The common stable fly is the most common cause, however, blackflies, deer flies, and houseflies can also bite.
Fly bites cause a common condition in outside dogs, often called "fly strike." It most frequently occurs in dogs that live near farms with livestock, and occurs when flies land on and bite the tips and top surface of the dog's ears. They may also bite the bridge of the nose. They are biting the dog to obtain a blood meal and the bite is often painful. Because of the thin skin and hair on the ears and the inability of the dog to defend his ears, flies will seek out this location. Some of these dogs will endure hundreds of bites a day for weeks at a time. Anyone that has ever been bitten by a stable fly knows how painful the bite is. If you see blood spots or flies congregating near your dog's ears then you can assume fly bites are occurring. The bite wounds attract flies which may lay their eggs in the damaged tissue. These will later hatch into maggots.
Cats are much less likely to acquire fly bites.
The ears, especially at the margins, will have painful bumps, sometimes with scabs or bleeding.
Whether you live in the city or country, do not underestimate the pain that these flies can inflict and be sure to initiate treatment at the first sign of fly bites on your pet. Gently cleanse the ear with warm water and a mild antiseptic soap. Then, apply a topical antibiotic ointment which will help to control any infection which may be present. If the fly bites are severe, or maggots are present, veterinary attention is needed.
The most important treatment for this problem is prevention, which consists of applying a topical insecticide to the dog's ears. Pyrethrin or permethrin sprays and ointments are very effective in repelling flies. Homemade solutions made by adding DEET or pyrethrin to petroleum jelly also work well. Moving the pet indoors during the heat of the day also will help. Spraying the dog's outside living quarters will also help keep fly numbers to a minimum. Remove any materials which may attract flies, e.g., fecal material, uneaten pet food, garbage, etc. Keep your pet clean and well-groomed, removing any urine-soaked or fecal-contaminated hair.
General treatment: If severe, the ear may be washed with a wound cleanser under sedation or anesthesia. The maggots, if any, will be removed, as will any dead or dying tissue.
Supportive treatment: None usually required, although a mild pain reliever may be prescribed. Rarely are systemic (oral or injectable) antibiotics necessary, unless the fly bites are severe.
Specific treatment: Unavailable
Good, unless maggot infestation is severe. The ear may be scarred and remain thickened.