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Most spider bites on horses go unnoticed, healing relatively quickly with little or no pain or scarring. Bites from a recluse family spider, such as the brown recluse, produce a venom that contains sphingomyelinase D, a potent toxin that causes necrosis of the animal’s skin and soft tissues. This can cause large open wounds to develop around the area of the bite, which can be prone to developing secondary infections. The brown recluse spider, also known as the fiddleback spider, are generally found in warmer climates, and bites are more common in the autumn as they seek shelter from the cooler weather.
The brown recluse, also known as the fiddleback spider due to the distinctive fiddle-shaped marking on its back, produces venom that can cause necrosis of the skin and the underlying tissues.
In the majority of equine cases, brown recluse bites are unremarkable, healing fairly quickly with little to no pain or scarring. If relatively large quantities of the venom are injected with the bite or if the horse is sensitive to the venom, a small white blister will form at the site of the bite which typically develops into ragged open lesions within just a few days that grow outwards as further tissue damage is caused by the necrotizing toxin. If left untreated, tissue damage can become significant and secondary infections often develop.
The brown recluse is not the only toxic recluse spider that can be found in North America. The recluse family can be distinguished from most other spider families by the number of its eyes as recluse spiders have six eyes arranged in pairs rather than the usual eight. Some of the cousins of the brown recluse include:
Arizona brown spider (Loxosceles arizonica) - Only found in Arizona, this spider appears to feed mostly on ants.
Desert recluse (Loxosceles deserta) - This spider is often misidentified as a brown recluse, but lives farther west in California, Nevada, Utah, and Arizona
The spiders in the Loxoceles family produce a toxin that frequently causes growing patches of necrosis of the skin that extend out from the area of the bite, although systemic responses for equines are rare and relatively mild. Horses generally encounter these spiders in the corners of barns and tack rooms, and reports of bites tend to increase as the weather cools and they come inside to keep warm.
Spider bites on horses are most frequently found on the muzzle or head area, although the legs and feet are also a common target. The veterinarian will examine the damaged area to evaluate the amount of damage. In early stages, this means assessing the amount of swelling around the area as well as the horse's response to having the area touched or handled. Recluse bites generally start out as painless, however as the wounds grow or get infected they can become quite painful. In more advanced stages they will be assessing the open wound, checking to see if there is any evidence of a secondary infection and evaluating the extent of the dead and dying tissues; samples of tissue may be taken at this time to test for secondary infections.
Recluse bites are frequently surrounded by black, deadened skin. Your veterinarian will also perform a full physical examination and will typically inquire 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.
For spider bites that are caught early cold therapy is usually the first course of action after cleaning the bite with soap and water; applying ice to the area can reduce swelling and inflammation as well as slowing the spread of the venom itself. If the horse is experiencing any pain, pain management drugs such as NSAIDs, such as Phenylbutazone (bute) or flunixin meglumine (Banamine), may be administered to the patient either by injection, oral medication, or as a liposomal cream.
More advanced bites will require that the veterinarian remove any dead, necrotic skin or tissue to prevent the wound from becoming larger or deeper. If any bacterial or fungal infections are found in the open wound left by the brown recluse bite, then either antibacterial or anti-fungal medications will also be prescribed. Dapsone, a leprosy drug designed to inhibit the function of white blood cells, may be recommended to help treat severe brown recluse bites as well.
Although most spider bites are of little consequence, recluse spiders and widow spiders both produce venom that can cause horses to have a toxic reaction. 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. 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.
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