This is the season our practice sees a lot of cases of itchy, crusty, flaky skin in horses. There is more than one cause for crusty skin but a major source is fly bite hypersensitivity. This problem is called names like sweet itch, ventral midline dermatitis, summer itch, and psoriasis (figure 1 and 2). It is a problem that can be difficult to treat because there is no one medication that is curative. Using systemic and topical treatments, we are often able to control the problem, although there is often recurrence annually. Here is what you should know if you think your horse is affected.

are on the underbelly of your horse. One major aspect of treating fly bite hypersensitivity is to stop the flies from biting. There are many ways to do this, ranging from topical fly sprays to protective coverings on the horse (fly mask, fly sheet, or belly cover). Placing fans in stalls also helps discourage flies from staying in the area and biting horses. Cattle ear tags impregnated with fly repellent can be tied to horse's mane, tail and halters. Insect growth regulators may be mixed in with the feed and passed into the manure reducing your resident fly population in a stable setting (Solitude (R) by Pfizer). Several spot on fly control products are available and similar to the ones popular in flea control in dogs. In the early Spring, many people put out "Fly-Predators" which are little insects that eat the fly larvae (www.spalding-labs.com) Often times using multiple defense systems against flies is necessary when a horse is known to be sensitive.

The most commonly affected areas are the belly (ventral midline dermatitis), the mane and tail (dorsal dermatitis), or a combination of both. Where the horse is affected has to do with where the specific species of fly likes to feed. In either ventral or dorsal dermatitis the face may also be affected. The main sign noticed by owners of affected horses is that the horse is itchy. They may find the horse is rubbing its tail against the fence, rubbing its belly on the ground, or that the horse seems agitated. Sometimes owners do not even see the horse actively scratching but notice the tail head looks ratted or the belly is hairless and crusted. Severely affected horses can be so itchy that they cannot be worked.

If you ask 10 veterinarians how to treat fly bite hypersensitivity, you will get 10 different answers. This is because there is not one treatment that is universally effective. The most important aspects for treatment are fly control, stopping the horse from itching, and clearing secondary infections. Since the whole process is initiated from the fly bite, if you can stop the fly from biting, this condition will not develop. This is easier said than done because a sensitive horse will not need many bites to become affected. It is very difficult to protect a horse at all times, but significantly lessening the fly burden will help the horse enormously.

The next aspect of treatment is to control itching. This is frequently accomplished with topical steroids, systemic steroids, or a combination of both treatments. The most common oral medications are prednisolone and dexamethasone. Intramuscular Depo-medrol has also been effective as a long acting itch relief (usually 2-4 weeks duration). Steroid use can lead to side effects and should be done with appropriate doses as prescribed by a veterinarian.

The third aspect of treatment is the control of secondary infection whether bacterial or fungal. Secondary infections do not necessarily occur in every case of fly bite sensitivity. If the affected areas ooze with white discharge or have a bad smell, it is likely that a bacterial infection has set up. Your veterinarian will decide if the infection can be cleared through treatment topically with shampoos or sprays or if it is more significant infection that requires a systemic antibiotic. Fungal infections may be more difficult to recognize by visual inspection of the affected area. Sometimes the skin lesions may be circular in nature. By pulling out just a few hairs (with roots intact) in an affected area, a culture for fungus can easily be performed. This may become necessary if a horse is not improving with typical treatments.

Another key to success is starting treatment early. Is it much easier to manage this problem early on when there is one small affected area than when the whole belly is one large crust. If a horse is known to be sensitive to fly bites, these measures should be started early in the fly season to avoid a horse becoming severely affected.

There is evidence that fly bite hypersensitivity in horses is genetic. If you have a mare or stallion that is severely affected, you may want to avoid breeding that animal. Additionally, if you have a horse that is affected, it may be worth letting the breeders know so that they can keep records or look for other affected family members.

Fly bite hypersensitivity in the horse can be a frustrating issue. Through measures to decrease fly exposure, stop itch, and clear secondary infections the problem can often be managed. Ongoing research will hopefully bring a cure to this challenging ailment in the future.