or has a fever, your veterinarian might suspect that your dog has been bitten.

An antivenin is available for black widow spider bites, but it is not a commonly carried product in most general practicing small animal hospitals. Specialty hospitals may have access to it. Otherwise, supportive care is necessary. Intravenous fluids are provided for support, and muscle relaxer medications can be given to help with tremors and twitching. Depending on the amount of venom injected, dogs may need this kind of therapy for a few days to several weeks! Vets will also treat spider bites with the same therapies used for allergic reactions, and pain medication will be necessary if the bite site is swollen or causing your pup discomfort.


Many spider bites are innocuous, but pet owners who live in warmer climates need to be wary of the black widow spider. The bite from a black widow spider can cause minor gastrointestinal symptoms or severe neurologic problems. If you suspect that your canine companion has been bitten by a black widow spider, be sure to contact your veterinarian right away for emergency treatment. If supportive care does not work, you may need to contact a specialist for access to black widow spider antivenin.  

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Meet The Author 

Dr. Erica Irish author of Black Widow Bites in Dogs

Dr. Erica Irish

Erica has worked in the veterinary field since 2006, starting out as a veterinary technician before graduating from the UF College of Veterinary Medicine in 2013. As a general practitioner in an animal hospital, she has many interests and is especially interested in dermatology, cardiology, internal and integrative medicine

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