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The venom in bee stings is toxic but generally requires extraordinarily large doses to be fatal for most mammals, approximately ten stings per pound of body weight. Although the venom from a single bee sting is not typically enough to trigger more than a slight swelling around the site of the sting, some horses may develop an allergy, triggering rapid breathing and pulse, hives, fever, and in severe cases a potentially fatal reaction known as anaphylactic shock.
The venom in bee stings is not generally strong enough to produce more than an uncomfortable welt around the area of the sting unless the horse has developed an allergy to the compounds in the venom.
Horses that experience an allergic reaction to a bee still may present initially with rapid pulse and breathing, anxiety, and sweating or a low-grade fever with swelling around the area of the sting. Any horse with a resting heart rate over 48 beats per minute or a rectal temperature over 101.5 should be evaluated by a veterinarian. Hives may also show up several hours after the horse has been stung, either at the sting or on the side of the neck, and then moving across the throat and shoulders. Early welts, which may or may not itch, typically start as small as half an inch in diameter, however, they frequently increase in size and often converge as the reaction progresses.
Bees and wasps have very different venom, and an animal that is allergic to one is often not allergic to the other, although there are some challenges to determining which your horse is allergic to. Examining the site of the sting may help as honeybees and bumblebees will frequently leave their stinger in the wound and wasps and hornets never will. Horses that are sensitive to bee venom are reacting to either the Phospholipase A2 or mellitin that they produce, whereas horses that are sensitive to the venom of wasps or hornets are reacting to antigen 5. Unfortunately, serum tests frequently identify patients as allergic to both venoms, even when the allergy is only to one, and intradermal tests for venom are more difficult to read than those for environmental or contact allergens.
When the immune system is stimulated by a particular protein, such as the venom from a bee sting, it can sometimes spur specialized cells of the immune system known as mast cells into action. When triggered, the mast cells release histamine, a naturally occurring compound that has an inflammatory effect on the tissues and membranes that it comes into contact with. In rare cases, allergies that are related to insect stings or bites may induce an anaphylactic reaction in which the horse experiences a dramatic drop in blood pressure as well as respiratory distress due to swelling of the lung tissues. Anaphylactic shock can occur rapidly and is frequently fatal not treated quickly.
If your horse experiences an anaphylactic reaction shortly after being stung by bees, the timing will help your veterinarian determine that the allergic reaction may be related to the venom. Signs of milder reactions may be delayed by several hours, and bee stings may not be noticed right away, making it more challenging to determine the origin of the symptoms. The symptoms are fairly non-specific and may indicate a number of different diseases or disorders, so the visit is likely to start with a full physical examination, which is liable to reveal if any stings are present. Standard blood tests may expose increased levels of eosinophils, a specialized white blood cell that may indicate an allergic reaction, although eosinophils may also be raised in horses that are hosting parasites in their GI tract. Intradermal testing and serum testing can both be employed to help define an individual horse’s sensitivities, however, intradermal testing is more common.
Serum testing is typically simpler, a sample of blood is extracted from the animal and used to test against allergens; intradermal testing is more invasive, but also more reliable. The intradermal procedure involves sedating the patient and shaving a patch on their necks, then injecting a minuscule amount of common or suspected allergens under their skin in a grid pattern. The injections will then be evaluated at several intervals to ascertain which allergen is triggering the response, starting at fifteen minutes from injection until twenty-four hours after being injected.
Cold water is often a useful technique to reduce allergy-related fever and inflammation until your veterinarian has been able to examine the animal to ascertain an appropriate treatment plan, and if the site of the sting is known, applying ice to it for ten minutes will both ease pain and cool the wound. Treatment plans for allergic reactions may include the administration of corticosteroids and antihistamines to help reduce the symptoms that are related to allergic reactions, and anti-inflammatory medication will often be recommended to reduce the associated fever, swelling, and pain.
If the horse has severe allergies to bee venom and is showing signs of anaphylactic shock, supportive treatments are likely to be started without delay and epinephrine will be injected directly into the animal’s jugular in order to reduce the swelling in the lungs enough to allow the horse to breathe. With any moderate to severe reaction, an IV line is likely set up to provide the large volumes of water that are needed to support the large animal's circulatory system adequately, as well as to administer any additional medications that are required.
In order to prevent new reactions to bee or wasp venom, horses may be further treated with immunotherapy. Immunotherapy for animals can take the form of either injections or drops, and it contains small amounts of the allergen in order to desensitize the animal. The compounds that are found in the shots or drops that are given to the animal are specially formulated for the animal to be treated and typically requires administration on a frequent basis for several weeks to months before the body is able to tolerate the venom. It may take six months to a year before the benefits of immunotherapy are realized.
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