Black Fly Bite on Dogs

Rhiannon Koehler, DVM
By Rhiannon Koehler, DVM on Oct. 4, 2023
woman playing with dog on lawn.

Flies are rather annoying insects, both to us and our dogs. However, the black fly in particular can be more trouble for our canine companions.

Black fly bites on dogs may cause extreme itchiness, pain, and sometimes severe allergic reactions, so it’s important to know what these insects are and what signs to look for.

What Are Black Fly Bites on Dogs?

The black fly is a small, winged insect with a black, gray, or yellowish-brown body. Black flies, also called buffalo gnats, are especially active in the humid spring and summer months. In sub-tropical areas, black flies may be active year-round.

Female black flies are aggressive biters. Unlike mosquitoes, these flies don’t cut into a blood vessel when they bite. Instead, they cut the skin and drink up the small pool of blood that forms. Male black flies don’t bite.

During black fly season, swarms of black flies surround and bite dogs, causing small, itchy, bloody spots on the body (especially the underside and inner thighs), ears, and face. These swarms usually happen during the daytime when the air is calm.

What Do Black Fly Bites Look Like on Dogs?

Black fly bites on dogs are typically found in areas with less fur, such as the underside of the abdomen, the insides of the hind legs, or the ears.

These bites can be extremely itchy. Sometimes, the bites cause scabbing and open sores or dark red bumps.

Dogs with black fly bites often develop a very specific rash on their underside and inner thighs, which is actually a collection of black fly bites. The individual bites have a pinpoint puncture, surrounded by a paler area of swelling, and then a red outer rim. Each bite forms a bullseye or target structure.

During black fly season, swarms of black flies surround and bite dogs, causing small, itchy, bloody spots on the body."

Because of the unique look of the bites, your veterinarian usually knows your pet has been swarmed by black flies after seeing the rash.

Rarely, dogs can also have allergic reactions to black fly bites. If your dog has hives on other parts of their body, has facial swelling, or develops sudden vomiting, visit a veterinarian right away. Sudden allergic reactions or anaphylaxis can be deadly if left untreated.

Difference Between Black Fly Bite and Tick Bite on Dog

When you hear “bullseye lesion,” you may think of the characteristic rash caused by a tick passing on Lyme disease in humans. Because of this, pet parents often think the bite of a black fly is a tick bite. However, tick bites on dogs don’t cause the target lesion.

Keep in mind that black flies often attack in swarms, so you’ll usually see multiple bites in one area of the body. Tick bites are usually more solitary.

Black flies also don’t latch onto dogs. They bite, feed, and fly away. Ticks, on the other hand, attach to the pet while they’re feeding, so you often find them still attached to the dog’s skin.

Both black fly bites and tick bites can be itchy. Ticks also carry diseases to dogs like Lyme disease or ehrlichiosis, while black flies do not carry diseases that affect dogs.

Infected Black Fly Bite on Dog

Any insect bite can get infected if the dog scratches or chews at the bite, including black fly bites.

The key sign you’ll want to watch for is drainage or pus. Clear fluid may be normal, particularly if the bite formed a sore. However, if the fluid is white, yellow, green, or brown, the bite is probably infected.

Infected bites may be more swollen than others, painful to the touch, and slower to heal.

If you think a bite may be infected, it’s best to talk with your veterinarian. Some infected bites may need antibiotics.

Examples of antibiotics your veterinarian may recommend giving by mouth include cephalexin or cefpodoxime. Examples of topical antibiotics include Animax® ointment or antibiotic shampoos. Don’t use triple antibiotics meant for human use. Your veterinarian will decide which antibiotics are best for your pet’s case.

Treatment of Black Fly Bites on Dogs

Treatment of black fly bites is focused on relieving itchiness and discomfort, promoting healing, and preventing infection.

If your pet isn’t excessively itchy or in pain, these bites usually heal without the need for veterinary attention. Ensure you’re keeping your pet from biting or scratching. You can do this by placing a recovery cone on your pet and having them wear protective clothing over bites.

At-home steps you can take to help relieve discomfort and itchiness include:

  • Applying a cold compress like an ice pack to the bites. Make sure to put a cloth between the ice pack and the skin.
  • Placing hydrocortisone cream directly on the bites.

You may be tempted to give your pet a soothing oatmeal bath. While you can bathe your dog, there are several things to keep in mind:

  • Warm or hot water can increase histamine release, making your pet itchier.
  • Moisture may lead to infection and encourage your pet to lick the area. Make sure you gently dry your dog after you bathe them.

If your pet is excessively itchy and uncomfortable, contact your veterinarian. They may recommend over-the-counter antihistamines like diphenhydramine (Benadryl®) or cetirizine (Zyrtec®). Follow your veterinarian’s dosing recommendations as they may be different from the directions on the bottle.

If the bites are infected, antibiotics may be recommended. If this is the case, follow your veterinarian’s directions to give antibiotics correctly.

In rare cases of severe allergic reaction or anaphylaxis, you must seek emergency treatment for your pet. Do not use an EpiPen prescribed for a human on your pet.

How to Protect Your Dog from Black Fly Bites

Preventative measures to protect your dog from black fly bites include:

  • Avoiding running bodies of water like rivers, streams, and creeks during black fly season. Black flies breed in running water.

  • Avoid woodlands and farms during black fly season.

  • Avoid leaving your dog outside in the several hours after sunrise and several hours before sunset. Swarms occur during the daytime, typically in the morning or the late afternoon/evening.

  • Consider protective clothing. Light clothes that protect the dog’s underbelly and thighs may prevent swarms from getting to the parts of the body they like best.

  • You can put SWAT® fly repellent on your dog’s ears. It’s especially helpful for preventing bites on the ears.

No DEET-containing bug sprays are currently FDA-approved for use in dogs. You can try commercially available over-the-counter bug sprays meant for dogs, but these aren’t as effective as avoiding swarms altogether.

Black Fly Bite on Dogs FAQS

Do black fly bites hurt dogs?

Yes, black fly bites are painful. However, dogs vary in how they respond to the bites. Some dogs seem relatively unfazed by black fly bites while others are very sensitive.

Do black fly bites on dogs heal on their own?

Black fly bites often heal on their own and may not need veterinary attention. Keep your pet from scratching or chewing at the bites. Get veterinary care if you’re worried about allergic reactions or infection, the bites aren’t healing, or the pet is scratching or chewing too much.

Featured Image: Getty/Bogdanhoda

Rhiannon Koehler, DVM


Rhiannon Koehler, DVM


Dr. Rhiannon Koehler is a veterinarian and freelance medical writer. She received her Doctor of Veterinary Medicine and Master of Public...

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