Short answer: the next step is a visit with your vet. Itching and hair loss are signs, not a diagnosis. Fleas, other skin parasites, skin infections and allergies are the usual causes, and they can look alike. Your vet will check one thing at a time, and treatment depends on what the cause turns out to be.
What you might notice
Scratching, licking, chewing or rubbing more than usual. Red skin, bald or thin patches, scabs, sore spots, a smell from the skin or ears, or fur stained by licking. Some dogs scratch and lick when no one is looking, so a patch of damp or stained fur can be the first clue.
Try to note where it is, when it started, and whether it changes with the season. That helps your vet more than anything else you can bring.
Common causes
Fleas. This is the first thing a vet checks. Some dogs are allergic to flea bites, and in those dogs a single bite can set off a lot of itching.
Other skin parasites and infections. Mites, lice, ringworm, and bacterial or yeast infections can all make skin itchy and patchy. Scratching can also cause an infection on top of the first problem, which is one reason not to wait it out.
Allergies. A dog can react to things in the air such as pollen, mold and dust mites, to something in the food, or to something the skin touches, like a cleaning product or bedding. Allergies often start after about six months of age. The itching can be in one spot or all over, and it may be worse in some seasons.
Other causes of hair loss. Hormone problems can thin the coat. A poorly fitted collar can rub fur away in one spot. Normal shedding should not leave bald or patchy areas, so patches are worth a call.
How your vet will work it out
There is no single test for most allergies. Your vet will ask questions, look over the skin and coat, and rule out fleas, mites and infections first. They may take skin samples or comb the coat to look closer. If a food allergy is suspected, they will plan a diet trial that runs for several weeks, with no other treats or table food during that time. It asks for patience, and it is how a food allergy is found.
If an allergy is found, it often needs long-term care with regular check-ins, and your vet can adjust the plan as you go.
What to do now
Book a visit with your vet, even if your dog seems cheerful. Before you go, jot down when it started, where it is, any change in food or treats, what flea prevention your dog has had and when, and anything else you have noticed, such as weight, thirst or energy.
Please hold off on creams, shampoos, supplements or medicine until your vet has seen the skin, and do not use anything made for people. The right treatment depends on the cause, and your vet will choose it for your dog.
If your dog has a swollen face or muzzle, hives, trouble breathing, or seems weak or collapses, go to an emergency vet straight away. This is less common, and your vet or an emergency vet can help quickly.
Making it easier
Keep up with the flea prevention your vet recommends, and keep a short note on your phone of what you see and when. A few photos of the skin on different days can show your vet how it changes.
General education only, not veterinary advice or a plan for your individual dog. If your dog is itching, losing hair or has skin that looks sore, please contact a vet. See our legal page.
Sources & care notes
Sources: Merck Veterinary Manual: itching, allergies, hair loss. AVMA, allergies in pets. VCA Animal Hospitals: allergies in dogs, allergic reactions. PDSA: itchy skin, hair loss, severe allergic reactions.
This article is general education, not veterinary advice. If your dog seems unwell, is in pain, or something changed suddenly, call your vet. If it could be an emergency, do not wait.
Frequently asked questions
Could it be an allergy?
Possibly, but itching and hair loss are signs, not a medical conclusion. Fleas, other skin parasites, skin infections and allergies can look similar, so the next step is a visit with your vet.
What should I note for the vet?
Where it is, when it started, and whether it changes with the season, plus red skin, thin patches, scabs or a smell from the skin or ears.
Why do fleas matter so much?
They are the first thing a vet checks, and some dogs are allergic to flea bites, so a single bite can set off a lot of itching.
How will the vet work it out?
Through an examination and the tests they recommend, aimed at the likely causes they can see.
