Vet Advice: Dog Flea Allergy and What to Do About It

What to do about a dog who is crazy itchy
By Sara Greenslit DVM, March 2017, Updated June 2021
Flea allergy dermatitis

The first thing you notice is hair loss along your dog’s neck, spine and thighs. The skin is flecked with scabs and hot to the touch. Then there’s the scratching: automatic, back-foot-reaching, irrepressible. You may—or may not— see live fleas, or only scant flea dirt (specks of digested blood).

Often, a client will say to me, “But my other cat/dog is just fine.”

That’s just it, though—not all pets are allergic to fleas. But for the ones who are, the suffering can be extreme. Itching causes a distinct distress; as Dante wrote in his 14th-century epic poem Inferno: “As every one was plying fast the bite/Of nails upon himself, for the great rage/Of itching which no other succor had.”

Flea allergy dermatitis (FAD) arises when a dog’s immune system overreacts to flea saliva. The severity of the itching doesn’t necessarily correlate to the number of flea bites the dog’s dealing with; sometimes, it only takes a few to generate a whole lot of scratching. Many dogs will also have secondary bacterial and yeast infections as well as environmental allergies, all of which aggravate the itch.


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William Oldenhoff, DVM, DACVD, is a dermatologist at LeadER Animal Specialty Hospital in Cooper City, Fla. He prefers to attack Flea allergy dermatitis from several different directions. The first step is the year-round use of a flea preventive, which takes a while to resolve the infestation because the flea life cycle ranges from one to two months, depending on environmental conditions, and pupa can survive for up to a year before becoming adults.

Oldenhoff also advocates thorough house cleaning. “Vacuum all surfaces, paying particular attention to the areas just adjacent to walls and corners, and under furniture. Be sure to clean the furniture as well, and launder any bedding the dog sleeps on.” He does not recommend having the house itself sprayed or otherwise treated, since flea preventives and meticulous environmental cleaning are usually adequate.

Oldenhoff’s third treatment tier includes a systemic such as oclacitinib or steroids to relieve the itching. However, he cautions, the fact that the dog stops his mad scratching doesn’t mean the fleas are gone. “When these therapies are prescribed, the owner must be made aware that the pet will be feeling much more comfortable, but the flea infestation is still present, and thus flea control must still continue.”

Other Flea allergy dermatitis treatment considerations include medications for secondary bacterial and yeast infections, and a dewormer for tapeworms. Fleas harbor Dipylidium caninum larvae, aka the flea tapeworm. If an infected flea is ingested—for example, as the dog grooms himself—the larvae develop into adult tapeworms in the dog’s intestines. According to the Center for Disease Control, the risk of a Dipylidium infection is rare in humans, and the infection rarely causes harm to either humans or animals; treatment with oral praziquantel is simple and effective.

If flea control depends on flea products, and these have been used for many years, do fleas develop resistance? Hypothetically, they could, or the problem may be one of perception. As Michael W. Dryden, DVM, PhD, notes in Clinician’s Brief, “The continued emergence of fleas in a home and the presence of fleas on pets for several weeks after treatment [are] actually quite normal. In fact, the problem often worsens before it improves, depending on the number of eggs deposited and survivability of larvae.”

“In theory, resistance would be more likely with the products that have been on the market longer,” Oldenhoff says. “For this reason, I have recently been recommending the new oral flea and tick preventives,” among them, Nexguard, Bravecto and Simparica (isoxazoline acaricide/insecticides, only for use with dogs). He believes compliance is better with these products because there are fewer steps that can go wrong in the process: “Did I put the full vial on my pet? Did I get it right on the skin? Did I bathe my pet too quickly, thus washing it off?” Note that isoxazolines are only available by veterinary prescription and are among those drugs for which federal law requires an annual exam to be renewed.

According to Dryden, a dog’s perceived resistance to flea products may also be the result of his exposure to feral cats and urban wildlife, or to other pets in the household (all of whom need to be treated year-round, or the infestation will continue).

If you’re looking for a natural solution to a flea infestation, be aware that “natural” does not mean “non-toxic,” nor does it mean effective. Some people use diatomaceous earth or boric acid in their homes, particularly on carpet and around baseboards. These pesticides may kill some of the larvae to reduce the overall flea population, but it won’t be enough to gain full control of an infestation. The products can be harmful if applied directly on animals as well.

Both diatomaceous earth and boric acid have effects on humans and animals. Diatomaceous earth, a type of silica, can be an irritant to the eyes, skin and respiratory tract. Boric acid can be corrosive to the eyes and skin, cause vomiting and diarrhea, respiratory difficulties, and (in large quantities) seizures and coma. You can take steps to reduce your and your pets’ exposure to these natural pesticides during their application, but in the end, basic house cleaning is your best tool.

You’ll also see flea collars made with essential oils. These collars are sold as unregistered repellants and not true insecticides, so the Environmental Protection Agency doesn’t require their manufacturers to provide data regarding their usefulness (Cats are especially sensitive to essential oils, so caution is advised.)

Other integrative options include allergy shots and fish oil. Because they’re based on the patient’s specific tested allergens, allergy shots may help with a dog’s concurrent atopy (the likely genetic tendency to develop allergic diseases), but will not directly affect Flea allergy dermatitis. Adding fish oil to a dog’s diet after initial treatments have controlled the Flea allergy dermatitis may help reduce inflammation, but applying oils—such as coconut oil—directly to the skin isn’t recommended. “I have seen animals with microbial overgrowth that I suspect was exacerbated by coconut oil application,” Oldenhoff says.

A flea infestation can cause physical and emotional strain. No one wants fleas—not you, not your pets, not your allergic pets. We all wish there were a simple, one-step approach to creating a comfortable, flea-free home. So, the next time you pull out your vacuum and the dogs run away, tell them, “It’s for your own good, my dears.”

Article first appeared in The Bark, Issue 89: Spring 2017

Illustration: iStock

Sara Greenslit, DVM, CVA, is a small-animal  veterinarian and writer who lives and practices  in Madison, Wisc.