Black widow and brown recluse spiders can cause necrotizing lesions as well as severe pain and itching.
Black Widow Spider
There are a variety of black widow species found in North America, each with a slight variation in appearance. Most are recognized by their black coloration and red hourglass pattern on their underbelly.
Black widows are not aggressive spiders, but will bite if threatened. Their venom is a neurotoxin that causes paralysis, muscle rigidity and breathing difficulty. In horses, the effects are typically experienced locally.
Brown Recluse Spider
A brown recluse spider are more difficult to identify. This spider is generally a light brown color with a dark violin shaped spot along its back. Unlike most spiders, brown recluse species possess three (as opposed to four) pairs of eyes. They typically hide during the day in dark areas in homes, barns and tack rooms.
The venom of a brown recluse is powerful hemotoxin which usually results in local tissue necrosis (similar to that of snake venom).
If you suspect that your horse has been bitten by a poisonous snake or spider, contact your veterinarian immediately.