Dog Losing Hair in Patches: 8 Causes & When It's Serious

By Sarah Bennett, Certified Animal Nutritionist

Understanding Patchy Hair Loss in Dogs

Seeing your dog develop bald patches can be alarming, and rightfully so — focal or patchy alopecia (the medical term for hair loss) is rarely a cosmetic issue alone. It is always a symptom, not a disease in itself. The underlying causes range from highly contagious parasitic infections to complex hormonal disorders, and the treatment for each is entirely different. This guide covers the eight most common causes, what distinguishes them, and the warning signs that mean you should not wait for a vet appointment.

Cause 1: Demodectic Mange (Demodex)

Demodectic mange is caused by an overpopulation of Demodex canis mites — microscopic parasites that live in hair follicles. Small numbers of Demodex mites are a normal part of canine skin flora. Problems arise when the immune system cannot keep their numbers in check, which happens most often in puppies (whose immune systems are still maturing), dogs on immunosuppressive medications, or dogs with underlying diseases.

Localised demodicosis typically appears as one to five small, scaly, hairless patches around the face and forelegs. Most puppies with this form resolve spontaneously. Generalised demodicosis affects large areas of the body, causes significant inflammation, and requires extended medical treatment — usually a veterinary-prescribed oral isoxazoline parasite preventative such as fluralaner or afoxolaner. Demodex is not contagious between adult dogs or to humans.

Cause 2: Sarcoptic Mange (Scabies)

Sarcoptes scabiei mites burrow into the skin and trigger an intense hypersensitivity reaction. Unlike Demodex, sarcoptic mange is highly contagious — between dogs, to cats, and to humans (where it causes a self-limiting itchy rash). Hair loss typically starts at the ear margins, elbows, hocks, and face before spreading. The defining feature is relentless, severe itching that seems disproportionate to the visible skin changes. Diagnosis is by skin scraping, though the mites are notoriously difficult to find; many vets treat empirically based on clinical signs. Treatment involves prescription antiparasitic medication and environmental decontamination.

Cause 3: Ringworm (Dermatophytosis)

Despite its name, ringworm is a fungal infection caused by dermatophytes — most commonly Microsporum canis in dogs. It produces circular or irregular patches of hair loss with a scaly, sometimes reddened border. The centre of the lesion may appear to regrow hair as the active infection moves outward, creating the "ring" appearance. Ringworm is zoonotic (transmissible to humans) and can spread between pets in a household. Young, elderly, and immunocompromised dogs are most susceptible. Diagnosis is by Wood's lamp examination (about 50% of M. canis cases fluoresce green), fungal culture, or PCR. Treatment involves antifungal medications — topical for localised cases, systemic for widespread disease — plus rigorous environmental cleaning.

Cause 4: Environmental and Food Allergies

Allergic dogs scratch, rub, and chew at themselves chronically, and this self-trauma gradually damages hair follicles and causes patchy hair loss. The pattern is typically bilateral (affecting both sides of the body symmetrically) and concentrated where the dog can reach — paws, face, axillae (armpits), groin, and the base of the tail. Unlike mange or ringworm, allergic alopecia is accompanied by marked itchiness and often recurs seasonally (environmental) or year-round (food). Identifying and eliminating the allergen — through elimination diets or allergy testing — is the only way to address the root cause.

Cause 5: Cushing's Disease (Hyperadrenocorticism)

Cushing's disease results from chronically elevated cortisol levels, usually due to a benign tumour of the pituitary gland (85–90% of cases) or the adrenal gland. Excess cortisol breaks down skin and muscle, causing a classic presentation: bilateral, symmetrical hair loss over the flanks and trunk (while the head and legs are often spared), a pot-bellied appearance, thin and fragile skin, and calcinosis cutis (calcium deposits under the skin). Accompanying signs include polydipsia (increased thirst), polyuria (frequent urination), polyphagia (increased appetite), and lethargy. Cushing's disease typically affects middle-aged to older dogs and requires blood and urine testing for diagnosis. Treatment options include medical management with trilostane and, in select cases, surgery or radiation.

Cause 6: Hypothyroidism

The thyroid gland regulates metabolism, and when it underperforms, virtually every body system is affected — including the skin and coat. Hypothyroidism causes bilateral, symmetrical hair loss (often starting at the tail and flanks), a dull, brittle coat, thickened skin, and reduced ability to regrow shed hair. Unlike Cushing's, hypothyroid dogs tend to be lethargic, cold-intolerant, and prone to weight gain without an increase in food intake. The condition is diagnosed with a blood panel measuring thyroid hormone levels (T4, free T4, TSH) and treated with daily oral levothyroxine — a medication most dogs tolerate well and require for life.

Cause 7: Alopecia X (Black Skin Disease)

Alopecia X is a poorly understood, non-inflammatory hair loss condition that disproportionately affects Nordic spitz breeds — Pomeranians, Chow Chows, Keeshonds, and Samoyeds — as well as Miniature Poodles. Hair loss is symmetrical and progressive, beginning at the rump and thighs and eventually extending over the trunk. Critically, the skin itself is healthy — no itching, scaling, or inflammation — and affected dogs are systemically well. The name "Black Skin Disease" refers to the hyperpigmentation that develops on the hairless skin over time. The cause is believed to involve sex hormone imbalance or abnormal response to growth hormone, though research is ongoing. Spaying or neutering resolves some cases; others respond to melatonin supplementation. The condition is cosmetic and does not affect quality of life.

Cause 8: Pattern Baldness and Post-Clipping Alopecia

Pattern alopecia (pinnal alopecia) is a breed-specific, non-inflammatory hair loss that affects predictable locations: the ear pinnae in Dachshunds, the ventral neck and chest in Chihuahuas, and the temples in Whippets. It is a cosmetic condition with no known treatment, though it does not progress to generalised alopecia and causes no discomfort. Post-clipping alopecia is a separate phenomenon seen primarily in Nordic breeds after surgical or cosmetic clipping — the fur fails to regrow at the clipped site for months to years. Both conditions are benign diagnoses of exclusion, made after ruling out hormonal disease and dermatophytosis.

When Hair Loss Is Serious

Patchy hair loss should be evaluated promptly when any of the following apply:

• Rapid spread — lesions that double in size within days suggest an active infectious or inflammatory process.
• Skin lesions — crusting, scaling, pustules, open sores, or pigment changes indicate more than simple self-trauma.
• Systemic symptoms — weight gain or loss, changes in thirst or urination, lethargy, or behavioural changes alongside hair loss point to hormonal or internal disease.
• Household contacts affected — if other pets or people in the home develop skin rashes or itching, zoonotic ringworm or sarcoptic mange must be ruled out immediately.
• Puppy or immunocompromised dog — these populations are at higher risk for generalised demodicosis, which can be life-threatening if secondary infections develop.