Hives are patches of swelling that can vary in size and can occur anywhere on a horse's body, but most commonly occur on the neck, the sides and the upper areas of the legs. Hives can cause itching and make a horse very uncomfortable. Determining the causes of hives on horses requires a careful assessment of a horse's activities and exposures to things in the environment that can cause an allergic reaction, resulting in hives.
To determine if diet causes hives on horses, a veterinarian will take a medical history from the horse's owner, detailing what types of commercial feed, hay and nutritional supplements that the horse is consuming, as well as the type of pasture grass that the horse is grazing on. If the medical history doesn't reveal a cause, horses can be put on an elimination diet, which begins with one protein source and one carbohydrate source to which horses have not been previously exposed. If high-quality pasture grass is available, other sources of food can be stopped, and if the horse's condition improves during the diet period, pasture grass can be eliminated as the source of the hives. Horses that do not show improvement in four to six weeks should not be allowed access to pasture grass for another six weeks, and should be introduced gradually to new foods.
Another cause of hives on horses, especially Arabian and thoroughbred horses, is an allergic disease called atopy. Thoroughbred and Arabian horses have a genetic predisposition to this disease, which usually presents by age 4, and also appears in older horses after a move to a new environment and exposure to allergens. Allergies can be seasonal, such as pollens, moulds, grasses or trees. Horses may also be allergic to material in saddle pads or horse blankets.
A common cause of hives on horses in the spring and summer is insect biters; flies and mosquitoes are the most frequent offenders. Swelling can also result if horses are bitten on the midline, which is under the belly; the udder; or the front skin covering the penis. To keep from attracting insects to the barn area, the owner should maintain clean stalls and locate the manure pile far away from the barn area. Horses should stay indoors during peak insect hours, such as dusk and dawn.
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