Mosquitoes and several kinds of biting flies, among them stable flies, black flies, biting midges and sand flies, feed on blood. Their bites irritate and hurt, repeated bites can leave open sores, and some of these insects carry disease. [1][5] On dogs and cats the skin damage shows up mostly on the ears. This guide covers what the bites do to the skin, the mosquito bite allergy some cats have, and how to cut down on bites. Maggots in wounds (fly strike) and bot fly larvae under the skin are covered in maggots and warbles in dogs and cats.
Fly bites on a dog’s ears
In dogs, the best known problem is the stable fly, a fly that looks much like a house fly but feeds on blood. Both males and females bite, usually twice a day, and in dogs they go mainly for the ear flap. Vets call the resulting sores fly strike dermatitis (fly bite dermatitis). [1]
What it looks like: [1]
- Redness and dark, crusty material left by blood and fluid oozing from the bites.
- Crusts, raw patches and small ulcers that can turn into open wounds with repeated bites.
- In dogs with upright ears, the sores sit on the tip of the ear flap. In dogs with floppy ears, they sit on the folded edge.
Dogs that live outdoors, especially dogs kept penned near horses, donkeys or farm animals, are more likely to get these sores. The sores only last while the bites go on, and they tend to clear once the flies stop biting. [1] Before calling it fly bite dermatitis, vets rule out look-alikes such as mange and ear mites. [1]
Mosquito bite allergy in cats
Some cats do more than itch after a mosquito bite. Mosquito bite hypersensitivity is an allergic reaction to the bites, related to flea allergy. [2]
What it can look like: [2]
- Small crusty bumps (miliary dermatitis), the most common sign.
- Itch on the head, neck and ears, with sores gathered on the outer side of the ear flaps and along the bridge of the nose.
- Now and then the paw pads are involved.
- Less often, raised eosinophilic sores such as plaques or granulomas.
The problem is reported more often in cats that go outside in places where mosquitoes are common. [2]
A study in Veterinary Dermatology described 26 cats with mosquito bite hypersensitivity. Most were short-haired cats that roamed freely outdoors, and the typical signs were seasonal (summer) and appeared on both ear flaps in a matching pattern, as small crusty bumps. [3] When five of these cats and three healthy cats were bitten by mosquitoes on purpose, all bite sites showed welts within 20 minutes, and bumps appeared over 12 to 48 hours. The healthy cats showed only slight, short-lived redness after being bitten. [3] The authors concluded that the reaction is triggered by an allergic (type I hypersensitivity) reaction to mosquito antigens. [3]
Crusty sores on a cat’s ears and face can have other causes too, such as mange, ear mites and the eosinophilic granuloma complex. Skin patterns like these point toward allergy but are not a diagnosis on their own, so vets first look for parasites and infections. [2]
Diseases spread by mosquitoes and flies
- Heartworm. Heartworm disease is spread only through the bite of a mosquito. Dogs are the main target, and cats can be infected too, though they are less susceptible than dogs. [4]
- Leishmaniasis. Sand flies carry Leishmania parasites, and dogs are an important host for the species that infects dogs and people around the Mediterranean. [5]
- Stable flies can also carry some germs from one animal to another, mostly among farm animals. [6]
Your vet can tell you which of these are a concern where you live.
How vets handle bite reactions
- Dogs with fly bite sores on the ears: the main aim is to stop the bites, because the sores heal on their own once the flies are kept off. [1] Your vet can advise on fly protection that is right for your dog.
- Cats with mosquito bite allergy: avoiding mosquitoes is the mainstay. Ideally the cat stays indoors, where it meets fewer mosquitoes, and your vet may treat the itch when it flares. [2]
A safety note for cats. Many insect repellents made for people are unsafe for cats, and some insecticides used on dogs are toxic to cats. [2] Do not use any product on a pet unless its label clearly says it is for animals, [7] and ask your vet before putting any repellent or insecticide on a cat.
Cutting down on bites
- Get rid of standing water around the home, so mosquitoes have fewer places to breed. [2]
- Keep a mosquito-sensitive cat indoors. [2]
- Clean up where stable flies breed. Stable flies breed in rotting material like manure and dirty bedding, so regular clearing of manure and old bedding is the simplest step against them. [6]
- Think about where your dog spends time. Dogs kept outdoors next to horses or farm animals get bitten more. [1]
When to see your vet
- Crusty, bleeding or ulcerated ear tips, or sores on the folded edge of floppy ears. [1]
- A cat with crusty bumps or sores on the ears or the bridge of the nose, especially if they spread or come back every summer. [2][3]
- Many bites at once, or a pet that seems unwell after being bitten.
- Maggots in a wound or a swelling with a breathing hole: see maggots and warbles in dogs and cats, and have your vet deal with it.
- Before using any insect repellent or insecticide on a cat. [2][7]
FAQ
Why are my dog’s ear tips crusty and bleeding?
One cause is fly bites. Stable flies bite dogs mainly on the ear flap, leaving redness and dark crusts on the tips of upright ears or the folded edge of floppy ears. [1] Mange, ear mites and other conditions can look similar, so have your vet look at it. [1]
Can cats be allergic to mosquito bites?
Yes. Mosquito bite hypersensitivity is an allergic reaction that usually shows as small crusty bumps on the outer ear flaps and the bridge of the nose. [2][3]
Can I use my dog’s insect repellent on my cat?
No, not without asking your vet. Some insecticides used on dogs are toxic to cats, and many repellents made for people are not safe for cats either. [2]
Are fly bites dangerous to my dog?
The bites hurt and repeated bites can cause open sores. [1] Some biting insects also spread disease: mosquitoes spread heartworm to dogs and cats. [4]
The bottom line
Biting flies and mosquitoes hurt, and repeated bites can leave sores. [1] In dogs, stable flies cause crusty, bleeding sores on the ear tips or the folded edge of floppy ears, and the sores clear once the bites stop. [1] In cats, mosquito bite allergy causes small crusty bumps on the outer ears and nose, and keeping the cat indoors is the main step. [2][3] Mosquitoes also spread heartworm. [4] Many repellents made for people, and some insecticides used on dogs, are not safe for cats, so ask your vet first. [2]
This article is for general information and is not a substitute for a veterinary exam. If you are worried about your pet’s skin, see your veterinarian.
Sources
- Castilla-Castaño E, Moog F, Mandin-Cabaret C, Pressanti C, Cadiergues MC. Control of fly strike dermatitis in dogs with a topically applied combination of imidacloprid and permethrin: a prospective open-label controlled clinical trial. Parasit Vectors. 2019;12(1):132. doi:10.1186/s13071-019-3356-4. Open access (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC6429772/
- Diesel A. Cutaneous Hypersensitivity Dermatoses in the Feline Patient: A Review of Allergic Skin Disease in Cats. Vet Sci. 2017;4(2):25. doi:10.3390/vetsci4020025. Open access (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC5606602/
- Nagata M, Ishida T. Cutaneous reactivity to mosquito bites and its antigens in cats. Vet Dermatol. 1997;8(1):19-26. PMID 34645030. https://pubmed.ncbi.nlm.nih.gov/34645030/
- Keep the Worms Out of Your Pet’s Heart! The Facts About Heartworm Disease. U.S. Food and Drug Administration, Center for Veterinary Medicine. https://www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease
- Touati A, Idres T, De Champs C, Basher NS. Emerging Arthropod-Borne Infections in Temperate Regions: Comparative Synthesis Across Mosquitoes, Ticks, Sandflies, and Biting Midges. Insects. 2026;17(3):311. doi:10.3390/insects17030311. Open access (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC13026707/
- González MA, Duvallet G, Morel D, de Blas I, Barrio E, Ruiz-Arrondo I. An Integrated Pest Management Strategy Approach for the Management of the Stable Fly Stomoxys calcitrans (Diptera: Muscidae). Insects. 2024;15(4):222. doi:10.3390/insects15040222. Open access (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC11050470/
- Using Insect Repellents Safely and Effectively. U.S. Environmental Protection Agency. https://www.epa.gov/insect-repellents/using-insect-repellents-safely-and-effectively