Black Widow Spider Bite Poisoning in Dogs

PetMD Editorial
By PetMD Editorial
Published: February 22, 2010

Black Widow Spider Venom Toxicosis in Dogs

In the U.S., the three significant species of Latrodectus, or widow spiders, to watch for are the Western widow, the Northern widow, and the Southern widow spiders, all of which are highly venomous to domestic pets and humans. The Western widow is found in western regions of the United Sates; the Northern widow is found in the northernmost U.S. and southeast Canada; and the Southern widow is found in the southeast U.S., from Florida to New York and into many of the southwest states. They are found in every state except Alaska and are often found residing around buildings and human habitations.

The female measures about 2–2.5 cm in length and is the larger of the genders. She is shiny black with a red or red-orange hourglass shaped mark on the underside of her abdomen, and in some females, there is also a patch of red on the top of the abdomen, above the spinnerets. The immature female is more of a brown color with red to orange or yellow stripes across the top abdomen that change into the hourglass shape as she ages and darkens to black. The male is significantly smaller, about half the size, with light brown coloring, and lacking the red hourglass marking the female is recognized for. The male is not regarded as a threat, as it is typically the female that bites.

Bites may be dry, with no venom injected. The venom is a potent neurotoxin, opening channels at the presynaptic nerve terminal and causing massive release of acetylcholine and norepinephrine, both of which can cause sustained muscular spasms and paralysis. A dog may be bitten while indoors or outdoors, as black widows are known to frequent both. No one dog is more at risk than another, but young and old dogs are at an increased risk for complications due to their weaker immune systems.

Symptoms and Types

  • Early, marked paralysis
  • Muscle tremors and cramping
  • Abdominal rigidity
  • Severe muscle pain in the back, chest and abdomen, manifested by howling, whining and loud vocalizations
  • Trouble breathing, respiratory collapse due to abdominal muscle paralysis
  • Excessive salivation and restlessness
  • Increased blood pressure and heart rate
  • Incoordination and inability to stand (ataxia)
  • Vomiting - not unusual for a dog to vomit up the actual spider
  • Diarrhea
  • Death, if anti-venom treatment is not given quickly


  • Very young or old dogs are at increased risk of severe reactions because of weaker immune systems
  • Dogs with systemic high blood pressure are at increased risk for a fatal outcome


This disease is difficult to diagnose because the symptoms may be caused by other diseases. A complete blood profile will be conducted, including a chemical blood profile, a complete blood count, and a urinalysis, as well as a thorough physical exam of your dog. Your doctor may also want to test a sample of your dog's stool. If possible, take a sample of your dog's stool and/or vomit with you to the veterinary clinic, this can help your doctor to make the diagnosis that much faster. Your veterinarian will be looking for wounds to the skin, at which point a bite mark may be found. Muscle rigidity and abdominal rigidity are classic signs of black widow envenomation.


Your dog will be hospitalized and given supportive care. Oxygen may be given to assist in breathing, and intravenous fluids given to lower blood pressure. Anti-venom drugs are available and your veterinarian will administer them while monitoring carefully for reactions. The muscle spasms and severe pain will be controlled by intravenous administration of medications that will relax the muscles and bring relief from pain, allowing your dog to rest and recover from the spider poison.

Living and Management

Your veterinarian will want to monitor the wound site weekly until it has healed. The prognosis may be uncertain for days. Weakness, fatigue, and insomnia may persist for months.

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