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What is Black Widow Spider Bite?

Spiders in the Latrodectus family, also known as widow spiders, are equipped with powerful neurotoxins that can debilitate both prey and enemies. This family of spiders has developed multiple forms of venom so that it is toxic not only to vertebrates but to insects and crustaceans as well. Only the mature female of the species gets large enough for their bite to pierce the skin of most horses.

Black widows are a web spinning spider that spins their webs in areas that are dark, warm, and dry, like most barns. Most varieties of widow spider can be identified by a distinctive red hourglass on their abdomen, although some species have a more broken or rod-shaped red mark. A bite from a widow spider should prompt immediate contact with your horse’s veterinarian.

Spiders in the Latrodectus family, such as the black widow, are equipped with a potent neurotoxin that can cause painful cramping and malaise in horses. Fatal black widow bites are rare for equines.

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Symptoms of Black Widow Spider Bite in Horses

Black widow bites are generally painful right from the start and swell rapidly. Quite often the hair around the bite will fall off and leave a tender, raw circle of raised skin, which is frequently both painful and itchy. Horses often itch and rub this spot which can cause a thick fluid to ooze out, which can be either clear, yellowish, or greenish. Systemic symptoms of the bite can include:

  • Colic 
  • Difficulty swallowing
  • Fever
  • Increase in blood pressure and heart rate
  • Loss of appetite
  • Muscle contraction
  • Paralysis
  • Respiratory distress
  • Restlessness

Types

There are five species of widow spiders found in the US. Brown widow spiders (Latrodectus geometricus) were found exclusively in Florida until around the year 2000 when they began to spread. This spider is brown with an orange hourglass with venom that is just as potent as the black widow, however, they deliver less of it in their bite than the black widow. 

  • Northern widow spider (Latrodectus various) is common in the northern U.S.; its hourglass is not joined and looks more like two large spots on its abdomen
  • Red-legged widow spider (Latrodectus bishopi) is a threatened species of widow found in central and southern Florida; red-legged widows have reddish-brown bodies and legs, with a black abdomen and the red hourglass is replaced with a small red bar
  • Southern black widow spider (Latrodectus mactans) is the most common of the widow spiders; it is black with the distinctive red hourglass pattern on the abdomenand nly mature females have large enough fangs to deliver the venom through horsehide
  • Western black widow spider (Latrodectus Hesperus) is found in the western U.S.; it is similar in appearance to the southern black widow, but it is mainly found in the western regions of the U.S

Causes of Black Widow Spider Bite in Horses

The threat in black widow bites lies in the venom. The venom produced by the Latrodectus  is called a protein known as latrotoxin. These specialized toxins are designed to target either insects, crustaceans, or vertebrates. The latrotoxins designed to target vertebrates are classified as an a-latrotoxin. Once the venom is absorbed into the bloodstream, it causes involuntary muscle spasms and severe pain as it targets the nerve endings. Only adult female spiders have long enough fangs to penetrate either human or horse skin.

Diagnosis of Black Widow Spider Bite in Horses

Horses most commonly receive spider bites on the muzzle or head area, although the legs and feet are also a frequent target. The veterinarian will generally start by examining the area of the bite to evaluate the amount of damage, which typically means assessing the amount of swelling around the area as well as the horse's response to having the area touched or handled. Fluid from the wound may be cultured as well to ensure that no additional infections have taken hold, and the wound itself will be examined to determine the extent of the localized damage. Your veterinarian will typically perform a full physical examination at this time as well as inquiring about the horse’s living environment and if any spiders or insects have been seen recently.

Information regarding any other supplements or prescriptions that have been administered to your horse will also be requested at this time as this information helps to uncover any alternate toxins or drug interactions that may induce similar symptoms. It is relatively rare for enough of the toxin to be injected into the animal to cause the more severe signs of black widow bites, but it can occur, particularly in horses that are sensitive to the venom.

Treatment of Black Widow Spider Bite in Horses

If your horse is in distress when the veterinarian is able to examine it, supportive treatments are likely to begin as soon as possible. Fluids and electrolytes will be administered intravenously in order to prevent dehydration, and if respiratory distress occurs, oxygen will usually be offered. If the bite has caused an open wound, there is a risk of infection, and antibiotics are frequently prescribed to either clear up or prevent bacterial infection.

Historically, calcium gluconate solutions have been offered as well to help counteract the effects, however, this treatment has been shown to be less effective than previously thought and is falling out of favor with the medical toxicology community. Medications such as NSAIDs, designed to reduce pain and inflammation, will typically be administered at this time. Although antivenin is available for black widow bites and may be used if it is available, it is not commonly kept on hand.

Recovery of Black Widow Spider Bite in Horses

Most spider bites are of little consequence, however, recluse spiders and widow spiders both produce venom that can cause horses to have a toxic reaction. Healing from an adverse reaction to either of these spiders may be extended, particularly if large wounds develop, so it is best to avoid spider bites if at all possible. Although spiders are present to some extent in most barns, there are some steps you can take in order to reduce the possibility of a bite to your horse or yourself.

Both of these types of spider are non-aggressive, only biting when threatened or when pressed to the skin of the horse, so checking for spiders before putting blankets or tack on your horse will help to prevent bites, as will cleaning spider webs and cobwebs from the stables on a regular basis, making it unwelcoming for new spiders.