Merck's vet manual names parasites, infections and allergies as the most common causes of itching in dogs. Vets work through them in that order: parasites first, then infections, then allergy. Allergy comes last because atopic dermatitis, the allergy to things like pollen, mold and dust, is diagnosed by ruling everything else out.
Steroids calm the itch while that happens. They do not kill fleas or mites, and Merck lists steroid use as one of the most common reasons skin infections keep coming back. So when an itch has gone on for months, the useful question for the vet is which of these causes have been ruled out, and how.
Where a dog itches is one of the first clues. These are the patterns Merck describes.
| Where it itches | What it points to |
|---|---|
| The lower back, the base of the tail and the back of the thighs | Flea allergy. A dog with it can have almost no fleas on it. |
| The belly, chest, ears, elbows and hocks (the back ankles), starting suddenly and very itchy | Sarcoptic mange, the scabies mite. It spreads to other animals, and people can catch it too. |
| The paws, face, ears, front legs, armpits and belly | Atopic dermatitis, the allergy to pollen, mold and dust. Food allergy affects much the same places. |
| The ears, again and again | An ear infection with a cause underneath. Allergies are on Merck's list of causes, and ear infections are one of the common signs of food allergy. |
| The bottom, dragged or rubbed along things | Full anal sacs or tapeworm, which Petipedia's scooting page covers. Food allergy can reach the skin around the bottom too, less often. |
| The paws and face, licked and rubbed | Merck says this is common when a yeast or bacterial infection sits on top of the itch. |
Merck also gives a checklist vets use to recognize atopic dermatitis, and two items on it matter here: the itch gets better on steroids, and the lower back is left alone. A dog whose itch did not improve on steroids, or whose itch is centered on the lower back and the base of the tail, does not match those two items. That is a reason to go back over fleas, mites and infection before settling on an allergy.
Flea allergy is an allergy to the flea's saliva. Merck's classic picture is crusted, itchy skin over the hips, the base of the tail and the thighs. It adds the part owners do not expect: a very sensitive dog grooms so much that it is likely to be almost free of fleas, and it is usually difficult to find any evidence of them.
To look anyway, comb the lower back with a fine flea comb and tap what comes out onto a damp white paper towel. Flea droppings dissolve there into reddish brown smears.
Merck says skin and blood tests for flea allergy are no longer recommended. What works is flea control done properly, and Merck counts the dog's response to it as a diagnostic test in itself. Done properly means every dog and cat in the house, all year, for life, and Merck says it can take six weeks to three months to bring an infestation under control. Cats in the house get a product made for cats. Petipedia's page on fleas that keep coming back has the CDC's warning about dog products on cats, and why a treatment can look like it failed when it has not.
Sarcoptic mange, or scabies, is known for how hard it itches. Merck describes intense itching that usually comes on suddenly, starting on the belly, chest, ears, elbows and hocks and spreading over the body if it is not treated. It passes easily between animals by contact, and people who are in contact with an infested dog can catch it too.
The mites are often missed. Merck says negative skin scrapings sometimes make it hard to diagnose, and that the mites can be hard to find on a dog that is bathed regularly. When the signs fit and no mites are found, Merck says a trial treatment is warranted, and the international committee on allergic diseases of animals says the same in its guidelines: a trial treatment may be needed to rule these mites out before calling the itch an allergy. Merck's list of drugs that treat scabies includes afoxolaner, fluralaner and sarolaner, the ingredients in NexGard, Bravecto and Simparica, and selamectin, the ingredient in Revolution.
Demodex is the other mite. In its mild, local form it causes a few small patches that are bald, red and scaly, and itching is mild or absent. When it spreads over the body of an adult dog, Merck says it is often linked to an illness that has weakened the immune system, such as cancer, Cushing disease, an underactive thyroid or diabetes. It is found with deep skin scrapings, and Merck says steroids must not be used at all in a dog with demodex, which is one more reason the scraping comes before the steroids.
Scratching and the inflammation underneath it can let bacteria and yeast into the skin. Merck's point is that the infection can keep the itch going after whatever started it is long gone. So the second step is to look for infection and treat it fully. Merck describes a 21 to 30 day course of antibiotics, and for skin infections at least three weeks and preferably four, with eight to twelve weeks or longer for deep or repeated ones. The course runs its full length even when the skin looks better after a few days.
The result of that course tells the vet what comes next. In Merck's words, if the itching goes away after antibiotics, the cause was a microbial infection, and if the itching is unchanged or only somewhat better, the most likely underlying cause may be an allergy.
Merck describes a deep infection as pain, crusting, odor, and secretions of blood and pus. It names fleas, allergies, an underactive thyroid and Cushing disease among the causes underneath skin infections that keep coming back, along with steroid use and antibiotic courses that were not right for the infection. And a medicated shampoo usually has to stay on the coat for ten minutes to work, according to the same Merck page.
Ears follow the same rule. Merck says vets often take a sample of the discharge to look at under the microscope or to culture, that dogs with bacterial and yeast ear infections usually need rechecks until no infection shows, often two to four weeks, and that unless every cause is treated, the condition often returns. Petipedia's page on head shaking has the rest.
Food allergy looks much like the pollen and dust allergy, with one difference Merck points out: the itch barely changes from one season to the next. It affects the ears, feet and belly, and less often the skin around the bottom, and ear infections and infections that keep coming back are common with it. Merck says the itch can be as severe as scabies. It usually starts in young adults, but it has been reported in dogs from under six months to 13 years old, and studies of itchy dogs have found it in anywhere from 9 to 40 percent of them. The food allergens Merck names as most common in dogs are beef, dairy, chicken, wheat and lamb.
No blood, skin or hair test finds it. Merck says none of them reliably gives accurate results. And in a 2019 study, researchers sent samples to a hair and saliva allergy test from an allergic dog, a healthy dog, fake fur and plain water, and the results were no different from chance. Their conclusion was that hair and saliva testing should not be used to diagnose allergies.
The test that works is an elimination diet: one food your vet chooses, and nothing else. That is either a hydrolyzed diet, made with the protein broken down into small pieces, or a protein the dog has never eaten, bought or home-cooked. The international committee's guidelines say a strict trial needs at least eight weeks to clear the signs in most dogs, and Merck says ten weeks may be needed to find 95 percent of them.
The diet has to be the only thing the dog eats. Merck says the dog eats only the food and treats the vet recommends, which rules out table food, flavored medicines and toothpaste. It also warns that chewable or flavored heartworm and flea medicines generally contain beef, pork or soy protein, whether or not the label lists it. Ask the vet what to use for flea and worm protection during the trial.
A pet-store food with a limited ingredient label is not the same thing. A 2018 review of the studies that tested pet foods in a lab found ingredients that were not on the label in 33 to 83 percent of the foods sold as novel or limited ingredient, while the hydrolyzed diets had one possible case. The authors add that the studies did not test whether those traces set off a reaction in allergic dogs.
If the itch clears, the old food goes back in to prove it, because Merck says a relapse is what confirms a food allergy. The itch usually returns within days, sometimes hours, but it can take 10 to 14 days, which is why most vets add back one food at a time, no more often than every two weeks. Merck adds that starting the diet together with a two to three week course of steroids may shorten the trial to as little as four weeks.
Atopic dermatitis is an allergy to things in the environment, such as pollen, mold and dust. Merck says it usually starts between six months and three years of age, affects the paws, face, ears, front legs, armpits and belly most, and is usually seasonal but can be seen all year. Merck lists the Boston Terrier, Boxer, Chinese Shar-Pei, Dalmatian, Golden Retriever, Labrador Retriever, Lhasa Apso, Scottish Terrier, Shih Tzu, West Highland White Terrier and Wire Fox Terrier among the breeds prone to it.
No test can prove it. Merck says the diagnosis rests on age, breed, signs and history, with the other causes excluded, and that allergy testing cannot diagnose allergies. What the skin and blood allergy tests are for is choosing what goes into allergy shots once the diagnosis is made. The international committee's guidelines say the same: the tests are not screening tests.
It is lifelong. Merck says it cannot be cured but can be managed, usually with several things at once: avoiding the allergens where possible, controlling the itch, bathing and coat care, keeping flare factors like fleas and infections under control, and allergy shots. These are the itch medicines Merck describes. Doses come from the vet.
Atopic dermatitis usually starts before the age of three, so a first itch late in life does not fit its usual pattern. Food allergy can still start late, with cases reported up to 13 years. Merck's list of what has to be ruled out before calling an itch atopic dermatitis also includes hormone diseases and cancer, and two of its pages point at them directly. Demodex mites that spread over an adult dog are often linked to cancer, Cushing disease, an underactive thyroid or diabetes. And an underactive thyroid and Cushing disease are among the causes of skin infections that keep coming back.
So for an older dog, ask whether blood tests for the thyroid and for Cushing disease make sense, and have any lump that has changed checked with a needle test. Petipedia's page on lumps explains the test.
A veterinary dermatologist is, in the words of the American College of Veterinary Dermatology, a veterinarian with additional training and expertise in skin, ear, hair and nail disorders, and a specialist in allergic disease, all skin diseases and ear diseases. The college lists constant licking, chewing, biting and scratching of the body, ears or feet among the reasons to see one. In the United States your vet can refer you, or you can contact one directly, and the college has a search for board-certified veterinary dermatologists. Elsewhere, ask your vet for a referral to a dermatology specialist. If the cost is the obstacle, Petipedia's page on paying the vet bill has the charities and the cheaper clinics.
Write down each treatment, the date it started, how long it ran and what changed, in the log on My Pet. The vet sheet prints it, so a new vet or a specialist can see what has already been tried.
Read 21 September 2026.