Bites from poisonous spiders such as the black widow (Latrodectus) or brown recluse (Loxosceles reclusa), vary in their severity depending on the location of the bite on the horse's body, size and age of the horse, and species of spider.

• Black widow spider: Bites from a black widow spider will inject acetylcholine and norepinephrine---which are potent neurotoxins, into the horse. This often results in muscle spasms and paralysis. A rigid and painful abdomen without tenderness is a classic sign of envenomation by the black widow spider.
• Brown recluse spider: The initial bite from a brown recluse spider is not painful, however within 2-6 hours after being bitten, signs of localized pain and redness will appear where the horse was bitten. Within 12 hours a blister will appear, which progresses into a classic "bull's eye" appearance. Within a week or 2 weeks the skin in the area of the bite will ulcerate and die. The tissue death can spread well beyond the initial bite wound, and in some cases of substantial tissue devitalization, an entire limb can be affected. The wound is very slow to heal, over a course of several months, which increases the risk of secondary infections.

What Black Widow Spiders Look Like

Female black widows (the ones that bite) are about three quarters to about an inch in length. They’re shiny black and have a red or red-orange hourglass-shaped mark on the underside of the abdomen. Some also have a red marking on the top of the abdomen, above the spinnerets (silk-spinning organs). Younger female spiders are brownish in color, with red, orange, or yellow stripes across the top of the abdomen. These markings change into the hourglass shape as the spider matures and changes to a black color. Widow spiders are commonly found in barns, wood and brush piles, and underneath water troughs.

What Brown Recluse Spiders Look Like

The brown recluse spider is also known as the “fiddle back” or “violin” spider after the violin-shaped marking on its back. It is brown in color, with a violin-shaped marking on the upper portion of the body, with the neck of the violin extending down to the base of the tail. Not all recluses have the marking, for example, young brown recluse spiders do not. These spiders also have a unique pattern of six eyes instead of three. They’re less than an inch in length, with very long legs. These spiders are reclusive and typically non-aggressive. They are most active at night, and bites usually occur when a spider gets trapped in bedding and a horse lays on it while in their stall.