cayenne-tick-542169_640M. Kathleen Shaw, DVM – Vermont Veterinary Medical Association

During the hot summer months, both large and small animals can suffer unwanted attention from biting flies and mosquitoes. These flies are not only the typical “house flies” we see, but species such as black flies, deer flies, and horse flies. In many cases, the mosquitoes and flies prefer to “strike” the sensitive and fairly thin skin of the animal’s ears. This has led to the common term “fly strike” as a means of describing the problem. Mosquitoes can carry disease to small and large animals. Hot days with little wind allow these insects to attack and cause more problems than days with a brisk breeze. Fly strike is uncomfortable for the animal and can be seen as seen as crusty and/or oozing lesions along the ear flaps: as the ears are continually wounded, more flies are attracted and bite.

Be particularly careful with older, long-haired pets who may not be very active when outside. In areas of the body prone to moisture and soiling, especially the rear end, it is common to find skin damage from the fly larvae (maggots). If you notice this problem, bring your pet to your veterinarian as soon as possible to avoid serious systemic illness.

In horses, biting flies can cause decreased performance and certain mosquitoes can transmit disease such as West Nile Virus (WNV) and Eastern/Western Equine Encephalitis Viruses (EEE/WEE). Besides this, they are annoying and painful to your horse, just as they are to you. There are many things you can do to protect your horse from these pesky insects. Help reduce the mosquito population by removing standing water (where mosquitoes breed), if possible add fish that eat mosquito larvae to water sources, and completely empty the stock tank periodically to rid it of any mosquito larvae. Keep horses stalled during peak mosquito feeding times of dusk and dawn and use mosquito repellants on them. Most products that repel mosquitoes will also repel biting flies. Many horse owners will attach insect proof face masks to their horses, to help prevent clusters of flies gathering around their eyes. Since wild birds are the source of WNV infection to mosquitoes, discourage them from roosting in the horse barn/stables. It is important to also talk with your veterinarian about vaccination for the viral diseases.

In our companion animals, fly strike can cause infection and pain. Mosquitoes can transmit potentially fatal heartworm disease to both cats and dogs. Cats in particular can be extremely sensitive to mosquito bites around their ears and face, leading to scabs, swelling, and itching. To help prevent fly strike and mosquito bites, try not to leave your pets outside all day long during hot summer days. Eliminate standing water to help reduce the mosquito population. While there are many products available to repel flies and mosquitoes, it is extremely important that you talk to your veterinarian before applying a human over the counter insect repellant: many of them contain ingredients that are toxic to dogs and cats. In addition, don’t assume that if the product is okay to use on dogs that you can use it in cats. There are many over the counter and dog-only products that can kill your cat. A monthly heartworm preventative (either a topical or a pill) can help prevent heartworm disease in your dog and cat, and some topical products now repel mosquitoes.

Remember, all of our livestock and companion animals are prone to mosquito bites and fly strike. For prevention, keep animals in at peak feeding times of these pests, do what you can to help eliminate standing water, and talk to your veterinarian about preventatives and repellents for our animals’ protection. For more information on biting insects and disease in our animals, visit


The Vermont Veterinary Medical Association (VVMA), founded in 1898, is a professional
organization of 330 veterinarians dedicated to compassionate animal care and quality medicine.