Ask the Vet, Sallie S. Hyman VMD, DACVIM: Shoo Fly! Horses vs. Flies ...

While people find their buzzing to be a nuisance, flies and gnats can cause several medical problems in our horses.  Not only are fly and gnat bites very irritating to horses, just like people some horses are more sensitive to them than other horses.  Further, flies can cause disease directly, mostly due to their bite, or by spreading disease that they are carrying.  Flies are excellent vectors for many diseases.

Sweet Itch:  Culicoides hypersensitivity, commonly called Sweet Itch or Queensland Itch, is cause by the bite of the culicoides midge (no-see-ums!).  Horses develop an allergy to the saliva of the culicoides midge.  The mane, tail, and ventral midline areas of the horse are most commonly affected.  Horses develop severe pruritis (itching) and will often create further skin damage with excessive rubbing. 

Deer Fly Hypersensitivity:  Deer fly hypersensitivity has also been seen in horses.  The allergens in the deer fly bite can cause severe hives to develop.

Eye Irritation: Eye irritation is very commonly seen during fly season.  Flies feed off of tears and bite the conjunctiva of the eye in the process.  Inflammation can develop that can eventually lead to blockage of the nasolacrimal duct and result in excessive tearing.  This, of course, leads to more feeding by flies.  Should you suspect a blocked tear duct in your horse, contact your veterinarian.  Your vet will examine your horse to make sure there is nothing else wrong with the eye and flush the nasolacrimal duct to open it up.  You may also be given some eye drops or ointment with antibiotics and steroids to help with the inflammation.  It is important for your veterinarian to examine and stain the eye before starting any eye treatment with steroids to make sure that there are no ulcers on the eye.

Corneal Ulcers: Corneal ulcers are common during fly season.  As horses try to rub away flies, they may accidentally scratch the corneal surface on fences or trees.  Any problem with the eye that includes tearing, cloudiness, holding the eye partially or fully shut, or swollen lids should be treated as an emergency.  Call your veterinarian to examine it. It is very important to get your horse on the correct medicine in order to treat possible bacterial and fungal infections.  Without correct treatment a simple ulcer can turn into a major problem.  As mentioned above, never use steroids in an ulcerated eye.  

Sarcoids:  Sarcoids are one of the most common skin tumors in the horse.  They can be flat roughened areas or protruding cauliflower like masses.  They are thought to be caused by a retrovirus or a papilloma virus.  Flies are suspected to be able to transfer the sarcoid cells from one horse to another by depositing them in traumatic skin wounds. 

Ear Papillomas:  Ear papillomas are virally induced skin tumors that appears as warts in the ear.  They are transmitted by black flies.

Summer Sores:  Habronemiasis, or summer sores, is casued by nemotode larve. Habronemia musca and Draschia microstoma are the species that affect horses.  The adults live in the horse's stomach.  The larvae are passed out into the feces where they are eaten by fly maggots.  The larvae stay within the developing fly and are then deposited on the lips, eyes, or wounds of the horse as the flies feed.  If the larvae are digested, they will develop into worms in the horse's stomach. However, if they are deposited in the eyes or traumatized skin, the larvae cannot develop and instead incite an inflammatory reaction.  The larvae become walled off in granulation tissue and a non-healing pink tissue mass ensues.  Yellow granules can be found within the mass.  These are dead and dying larvae.  As the flies like to feed in moist areas, Habronema lesions are commonly found on the lips, third eyelid, and penis.  The lesions are quite itchy, and the horse may cause secondary trauma from rubbing.

Pigeon Fever:  Pigeon fever, caused by the corynebacteria pseudotuberculosis, is also carried by flies.  The flies introduce the bacteria into the skin and underlying tissues.  it is then carried by the bloodstream to deeper tissues and lymph nodes.  Abscesses characterize the disease.  They are most commonly found in the pectoral (chest) region, causing the chest to look as if it is protruding, much like a pigeon's breast.  Your veterinarian should be consulted if you suspect your horse has pigeon fever.

Rain Rot:  Dermatophilosis, or rain rot, can be spread by flies although it is more likely to be transmitted from horse to horse by shared tack, brushes, and blankets.  This bacterial infection, caused by dermatophilous congolensis, results from the bacteria being introduced to moist, injured skin.  The matted, crusty scabs that lift off easily to reveal moist lesions underneath are unmistakable.  Your veterinarian should be consulted if you suspect your horse has rain rot.