Hot spots in dogs: why they appear overnight, and the first 24 hours

Quick answer

A dense coat that holds moisture against skin is where hot spots start. Most of them are found by hand during grooming, not spotted from across the room.

A hot spot is a patch of skin the dog has damaged itself: red, wet, hairless, sharply bordered, and often the size of a coin in the morning and the size of a hand by evening. The medical name is acute moist dermatitis.

Two things stop it, and both have to happen. The dog must be prevented from licking or scratching it, and the hair over and around it must be clipped away so the skin can dry. Everything else, including creams, works only after those two.

• Speed is the defining feature. A lesion that expands visibly within a day is a hot spot; one that has been there a fortnight is something else.
• The location tells you the cause. A hot spot on the cheek means look at the ear. One on the rump means look for fleas.
• A cone or a body suit is the treatment, not an optional extra. Twelve hours without it undoes two days of progress.
• Leaving the hair on to let it breathe is the exact opposite of what helps. Hair holds the moisture that feeds it.
• These lesions are genuinely painful, and many dogs need sedation to allow clipping. That is normal and not a sign of a difficult dog.

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Never use hydrogen peroxide, surgical spirit, tea tree oil, or human steroid or antifungal creams on a hot spot.

Peroxide and spirit damage the healing tissue and slow recovery. Tea tree oil is toxic to dogs and readily absorbed through broken skin. Thick barrier creams and petroleum jelly seal moisture in, which is the one thing the lesion needs removed. And any dog with a raw, painful area will lick the product off and swallow it.
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Decide in 60 seconds

Why they appear overnight

Skin is a barrier, and it works when it is dry. A healthy coat lets air move at skin level, and moisture evaporates.

Something starts an itch. A flea bite, an ear infection making the dog rub its face, a patch of allergic skin, a wet undercoat after swimming, a mat holding damp against the body, or pain in a joint underneath.

The dog licks and chews. Saliva keeps the surface wet, and the mechanical trauma strips the outermost layer of the skin barrier.

Now the resident bacteria that live harmlessly on every dog's skin, chiefly a Staphylococcus species, have what they need: a warm, moist, protein-rich surface with no intact barrier. They multiply quickly.

The bacterial overgrowth and the damage together produce inflammation, and inflammation itches and hurts. The dog licks more. That is the loop, and it is why these lesions expand at a rate that startles owners.

A dense double coat accelerates every step. It holds water at skin level long after the outer coat looks dry, it hides the lesion from view, and during a seasonal coat blow the trapped dead undercoat forms a felt that blocks airflow completely.

This is also why hot spots cluster in warm, humid weather. Higher ambient humidity slows evaporation, and the same wet patch stays wet for longer.

What the location is telling you

This is the most useful part of the assessment and the part most often skipped. A hot spot is a symptom. Where it sits usually names the underlying problem.

Two of these matter most in practice. Fleas remain the commonest underlying cause in most regions, and owners routinely rule them out because they have not seen one. On a dog with a dense coat and an allergy, a small number of fleas produce severe disease and are almost never seen. Absence of visible fleas is not absence of fleas.

The other is the ear. A cheek lesion with an unexamined ear will come straight back.

What it looks like, stage by stage

If the lesion is on the cheek, this is the examination that matters. Treating the skin and ignoring the ear guarantees a repeat.

Early, within hours.

A small area where the hair is stuck down and looks damp or spiky. The skin under it is pink and warm. The dog keeps turning to it, or rubs that part of its face on the carpet.

At this point the skin is often still intact. Clipping and drying alone may be enough.

Established, within a day.

Hair gone over a defined area. The skin is bright red, glistening and weeping a clear or straw-coloured fluid that dries into a crust and glues the surrounding hair down. The border with normal skin is sharp, which is one of the most useful identifying features.

It is painful. Many dogs that will tolerate anything else will snap when this area is touched, and that is pain rather than temperament.

Complicated.

Small pustules or crusted spots appearing in a ring around the main lesion, sometimes called satellite lesions, mean the infection has gone deeper into the hair follicles. The area may feel firm and thickened rather than eroded on the surface.

This version, sometimes seen on the cheeks of retrievers and other heavy-coated breeds, needs oral antibiotics. Topical treatment alone will not reach it.

Systemic signs.

A dog that is also dull, off its food, or feverish is not simply irritated. Large or deep lesions can produce genuine systemic illness, and that changes the urgency.

The look-alikes

Not everything red and raw is a hot spot, and the differences matter because two of these need a completely different plan.

Deep pyoderma or an abscess.

Firm, swollen, sometimes fluctuant under the fingers, often with a puncture wound in the history. An abscess is a pocket of pus that needs draining, not drying.

Demodicosis and other mange.

Usually less acutely painful, more diffuse, often with a greasy feel and a wider distribution. Diagnosed by skin scrapings.

Ringworm.

Scaly, expanding, usually far less painful and far less wet. It also spreads to people, which is a reason not to guess.

Mast cell tumour.

This is the important one. Mast cell tumours can look red, inflamed and ulcerated, and they characteristically change size, swelling and settling repeatedly, because the cells release histamine when disturbed.

Any lesion that recurs in exactly the same place, that does not resolve with correct treatment, or that keeps waxing and waning, needs a fine needle aspirate. Treating a mast cell tumour as a hot spot for months is a well-recognised way to lose time that mattered.

Lick granuloma.

Chronic rather than acute, on a limb, thickened, firm and hairless, built up over months of licking rather than days.

Burn or scald.

History usually gives it away, but a heat pad or hot water contact injury can look similar in the first day.

The first 24 hours

Clippers matter more than any of these. Scissors near an inflamed, mobile patch of skin cut the dog.

Stop the self-trauma first.

Nothing else works while the dog is still licking. A properly fitted cone that extends beyond the nose, an inflatable collar for a lesion the dog cannot reach with its mouth, or a body suit for a trunk lesion. Leave it on continuously, including overnight, until the vet says otherwise.

Owners consistently underestimate this. A dog left uncollared for one night can reverse several days of healing.

Clip the hair, widely.

Use clippers, not scissors. Clip well beyond the visible edge, because the lesion usually extends further under the coat than it appears to. The goal is dry skin and full visibility.

If the dog will not tolerate clipping, stop. Do not fight a painful dog for it. This is one of the most common reasons a vet will sedate, and sedation here is kindness rather than escalation.

Clean gently.

A dilute chlorhexidine-based antiseptic solution, applied with cotton wool or gauze, dabbed rather than scrubbed. Rinse, then pat dry, and let the area air.

Cool water compresses relieve pain and reduce inflammation. Warm ones do not.

Do not cover it.

Bandaging traps moisture. The whole strategy is drying.

Do not medicate from the cupboard.

Human painkillers are dangerous to dogs. Ibuprofen and paracetamol both cause serious harm, and paracetamol is lethal to cats in the same household. Pain relief here is a veterinary decision and it is usually part of the treatment.

Photograph it.

Take a picture with something for scale, in good light, before you start. A vet seeing how fast it grew makes better decisions, and if you are wrong about what it is, the record matters.

What the vet does

Expect the area to be clipped properly and cleaned, with sedation if the dog needs it.

Treatment is usually a topical preparation combining an antibacterial with a steroid to break the itch cycle, plus systemic pain relief. Oral antibiotics are added when the infection is deep, when there are satellite lesions, or when the dog has systemic signs. Cytology from the surface tells the vet which of those it is.

The second half of the appointment matters as much. Expect the ears to be examined and swabbed if the lesion is on the head, the coat to be checked for flea dirt, an assessment for joint pain if the lesion sits over a joint, and a conversation about allergic skin disease if this is not the first episode.

Bring these: when you first noticed it and how much it has grown since, a photograph from the beginning, what the dog was doing in the previous day or two including swimming, bathing, grooming or walking in long grass, the exact flea and tick product used and the date it was last applied for every pet in the house, any ear signs such as head shaking or odour, whether the dog has been slower on stairs or reluctant to jump, the diet, and where any previous hot spots were.

That last item, previous locations, is often the fastest route to the underlying diagnosis.

Preventing the next one

Short coat, visible skin, nothing trapped against the body. Clipping around a lesion is not damage, it is the first half of the treatment.

What never to do

Do not leave the hair on to let the wound breathe. Hair holds moisture, and moisture is the fuel.

Do not use scissors to trim around an inflamed lesion. The skin is mobile and swollen, it tents into the blades, and cutting a dog while trying to help it is a common and avoidable injury.

Do not skip the cone because the dog seems settled. Dogs lick at night, and a night of licking will undo everything.

Do not apply thick creams, petroleum jelly, sudocrem-type barrier products, or anything greasy. They seal the surface.

Do not use hydrogen peroxide or surgical spirit to clean it.

Do not use tea tree oil, or any essential oil, on a dog. Tea tree is toxic to dogs and broken skin absorbs it readily.

Do not give human anti-inflammatories or painkillers.

Do not treat the skin and ignore the ear, the fleas or the sore hip. That is the definition of a lesion that comes back.

Do not accept a third recurrence in the same spot without a fine needle aspirate. The differential list includes tumours that mimic this closely.

When to get help

Go the same day for a lesion that is expanding while you watch it, one that is too painful to touch, one with pustules or crusts appearing around it, one between the toes after walking in long grass, or any hot spot on a dog that is also lethargic, off food or feverish.

Book within a day or two for a smaller lesion that you have managed to clip and clean at home, because the underlying cause still needs identifying and topical treatment is usually prescription.

Ask specifically for the ear to be examined if the lesion is on the head, for a flea assessment if it is on the back half, and for joint assessment if it sits over a hip, shoulder or elbow.

Ask for a fine needle aspirate or biopsy if this lesion has appeared in the same place before, if it waxes and wanes, or if it has not healed on correct treatment.

Bring the photograph you took at the start, the dates and product names of flea prevention for every pet in the house, and the list of where previous episodes were. Those three things together usually identify the cause in one visit rather than three.