Widow Spider
Spiders are everywhere! There are over 30,000 species of spiders in the world.

The good news, though, is that in most cases, spider bites cause little more than local pain and inflammation. Most species of spider are unable to penetrate human or animal skin. Luckily in the U.S., there are only a few spider species whose bite can cause severe problems in people and pets. These are the widow spiders, the brown recluse spider and the hobo spider.

Widow Spiders

Widow (hourglass) spiders belong to the genus Latrodectus. Widow spiders get their name from the female's practice of killing the male after breeding, although this does not always occur with every species.

There are five species of widow spiders found in the U.S.:

  • The black or southern black widow spider (Latrodectus mactans) is found throughout the entire country. This is the typical widow spider that is black with the red hourglass pattern on the female’s ventral abdomen (tummy).
  • The western black widow spider (L. hesperus) is found in the western U.S.
  • The northern widow spider (L. various) is common in the northern U.S. Its hourglass is not joined and appears more like spots on its abdomen.
  • The red or red-legged widow spider (L. bishop) is found in central and southern Florida.
  • The brown widow spider (L. geometricus) is also found in Florida. This spider is brown with an orange hourglass.

Widow spiders like dark, nondisturbed places. They can be found inside buildings or outside in leaf litter. These spiders are not aggressive and will only bite defensively. Cats tend to get multiple or severe bites due to their propensity to poke at and harass the spider. Bites are typically from female spiders, as the male’s fangs are too short to effectively penetrate skin.

Widow spider venom is absorbed into the circulatory system where it causes neurologic signs. The bites are immediately painful. On nonhaired skin, a slight redness may be noted with two small puncture wounds 1–2 mm apart. In 30 minutes to two hours, muscle cramps will begin near the bite site and then spread to other large muscle groups. Pain peaks in two to three hours. High blood pressure and high heart rate are common. As the signs progress, vomiting, diarrhea, paralysis and death can occur. Cats are more susceptible to severe problems than dogs.

A single bite can cause life-threatening signs, but fortunately, “dry bites” (no venom released) are possible. The severity of the signs depends on two sets of factors — the spider and the victim. Spider-dependent factors include the size of the spider, the amount of venom injected and the time of the year (warmer temperatures appear to increase the toxicity of venom). Victim-dependent variables include the animal species, the size of the animal (smaller animals are more susceptible), the location of the bite, underlying health problems and age (younger and older victims show more severe signs).

Any suspected widow bite should be evaluated by a veterinarian. Treatment involves pain medications and muscle relaxants. There is an antivenin, but availability for animal use is sporadic at best, although it has been used in at least one cat with good results. Most animals recover in 48 to 72 hours, although some people report weakness and fatigue lasting for weeks to months. Fatalities are uncommon, with estimates ranging from <1% to 6% in humans.

Brown Recluse and Hobo Spiders

There are a few species of spiders in the U.S. that can cause a large area of cellular damage (necrosis) with their bites. The brown recluse (Loxosceles reclusa), or fiddleback spider, has a distinctive fiddle-shaped mark on its back. They are generally found in the south central U.S. (Texas through Georgia) but can be found as far north as Iowa, central Illinois and Indiana. The brown recluse is nocturnal and not aggressive. Other species of Loxosceles may produce wounds that are not as serious as the brown recluse's bite. Hobo spiders (Tegenaria agrestis) are found in Washington, Oregon and Idaho (as well as western Canada and Alaska), and can cause similar necrotic lesions like the brown recluse. They are large, aggressive brown spiders and build their webs at ground level or in basements.

Recluse spider venom contains compounds that destroy cell membranes. The venom attracts white blood cells, which also increases the cellular damage. If absorbed into the blood stream, the venom can break down red blood cells, causing anemia. The venom also affects coagulation and can cause bleeding.

Initially, the bites are only slightly painful. Over an eight-hour period, the bite will become red, swollen and tender. In people, you may notice a "bull's-eye” lesion, but this is rarely seen in cats and dogs due to their hair coat. Tissue around the bite begins to die, and a wound as large as 10 inches in diameter can occur. Healing is slow and may take months.

Animals are treated with pain medications and antibiotics. Multiple bandage changes may be needed over several weeks. Some wounds may even require surgical closure. Animals with anemia or clotting problems may need intravenous fluids and blood transfusions. Fortunately, most cases only show mild local signs, but if you suspect your pet has been bitten by one of these spider species, consult your veterinarian.


There are some species of tarantulas found in the U.S. that produce venom that may cause local pain. Tarantulas can be found in the wild in the southwestern, central or western regions of the country. They have also become increasingly popular as exotic pets in the home, where furry four-legged pets may encounter these equally furry eight-legged oddities.

It’s not just the mild venom that can cause problems for pets: Ingestion of the stiff hairs covering the spider’s legs by pets can also cause drooling, oral irritation, pain and vomiting. Tarantulas can actually “throw” these irritating hairs at targets when they feel that they are under threat, so you should never put your face close to a pet tarantula or allow your pet to get that close either. However, despite the Hollywood-inspired, fear-inducing reputation of tarantula spiders, no serious problems should be expected if your pet is bitten. Tarantulas are actually in more danger from your dog or cat than the spider is to them. Tarantulas can jump and may be injured in a fall. Furthermore, this jumping behavior makes them irresistible “toys” for cats and dogs, and tarantulas can die from a bite from a pet. However, as always, if you suspect your pet has had a run-in with a tarantula friend, be sure to consult your veterinarian. 

Keep a Friendly Distance

Spiders of all kinds are ubiquitous in the environment, and it can be hard to totally prevent your pet from coming in contact with them. They are also beneficial in helping to keep populations of other buggy pests at bay, so you don’t want to harm spiders that are simply minding their own business out-of-doors. However, you can do your best to minimize any risks to your pets by keeping them out of areas where spiders are noticeably present, such as basements, crawl spaces or outbuildings. Shake out any pet towels or blankets before using. Regularly dust and/or vacuum any webs away from living areas where your pet likes to play or rest. Finally, keep clutter to a minimum in order to cut down on any likely spider habitat in your home.

Read more Vetstreet articles written by Dr. Tina Wismer.