Noticing your dog losing hair in patches on the back is alarming, especially when the bald spots appear suddenly or start to spread.
Patchy hair loss along the spine, lower back, or near the tail often points to an underlying skin irritation.
We outline the common reasons for patchy hair loss on a dog’s back, what you can do, and when to seek veterinary help.
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Dog Losing Hair in Patches on Back — Why It Happens
A dog losing hair in patches on the back happens when skin becomes irritated or inflamed from allergies, parasites, fungal infections, hormonal imbalances, or bacterial overgrowth.
The back, especially toward the tail, is a common trouble area because fleas, mites, and environmental irritants tend to gather there.
Once the skin becomes itchy or painful, dogs scratch or chew aggressively, causing the hair to fall out and leaving uneven bald patches. Without treatment, the irritation often spreads, creating larger areas of hair loss.
Dog Losing Hair in Patches on Back: Common Causes
Flea Allergy Dermatitis
Flea allergy dermatitis (FAD) is the number one cause of patchy hair loss on a dog’s back, especially near the base of the tail. A dog allergic to flea saliva will have an exaggerated reaction to even a single flea bite. When a flea feeds, it injects saliva that triggers intense itching and swelling for sensitive dogs.
Dogs then scratch, chew, and pull out hair, especially on the lower back and tail area. Over time, the fur becomes thin or patchy, and the skin may develop scabs, crusty bumps, or darkened areas from chronic irritation.
Even if you don’t see fleas, they can still be the culprit—fleas hide exceptionally well in thick fur and spend most of their life cycle in carpets, bedding, and outdoor spaces. Dogs with flea allergies often require strict, year-round flea prevention.
Read more: Dog Losing Hair in Patches and Scabs (Why it happens)
Environmental or Food Allergies
Allergies to pollen, dust mites, grass, mold, or certain food proteins can cause hair loss specifically along the back.
When allergens contact the skin or circulate in the bloodstream, they trigger a release of inflammatory chemicals that make the skin itchy and inflamed.
Dogs scratch along their spine or chew near their tail, causing the hair to fall out in uneven patches. Food allergies may cause year-round itching, while environmental allergies often flare during certain seasons.
Because allergies weaken the skin barrier, secondary bacterial or yeast infections can develop — worsening the hair loss and creating red, irritated patches along the back.
Mange
Mites are another major cause of patchy back hair loss. Demodectic mange typically creates small round bald spots that appear smooth or scaly. These patches often begin on the face or legs but can spread to the back as the condition worsens or the dog scratches.
Sarcoptic mange (scabies) is extremely itchy and often begins along the ears, elbows, belly, and back. Dogs with scabies scratch nonstop, leading to widespread hair loss and thick crusts or scabs. Mange spreads quickly and can cause severe skin damage if left untreated.
Because mange can resemble allergies or fungal infections, a vet exam is necessary for diagnosis.
Hot Spots (Acute Moist Dermatitis)
Hot spots are painful, inflamed areas that often appear on the back, especially in dogs with thick coats or who swim frequently.
Once an area becomes moist or irritated, bacteria multiply rapidly, creating a swollen, red, sticky patch that the dog keeps licking or gnawing.
Hair around the hot spot falls out in clumps, leaving a bald area with a moist or scabby appearance. Hot spots can grow within hours and quickly become very painful. They often develop from:
• Allergies
• Flea bites
• Moisture trapped under the fur
• Scratching at a small irritation
Treating hot spots early helps prevent widespread patchy hair loss.
Ringworm (Fungal Infection)
Ringworm is a contagious fungus that commonly causes circular bald patches with rough, scaly edges.
While it can affect any area of the body, the back is a frequent location — especially in dogs that lay on contaminated surfaces or live in multi-pet households.
Ringworm lesions may start small and expand outward, leaving broken hairs and flaky skin. Dogs may scratch the patches, creating irritation and spreading fungal spores further along the back.
The condition spreads easily to other pets and even humans, so quick treatment is important.
Hormonal Disorders
Hormonal imbalances can cause symmetrical or irregular hair loss down the dog’s back. Hypothyroidism slows the hair growth cycle, making the coat thin, dry, and prone to shedding in patches.
Cushing’s disease causes excess cortisol, thinning the skin and making hair fall out easily. The hair loss is often most noticeable along the back, flanks, and tail.
Unlike allergies or parasites, hormonal hair loss usually does not itch — but infections may develop because the skin becomes fragile and poorly protected, leading to scabs or redness.
Read more: Dog Losing Hair on Legs (Common reasons explained)
What to Do If Your Dog Is Losing Hair in Patches on their Back
Start by gently checking your dog’s back for signs of fleas, redness, flakes, or bumps. Running a fine-tooth comb through the fur may reveal flea dirt or irritated skin. If fleas are suspected, begin a reliable flea treatment immediately and clean any bedding or carpets your dog uses.
If irritation is mild, you can trim surrounding hair to help the skin breathe. Using a gentle, fragrance-free shampoo or medicated antiseptic wash (recommended by your vet) can help soothe inflamed skin.
For dogs with potential allergies, switching to a bland diet or avoiding seasonal triggers may help—but sudden diet changes should be made gradually. Avoid applying human ointments or random over-the-counter creams; many seal in moisture or worsen irritation.
Omega-3 supplements, probiotics, and good grooming habits support healthy skin but are not substitutes for veterinary care when symptoms are significant.
When to Call or Visit Your Vet
It’s important to contact your vet anytime hair loss on the back spreads, worsens quickly, or begins to smell foul. Many causes, like mange or ringworm, require prescription medication to heal fully.
Seek veterinary care if your dog has:
• Red, raw, or bleeding patches
• Crusty scabs forming along the back
• Sudden hair loss with severe itching
• Thickened or darkened skin
• Circular bald areas
• Signs of pain when touched
• Hair loss combined with appetite loss or lethargy
Early diagnosis prevents conditions from becoming chronic or spreading to other pets.
Read more: Dog shedding excessively (What’s causing it?)
Key Takeaway
When a dog starts losing hair in patches on the back, it’s usually caused by fleas, allergies, mites, infections, or hormonal changes.
Because many conditions worsen quickly, early treatment is the best way to stop the itching, heal the skin, and prevent further hair loss.
With proper care and diagnosis, most dogs make a full recovery and regrow their coat normally.