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What is Flea Bite Dermatitis?

By Sheri Cyprus
Updated Jun 04, 2024
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Flea bite dermatitis is a skin condition in animals caused by an allergy to flea saliva. It produces itchiness and excessive scratching, which can lead to bare patches and skin lesions. When these insects bite, they inject a small quantity of saliva into the wound; this saliva contains more than 15 different substances that could provoke an allergic reaction in animals. The condition can be caused by a single bite, and so just one insect can produce a severe allergic response and prolonged discomfort in an animal. It is most commonly seen in domestic pets, such as dogs and cats, but other animals can be also be affected.

Fleas are most prevalent in summer and fall, so these seasons have the highest numbers of cases. All breeds of dogs and cats can be affected by flea bite dermatitis, but it is more likely to occur in young animals than in puppies, kittens or older pets. Although infestations can affect any animal, only a minority have this allergy.


The first symptom in an affected animal is likely to be scratching. All pets will scratch from time to time — it is a normal part of animal behavior — but very frequent scratching and chewing at particular places may be a sign of this allergy. Usually the lower back, inner thighs, abdomen and head are the worst affected areas. Fur may be lost from these parts, and the skin may develop tiny red lumps, along with abrasions caused by scratching; after a time it may thicken. The problem may be worse for cats, as their sharp claws can cause more damage to the skin.

Although these symptoms are distressing for the animal, and for its owner, the problem itself is not considered serious. If left untreated, however, heavily scratched areas may become infected. Some bacterial skin infections may be severe, and for this reason, any symptoms of flea bite dermatitis should be investigated as soon as possible.


The symptoms result from the animal’s immune system reacting to an antigen in the flea’s saliva. Confirmation of the condition requires a visit to a veterinarian, who can diagnose it with an intradermal allergy test. The vet injects a little of the antigen into the animal's skin and observes what happens. Basic blood tests are usually less accurate than the intradermal test in diagnosing the problem.

Pet owners can also check for fleas. The insects are more easily spotted in animals with light colored fur, but a useful technique is to stand the animal on a sheet of damp white paper or cloth, and run a fine comb through its fur. The insects, and their dark colored feces, will be clearly visible against the white surface.

Treatment and Prevention

Cool, but not cold, water baths may help a dog suffering from flea bite dermatitis; cats, however, are unlikely to tolerate this. A veterinarian will often prescribe hydrocortisone for allergic animals in order to help relieve some of the itching and redness of the skin. The animal should be monitored for secondary infections, as the scratching tends to open up the sores further, leaving them vulnerable to bacterial and fungal conditions. If infection is suspected, the veterinarian may prescribe antibiotics.

These measures treat the symptoms, but not the cause. The only way to eliminate the condition is to get rid of the fleas. This means using one of the many treatments suitable for the type of pet. Any other pets in the house should also be treated, as the insects can move easily from animal to animal. In addition, it may be necessary to treat parts of the house with a suitable insecticide, in order to destroy eggs and larvae, which may be present in carpets and furniture. Repeat treatments may be required.

Flea Habits and Life Cycle

An understanding of the way fleas develop, behave and reproduce can help to ensure their eradication. Adult females typically live for less than two months, but during that time, an individual insect may lay over 2,000 eggs, at a rate of up to 50 per day. The eggs are laid in the host animal’s fur, but tend to drop off onto carpets, cushions and bedding, which make ideal hiding places for the larvae, when they hatch. They feed mostly on the droppings of the adults, which tend to build up in the same places, and provide them with the iron and protein they need for growth.

In common with other insect larvae, the juveniles pupate, that is, go into an inactive phase within a cocoon, for about two to four weeks before emerging as adults. Although some insecticides can kill the eggs, the cocoons are very resistant. For this reason, several treatments may be required, although vacuuming of likely areas can be effective. Like mosquitoes, the adults feed on blood, but whereas a mosquito feeds only once, fleas do so frequently, throughout their lives. They will tend to migrate to parts of the animal that are difficult for it to reach during grooming, such as the back of the neck, but they are very mobile and can be found anywhere.

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Discussion Comments
By KaBoom — On May 03, 2012

It's interesting that humans and animals can actually be allergic to flea bites. Obviously being bitten by any kind of bug is unpleasant, but if you're allergic it's ten times worse!

I'm actually surprised I haven't heard more about flea bit dermatitis, since there are 15 different things in a flea's saliva that could cause a reaction. And just think, you'd only have to be allergic to one! But still, I don't know anyone who has had flea bit dermatitis, or who has dealt with it in their pet.

By JaneAir — On May 03, 2012

@ceilingcat - Yeah, keeping your pet indoors definitely helps keep fleas and ticks away. It's easier to do with a cat though, most dogs really seem to want to go outside.

Anyway, I just wanted to share another tip for flea treatment: make sure you get flea medicine that kills the eggs too. Not all flea treatments get rid of the eggs, which is not good. If you don't kill the eggs, they'll hatch and re-infest your pet (and possibly your home) with fleas. No one wants that! Then you have to treat the pet all over again.

By ceilingcat — On May 02, 2012

@John57 - That's a great tip to see if your dog or cat has fleas! Thanks for sharing.

I have a cat, and he's never gotten fleas. I do keep an eye out for excessive scratching though, just in case. He's also a totally indoor cat, so I think that's part of the reason he's never gotten fleas

On the other hand, my sister has a dog that she lets run around her yard a lot, and the dog recently got fleas and got flea bite dermatitis. I'm going to share some of the tips from the comments with her, so hopefully her dog won't have to go through that again.

By sunshined — On May 01, 2012

One of my dogs had flea bite dermatitis and I felt so bad for her. She had raw patches of skin where she had done so much scratching.

It is hard to comprehend how miserable something or someone can be from one tiny flea. My vet did the allergy test on my dog and it was positive.

Thankfully the steroids she gave my dog began to work very quickly. It took awhile for all the hair to grow back, but at least she was free from the misery of the constant itching and scratching.

I have a similar reaction when I get stung by a mosquito, so I have some kind idea of what my dog felt like. The area becomes red and swollen and the more you scratch at it, the more it seems to itch.

By John57 — On May 01, 2012

Any time I see my dogs or cat doing excessive scratching I am always afraid they have fleas.

My vet has a simple test he does to check for fleas. When they are on the stainless steel table he will comb out a few handfuls of their hair.

Then he simply takes a spray bottle of water and lightly sprays the hair and the debris on the table. If there are fleas, there will be tiny red spots of blood that show up when you spray the water.

Thankfully my pets have never had fleas. This is amazing to me because they spend a lot of time outside. He told me if one of them gets fleas, they will probably all get them, because they are very contagious.

By golf07 — On Apr 30, 2012

Once your pet gets fleas it can be hard to treat them and your home. I had a cat once that was absolutely miserable with fleas.

She would climb on top of the refrigerator because it was one place where there was no carpet.

We took her to the vet and he gave her a flea bath and we had to 'bomb' our house to get rid of the fleas.

It was quite an ordeal and we weren't able to go back inside for several hours. Ever since then I have tried to be pro-active when it comes to fleas and my pets.

It is much easier to prevent them than it is to try and treat them after they already have them.

By OeKc05 — On Apr 30, 2012

@ddljohn – Yes, humans can have flea bite dermatitis. Though many people just get one itchy spot like that a mosquito would cause, people with a flea allergy have a more intense reaction.

My little sister got bitten by a flea once, and she got several red, itchy bumps around the bite. This worsened into a purplish rash, and it didn't just stay in that one area. It spread all over the bottom of her leg, and she kept scratching the whole region.

My mother called the doctor for advice, and he told her that she should give my sister an antihistamine. She also should spread hydrocortisone cream on the rash to control the itching.

By wavy58 — On Apr 29, 2012

@Perdido – Thanks for the tip! I will give that six-month flea collar a try on my hunting dog.

He has been sticking his foot in his ear and scratching a lot lately. He moans while he does it, and though I have tried looking in there, I can't see anything but a patch of skin he has rubbed raw. I've tried cleaning the area, but that doesn't seem to help.

After reading this article, I think that he may have flea bite dermatitis. He also often sticks his foot in his mouth to bite it, so it isn't just confined to his ear. I know that he has fleas, despite my best efforts to keep them off of him, and I think that collar sounds like a good plan.

By orangey03 — On Apr 28, 2012

I don't know if this is a scientific fact, but in my experience, dogs with light colored fur and short hair are more susceptible to flea bite dermatitis. I have four dogs, two with long hair and two with very short, white-blonde hair, and the latter have way more sensitive skin.

Maybe the lack of pigmentation has something to do with it, but I know that they have had far more skin problems than my darker, furrier dogs. During flea season, they get red lesions and rashes on their thighs and around their tails, and I have to keep treating their skin.

At first, I thought that maybe I could just see the irritation better on my light dogs, but then I noticed that the other dogs really weren't scratching or licking much at all. I think it really does have to do with skin and fur tone.

By Perdido — On Apr 28, 2012

My poor hunting dog spent so much time in the woods that even his monthly spot treatment for fleas and ticks was not keeping them off. He broke out in a red, lumpy rash on his belly, and he could not stop scratching.

I could see that he was miserable, so I asked my vet what else I could do to keep the fleas away. She told me that I should get a high-quality flea collar that is designed to keep them off for six months. It cost a lot more than most department store flea collars, but that is because it works.

She gave me some medicine for his dermatitis, and I put the new flea collar on him. Within a few days, all his fleas were gone, and his ticks died, as well. He is so much happier and has much more energy, now that he doesn't have to focus on his itchy skin.

By ddljohn — On Apr 27, 2012

My cat is scratching more than usual, but I honestly can't see any fleas (she's dark brown so that makes it hard to see).

How will I know if my cat has fleas and/or flea bite dermatitis? How much scratching is too much? I've never seen a flea infection or allergy before so I have no idea what to look for.

And is there such a thing as flea bite dermatitis in humans? If my cat does have fleas, can the fleas bite me and potentially cause allergies too?

Also, can I put aloe vera gel on the areas on her skin she's scratching? I always use pure aloe vera gel when I gave allergic reactions on my skin. Not sure if that's safe for pets though. I guess she might lick it and swallow it, huh?

By serenesurface — On Apr 26, 2012

@turquoise-- I think a few select brands offer a special shampoo for flea bite dermatitis. It helps the itch and soothes the skin. If you can find it, it might be beneficial to bathe your dog with that.

You can also ask your vet for a corticosteroid drug. It's anti-inflammatory and works great for skin inflammations. My sister's dog took it for a couple of months when he had another type of skin allergy.

The best thing to do for the long-term is to try to keep your pet flea-free.

By turquoise — On Apr 26, 2012

My vet says that my dog has flea bite dermatitis. She has recently been treated for fleas and we've gotten rid of them. But while she had them, she itched the areas where they bit so much that the hair there has been lost. So she has these bald spots in certain places now.

My vet wasn't aware of her allergy until the bald spots appeared. We realized that the skin is abnormally red and slightly raised there.

I'm giving my dog baths more often and also putting anti-itch cream to prevent her from itching. But is there anything else I can do?

I want to treat this as quickly as possible because if my dog scratches any further, her skin is going to bleed and she will get an infection. Any suggestions?

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