The short answer
Itching is a sign, not a diagnosis. The Merck Veterinary Manual (U.S.) says the most common causes are parasites, infections and allergies, and that vets look for fleas and mites first, then infection, then allergy. Fleas are not only a summer problem in Canada: the Canadian Parasitology Expert Panel says the at-risk season is typically late spring to early fall but can run year-round in some regions. See your vet if your dog’s scratching, licking or chewing continues, if the skin is red, broken, smelly or wet, or if the ears are involved. A fast-growing sore needs a prompt vet visit. Sudden vomiting, heavy drooling or pale gums after a sting, injection or food is an emergency.
PawTrace is not a vet. This guide shares what we found in the sources listed below; ask your vet about your own dog.
Quick highlights
- Itching is a sign, not a diagnosis: vets look for fleas and mites first, then infection, then allergy.
- Fleas are not only a summer problem in Canada; the at-risk season can run year-round in some regions.
- A hot spot is a fast-growing red sore that needs a vet, and so does its cause.
- Do not start any human medicine without your vet.
Key takeaways
- Itching has four common sources: fleas and other parasites, skin infection, environmental allergy and food allergy. One dog can have several at once (Merck Veterinary Manual (U.S.)).
- Fleas come first on the vet’s list. Flea allergy dermatitis typically shows as itching and lesions on the lower back, tailhead and rear thighs (Merck, U.S.).
- Seasonal itching points to seasonal allergens. Year-round itching points to dust mites, food, or both (Merck, U.S.). In southern Ontario the tree pollen season starts in early April and ragweed runs from early to mid-August until the first frost (Allergy, Asthma & Clinical Immunology, 2018, a human allergy guide).
- A hot spot is a fast-growing red sore with pus on its surface, caused by the dog scratching or biting at an itch or pain. It needs a vet, and so does the cause (Virginia Tech (U.S.)).
- Only a diet trial of up to three months confirms food allergy. Blood and skin tests are not reliable for it, and the trial includes treats (Merck, U.S.).
- Merck (U.S.) says current research does not support antihistamines for itch relief in dogs. Do not start any human medicine without your vet.
- Hives or facial swelling: call your vet now. Sudden vomiting, diarrhea, heavy drooling or pale gums after a sting, injection or food: go to an emergency clinic.
What is causing the itch? The four usual suspects
The Merck Veterinary Manual (U.S.) describes itching as an unpleasant skin sensation that makes a dog want to scratch, and says it is a sign rather than a disease. A dog with an itch scratches, bites or licks its skin. The itch may be all over or in one area. Because infection can follow the first cause, Merck says the original trigger may be gone by the time itching develops. The table sets out what each cause tends to look like.
| Cause | What it tends to look like | Source |
|---|---|---|
| Fleas and flea allergy | Itch and crusted bumps on the mid to lower back, tailhead and rear thighs, with hair loss from licking, chewing or scratching. Flea allergy comes from a reaction to flea saliva. | Merck (U.S.), professional version, May 2026 |
| Mites and other parasites | Merck says vets exclude parasites first, including mites and fleas. Deep skin scrapings are used to rule out mite infections. | Merck (U.S.), itching; Merck (U.S.), pyoderma |
| Skin infection (bacterial or yeast) | Scaling, bald patches, welts at the hair bases, scabbing, odour and discharge. Itching of the feet and face is common when yeast and bacterial infection occur together. Warm, moist folds (lips, face, armpits, feet, neck) have more bacteria. | Merck (U.S.), itching; Merck (U.S.), pyoderma |
| Environmental allergy (atopic dermatitis) | Itching of the feet, face, ears, front legs and abdomen; licking or chewing paws; rubbing the face and eyes. Onset is generally between 6 months and 3 years. Signs are often seasonal but may occur all year. Any breed or mixed breed can be affected. | Merck (U.S.), allergies in dogs |
| Food allergy | Itching that varies little from one season to the next. Merck calls food allergy less common than airborne allergy. Recurring ear or skin infections can be part of the picture. | Merck (U.S.), allergies in dogs; Merck (U.S.), professional version, August 2025 |
Merck also estimates that 15% of dogs with atopic dermatitis develop nasal inflammation or asthma, and that in a small number of dogs recurring ear infections are the only sign. If your dog has ear trouble again and again, mention it to your vet even when the skin looks fine.
Fleas in Canada: not only a summer problem
The Canadian Parasitology Expert Panel (University of Saskatchewan; page dated May 2 2019) says flea numbers and species vary widely across Canada. Dogs in multi-pet homes, and dogs that spend time where many untreated pets gather, such as dog daycares and off-leash areas, are at greatest risk. In much of western Canada wildlife is the major source. The panel says dogs in flea-endemic areas should have monthly preventive treatment during the at-risk season, which is typically late spring, summer and early fall but can be year-round in some regions, and that flea-allergic dogs should be treated regularly.
Merck (U.S.) adds that in northern temperate regions the close link between pets, fleas and human homes allows a year-round problem, and that flea allergy dermatitis is most prevalent in summer. When fleas are found, the panel says every pet in the household should be treated, and that clearing an infestation can take five to six months of treatment, especially if the environment is not addressed. See our parasite prevention guide for the Ontario season by season, and our regional guide to ticks, Lyme and heartworm for tick risk where you live. Your vet chooses the product; we do not name any here.
Pollen, mould and the Canadian calendar
The Merck Veterinary Manual (U.S.) says dogs with seasonal itching are likely reacting to seasonal allergens, and that dogs with year-round allergic itching may have a dust mite allergy, a food allergy or both. The allergens it names are pollens, moulds and dust. We found no Canadian pollen calendar written for dogs. The best Canadian regional timing we found is a human allergy guide, Moote, Kim and Ellis in Allergy, Asthma & Clinical Immunology (2018). We offer it as a calendar of what is in the air, not proof of what affects your dog.
| Region | What the guide says is in the air | Source |
|---|---|---|
| Southern Ontario (our reading: Ottawa falls in this band) | Tree pollen season starts in early April. Grass season starts mid to late May, with peaks in the latter part of May and mid-June. Ragweed season begins early to mid-August, peaks in late August or early September and ends at the first frost. Moulds are present in spring, summer and fall. | Moote et al. 2018, Table 2 |
| Northern areas of Ontario and Quebec | Tree season can start four to six weeks later than in the south. | Moote et al. 2018, Table 2 |
| Prairies | Trees from the first week of April to June; grass from mid-May to the end of September. | Moote et al. 2018, Table 2 |
| BC coast | Tree pollen from early February to mid-July; grass from late April or early May to September. | Moote et al. 2018, Table 2 |
| Maritimes and Newfoundland and Labrador | Trees from late March to the last week of June; grass mid-May to the end of September; ragweed early August to the end of September. | Moote et al. 2018, Table 2 |
A practical use of this table is a symptom diary. Merck (U.S.) says vets match allergy test results with the allergens present at the time of year a dog has signs, and says the best way to judge allergy treatment is to compare similar seasons. Note the dates your dog starts and stops itching, the room or place, and any new food, shampoo or treat. Bring the diary to the appointment. That diary suggestion is ours, not Merck’s.
We found no source on whether winter heating or dry indoor air causes itching in dogs, so this guide makes no claim on it. If your dog itches in January, say so to your vet; fleas and dust mites are year-round possibilities in the sources above.
Hot spots: what they are and what to do
Virginia Tech’s veterinary college (July 21 2025, U.S.) says the veterinary term for a hot spot is pyotraumatic dermatitis. A hot spot appears as a red skin lesion with pus on its surface, tends to appear quickly and grow rapidly, and is itchy and often painful. It is caused by a dog scratching or biting in response to itch, discomfort or pain. The main primary causes are environmental allergy, fleas, flea bite allergy and sometimes anal sac disease.
- Cheek: check the ears. An ear infection can drive the scratching that creates a cheek hot spot.
- Back: look for fleas.
- Near the tail: ask your vet to check the anal sacs.
- What the vet does: clips the hair until healthy skin shows, cleans the area and prescribes a topical antiseptic. Sedation may be needed if the lesion is very painful. Virginia Tech says spring 2025 antimicrobial guidelines for canine skin infections concluded that topical treatment is the treatment of choice for true hot spots, which therefore do not require oral antibiotics.
- Prevention: manage the underlying cause. Virginia Tech says once the cause is controlled, a dog should not keep getting hot spots.
Leave treatment to your vet. Virginia Tech says hot spot lesions can resemble deep infections that need a different treatment approach, and only an exam can tell the two apart.
What your vet may do
Every dog’s work-up differs, and we give no drug names or doses here. The sources describe this general order:
| Step | What happens and why | Source |
|---|---|---|
| History and exam | A thorough skin history and physical exam. The vet looks for parasites first, then infection. | Merck (U.S.), itching |
| Skin tests | Skin scrapings to exclude mites; bacterial and fungal cultures in some cases. | Merck (U.S.), pyoderma |
| Treat an infection first | If infection is suspected, the vet may prescribe a 21 to 30 day course of antibiotics. If the itch resolves, infection was the cause. If it stays the same or improves only a little, allergy is the likelier underlying cause. Merck says to give the full prescribed course even if your dog looks better after a few days. | Merck (U.S.), itching; Merck (U.S.), pyoderma |
| Flea control | Treat the dog, the other pets and where needed the environment. Flea response is how vets identify sensitivity to insect bites. | Canadian Parasitology Expert Panel; Merck (U.S.), itching |
| Diet trial | A limited-ingredient or hydrolyzed diet, fed for up to three months, with no other foods, treats, flavoured medicines or toothpaste. If signs resolve, the old food is reintroduced to confirm the allergy. | Merck (U.S.), allergies in dogs |
| Allergy testing | Used to choose allergens for immunotherapy, not to diagnose atopy and not reliable for food allergy. Merck calls immunotherapy the preferred treatment of most veterinary dermatologists; improvement may take six months to a year, and it improves the condition but does not cure it. | Merck (U.S.), allergies in dogs |
| Itch control | Anti-itch medicines chosen by your vet, and bathing and coat care. Merck says steroids are often the most effective for itch but have side effects, and are inappropriate for itch caused by infection. | Merck (U.S.), itching; Merck (U.S.), allergies in dogs |
Red flags: when to go to the vet, and when to go now
| Sign | What to do | Source |
|---|---|---|
| Itching that continues, or returns when treatment stops | Book a vet visit. Itching is a sign and the cause needs finding. | Merck (U.S.), itching |
| Red, oozing sore that is growing fast; a painful lesion | See your vet promptly (that timing is our advice). Hot spots grow quickly and can resemble deeper infection. | Virginia Tech (U.S.) |
| Pain, crusting, odour, blood or pus from the skin | Vet visit. Merck lists these as signs of deep pyoderma. | Merck (U.S.), pyoderma |
| Repeated ear infections, or head shaking with ear redness | Vet visit. Ear infection can be the only sign of allergy and can drive hot spots. | Merck (U.S.), allergies in dogs; Virginia Tech (U.S.) |
| Hives or swelling of the face, lips, muzzle or near the eyes after a sting, medicine or food | Call your vet right away (that timing is our advice). Merck says hives and swelling generally develop within 20 minutes of exposure and are typically not life-threatening, but swelling can close the eyes. | Merck (U.S.), anaphylactic reactions |
| Sudden vomiting, diarrhea, heavy drooling, pale gums or cold limbs, seconds to minutes after a sting, injection or food | Emergency. Merck says anaphylaxis is rare and life-threatening, and that you should seek emergency veterinary help immediately. | Merck (U.S.), anaphylactic reactions |
Our guide to emergency signs every owner should know covers the wider list.
Myth vs fact
| Myth | Fact |
|---|---|
| Grain causes most food allergies in dogs. | Merck (U.S.) lists beef, dairy products, chicken, wheat and lamb as the most common food allergens in dogs, and soy, corn, egg, pork, fish and rice as less common. Wheat is the only grain among those five; the other four are meats or dairy. Food allergy is confirmed by a diet trial, not by guessing at an ingredient. |
| Hot spots come from a wet coat. | Virginia Tech (U.S.) says hot spots are not produced by moisture. They come from the dog scratching or biting at an underlying itch, pain or discomfort. |
| Antihistamines fix itching. | Merck (U.S.) says antihistamines are commonly used but their success is highly variable, and that current research does not support them for itching relief in dogs. Ask your vet before giving any medicine. |
| Fleas are a summer-only problem. | The Canadian Parasitology Expert Panel says the at-risk season is typically late spring to early fall but can be year-round in some regions. Merck (U.S.) says northern temperate regions can have a year-round problem because pets and fleas live close to human homes. |
| A blood or skin allergy test will show a food allergy. | Merck (U.S.) says blood and skin tests are not reliable for diagnosing food allergy. A diet trial is. |
| A change of food will clear itching. | Merck (U.S.) says a diet trial works only if every previously fed ingredient, treat and flavoured medicine is excluded, and it should be planned with your vet. A casual food swap tests nothing. |
Frequently asked questions
Can I give my dog human allergy medicine?
Ask your vet first. We give no doses. Merck (U.S.) says antihistamines are commonly used but that current research does not support them for itch relief in dogs, and that steroids carry side effects such as increased hunger, thirst and urination. Some itching is caused by infection, and Merck says steroids are inappropriate for that.
How long does a food allergy diet trial take?
Merck (U.S.) says the trial diet is fed for up to three months. If signs clear, the old ingredients are added back one at a time, for up to 14 days each, to find the trigger. Every treat, flavoured medicine and table scrap counts, so follow your vet’s plan exactly.
Is allergic itching curable?
Merck (U.S.) calls atopic dermatitis a lifelong disease that needs long-term management and regular vet exams. Allergy shots or drops improve the condition but do not cure it, and many dogs still need anti-itch medicine in flare-ups. Food allergy is managed by strict avoidance of the trigger.
Which dogs get allergies?
Merck (U.S.) lists breeds predisposed to airborne allergy, among them Golden Retrievers, Labrador Retrievers, Boxers, West Highland White Terriers and Shih Tzus, and says any dog of any breed or mix can be allergic. Onset is generally between 6 months and 3 years.
Why is my dog licking its paws?
Licking or chewing the paws and rubbing the face are listed by Merck (U.S.) as signs of atopy, but parasites and infection cause the same behaviour. Your vet will check for fleas, mites and yeast or bacterial infection first, since those are treated differently.
Does my dog’s food matter if the itch is not a food allergy?
A food change does not treat parasites, infection or environmental allergy. If your vet suspects food allergy, follow the diet trial exactly.
Next step
If your dog is scratching, licking or chewing, book a visit with your vet before trying home fixes. You can search for a clinic on our find a vet page. For groomers and other dog services near you, see our dog services directory.
Keep reading
- Parasite prevention through the Canadian seasons
- Ticks, Lyme and heartworm in Canada: a regional guide
- Emergency signs every owner should know
- What your puppy should eat, and how much
Sources
All links were checked on October 23 2026. Sources marked (U.S.) are not Canadian. We found no CVMA or Canadian vet college owner guide on canine skin allergy to cite; the Canadian sources here are the Canadian Parasitology Expert Panel and the regional pollen calendar.
- Merck Veterinary Manual (U.S.): Itching (pruritus) in dogs (last updated May 2025)
- Merck Veterinary Manual (U.S.): Allergies in dogs (last updated September 2024)
- Merck Veterinary Manual (U.S.), professional version: Cutaneous food allergy in animals (last updated August 2025)
- Merck Veterinary Manual (U.S.), professional version: Flea allergy dermatitis in dogs and cats (last updated May 2026)
- Merck Veterinary Manual (U.S.): Pyoderma in dogs (last updated September 2024)
- Merck Veterinary Manual (U.S.): Disorders involving anaphylactic reactions (Type I reactions, atopy) in dogs (last updated July 2026)
- Virginia Tech, Virginia-Maryland College of Veterinary Medicine (U.S.): That itches! A guide to canine hot spots (July 21 2025)
- Canadian Parasitology Expert Panel, University of Saskatchewan: Fleas (page dated May 2 2019)
- Moote W, Kim H, Ellis AK. Allergen-specific immunotherapy. Allergy, Asthma & Clinical Immunology 14 (Suppl 2), 53 (2018), Table 2: pollen and mould seasons by region of Canada (a human allergy guide)
General information only. This guide is not veterinary advice and cannot diagnose your dog. Ask your vet if your dog has itching, a skin sore or any allergy sign. Written by the PawTrace Team.
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