Why does my dog keep shaking its head?

The Merck Veterinary Manual lists head shaking, itching, pain and a bad smell among the signs of otitis externa, inflammation of the outer ear canal. Among its primary causes Merck names ear mites and foreign bodies, and among the factors that set an ear up for trouble it lists hanging ear flaps, narrow or hairy canals and excess moisture from the environment or from swimming. VCA adds that infected ears are often red and inflamed, may smell, and commonly produce a black or yellowish discharge.

Allergies are the hidden driver in many cases. VCA links allergies to as many as 43 percent of ear infections, especially chronic or recurring ones, and notes that ear mites are more common in puppies and often lead to a secondary bacterial or fungal infection. Dogs with large, floppy or hairy ears are more prone, VCA says, naming cocker spaniels, miniature poodles and Old English sheepdogs as examples, though any breed can be affected.

The Merck Veterinary Manual's pet owner section explains that an aural hematoma, a pocket of blood in the ear flap, most commonly forms when a dog shakes its head or scratches its ears hard enough to rupture a blood vessel. The flap turns puffy and tender, the swelling can partly block the canal, and treatment is usually surgical. Merck stresses that treating the underlying cause, such as an ear infection, is essential, or the shaking and the swelling return.

VCA draws the same line, warning that an untreated ear infection can lead to a hematoma that needs surgery. So a dog that shakes its head for days is not only uncomfortable; it is putting the flap itself at risk. Feel both flaps when you look in the ears, and if one feels thicker or softer than the other, book the vet that day rather than waiting to see whether it settles.

Every Dogtown bath ends with a blow-dry, so a floppy-eared dog goes home with dry ears rather than damp ones. A face trim at $10 tidies the hair around the eyes, muzzle and ears between full grooms, and the groomer sees the ears on every visit, so mention any head shaking at check-in. Plucking hair from inside the canal is not on our menu; our answer on ear plucking explains why current veterinary advice leaves healthy canal hair alone.

What the studio does not do is treat an ear. A red, smelly or painful ear, a swollen flap or a dog that will not let its head be touched goes to your vet first, and the groom is rebooked once the ear has healed; grooming needs no evaluation, so rebooking takes a minute online or in the app. After hours, Massachusetts Veterinary Referral Hospital in Woburn is open 24 hours, and the City of Gloucester lists local emergency veterinary offices.