Insect bite hypersensitivity

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Farm Forum

During the spring and summer many horse owners have to deal with insect hypersensitivities, also known as “sweet itch,” or “summer itch.” Certain insects’ bites, especially Culicoides (very small flies), are known to cause allergic reactions in some horses. Signs are usually first noticed when horses are relatively young, at about 2-4 years of age.

In temperate climates the signs of insect hypersensitivity usually begin in the spring as insects start to emerge. The most common sign is pruritis (itching). Little bumps or papules are also a common sign of hypersensitivity. The pruritis often worsens over the summer and then dissipates in the fall as the insects disappear. Horses may rub out their manes and tails or scratch so hard that they create abrasions on their bodies. In the case of Culicoides hypersensitivity, horses usually seem to be most itchy along their top line, although they can also be affected on the neck, shoulders, ears, face and belly. Sometimes the abrasions can develop secondary bacterial or fungal infections that have to be treated specifically. In horses that have been affected for several years, the repeated itching will cause scarring, hyperpigmentation (darkening of the skin) and hair loss.

The diagnosis of insect hypersensitivity is usually made based on clinical signs and history including the time of year that the signs are observed and the repeatability from year to year. Reduction of clinical signs after the institution of insect control measures is additional evidence of insect hypersensitivity. If a horse is highly suspected of having an insect hypersensitivity further testing can be performed to confirm the diagnosis, such as skin tests or serum analysis. Skin testing involves injecting whole-insect antigen into the skin (intradermally) and then examining the resulting localized skin reaction. Most horses will have some reaction to the injection, but those horses that have a true hypersensitivity will have a much more pronounced reaction. Skin testing does involve clipping hair in the neck area to enable visualization of the skin reaction, which is not ideal in competition horses. Alternatively, serum samples can be analyzed for specific antibodies to certain insects. Serum testing is not as reliable as skin testing, but if the results fit with the clinical history than it can be a very useful diagnostic tool.

Treatment of insect hypersensitivity includes several different tactics. First of all, the affected horse’s exposure to insects should be reduced as much as possible with changes in management. This includes the use of fly sprays to repel insects. The ingredients in fly sprays can vary and owners should ensure that they find a product with a relatively high concentration of pyrethroid-permethrin. The peak feeding times for Culicoides and mosquitoes are at dawn and dusk. Therefore, try to keep sensitive horses in the barn during these periods of the day. Strong fans are also effective at keeping many insects away. Fly masks and fly sheets can also be effective. Culicoides and mosquito larvae develop in standing water, so reducing stagnant water can reduce the insect populations. Fly larvae develop in manure piles, so frequent manure removal or pasture dragging is also effective. Several feed supplements that reduce the ability of insects to develop within manure are available on the market.

To treat the horses’ clinical signs, a veterinarian can prescribe steroids such as; dexamethasone or triamcinolone to reduce itching and irritation. Steroids are potent drugs and can have side effects, so horses should be weaned off of them as soon as possible or they should be used at the minimal effective dose. Dexamethasone is usually needed daily to every other day. Triamcinolone can be applied topically, as well as an injection of 40-80 mg (1-2 mls) in the muscle once every 2-6 weeks. Antihistamines can be effective in some cases and may negate the need for any steroid treatment or provide a longer term treatment option so that horses can be weaned off of steroids. Hydroxyzine is a common antihistamine, and horse owners have seen results with cetirizine (Zyrtec). Despite many horses having severe insect hypersensitivities the condition is very treatable especially with the convenience of a long acting steroid like triamcinolone.

Dr. Darin Peterson, DVM, was born and raised on a horse and cattle ranch in Rosholt, S.D., and received his B.S. in Animal Science from SDSU. He concentrates most of his work time with large animals. He can be reached at 701-347-5496 or dpeterson@cassvetservice.com.