Patchy hair loss in dogs almost always signals an underlying problem, whether it’s parasites, an infection, allergies, or a hormonal imbalance. The location, shape, and texture of the bald spots can tell you a lot about what’s going on. Some causes are minor and resolve quickly, while others need ongoing treatment.
Fleas and Allergies: The Most Common Culprit
Flea allergy dermatitis is one of the most frequent reasons dogs lose hair in patches, and it doesn’t take an infestation to cause it. A single flea bite can trigger intense itching that lasts for days in a sensitive dog. When fleas bite, they inject saliva into the skin, and the proteins in that saliva set off an allergic reaction. The dog scratches, chews, and licks the area until the hair falls out or breaks off.
The telltale pattern is hair loss from the middle of the back to the base of the tail and down the rear legs. Veterinarians sometimes call this the “flea triangle.” Dogs that already have environmental allergies to things like pollen, mold, or dust mites are much more likely to react severely to flea bites, so one problem often compounds the other.
Environmental and food allergies on their own can also cause patchy hair loss. These dogs tend to lick their paws, scratch their ears, and develop red, irritated skin in multiple areas. The hair loss is secondary: it happens because the dog is constantly traumatizing the skin through scratching and chewing, not because the hair follicles themselves are damaged.
Mites and Mange
Two types of mites cause distinct patterns of hair loss. Demodectic mange is caused by mites that live deep in hair follicles. It often shows up as small, localized bald patches on the face and front legs, especially in puppies and young dogs whose immune systems are still developing. In mild cases, a few patches may clear up on their own. In more severe cases, the patches spread and the skin becomes thickened and crusty.
Sarcoptic mange (scabies) is a different story. The areas most commonly affected are the ear margins, elbows, and hocks (the ankle joints on the hind legs). The skin turns red, develops crusts, and loses hair. These mites cause relentless itching, and affected dogs are often visibly miserable. Scabies is also contagious to other dogs and, temporarily, to people.
Ringworm: Not a Worm at All
Despite its name, ringworm is a fungal infection. Infected dogs develop bald, scaly patches with broken hairs. The patches may or may not have the classic circular shape people associate with ringworm. They can appear anywhere on the body and tend to spread outward over time. Ringworm is contagious to other pets and to humans, so early identification matters. Puppies, senior dogs, and dogs with weakened immune systems are most susceptible.
Hormonal Imbalances
When hair loss appears symmetrical on both sides of the body without much redness or itching, a hormonal problem is often the cause. Two conditions stand out.
Hypothyroidism (an underactive thyroid) slows down the hair growth cycle. Dogs with this condition often have a dull, thinning coat, gain weight without eating more, and become lethargic. The hair loss tends to appear on the trunk, tail, and along the flanks, and the skin may feel thickened or cool to the touch.
Cushing’s syndrome, where the body produces too much cortisol, also causes symmetrical hair loss. Dogs with Cushing’s typically drink and urinate more than usual, develop a pot-bellied appearance, and pant excessively. Both conditions are diagnosed through blood tests and are manageable with medication, though they require lifelong treatment.
Breed-Specific Hair Loss
Some breeds are genetically prone to hair loss conditions that aren’t caused by infection or hormones. Pomeranians and other spitz-type breeds can develop a condition called Alopecia X, where the coat gradually thins and falls out in a symmetrical pattern along the trunk while sparing the head, tail tip, and lower legs. The underlying cause involves abnormal hormone processing in the skin itself.
Irish water spaniels can lose hair on the neck, flanks, back, and tail, often with a color change from reddish brown to grayish brown. Other breeds, like Bulldogs and Boxers, are predisposed to seasonal flank alopecia, where patches appear on the sides of the body during certain times of year and may regrow on their own. If you have a purebred dog, it’s worth knowing whether your breed has a known predisposition.
Where the Bald Spots Are Matters
The location of the hair loss gives your vet important diagnostic clues:
- Base of the tail and lower back: Flea allergy dermatitis is the top suspect.
- Ear margins, elbows, and hocks: Points toward sarcoptic mange.
- Face and front legs (especially in young dogs): Suggests demodectic mange.
- Symmetrical patches on both sides of the trunk: Hormonal conditions like hypothyroidism or Cushing’s syndrome.
- Face, head, and ear flaps with well-defined borders: Could indicate alopecia areata, an immune-mediated condition where the body’s own defenses attack hair follicles.
- Scattered circular patches with scaly skin: Ringworm.
Also pay attention to whether the skin underneath looks normal or inflamed. Hair loss with red, crusty, or oozing skin suggests an active infection or parasite. Hair loss over smooth, otherwise healthy-looking skin points more toward hormonal or genetic causes.
How Vets Figure Out the Cause
A vet visit for patchy hair loss usually starts with a few straightforward tests. A skin scraping is the most common first step. The vet uses a blade to gently scrape the surface of the skin (or deeper into the follicle layer for mites like Demodex), places the sample on a glass slide with mineral oil, and examines it under a microscope. It looks like a mild abrasion afterward, similar to a skinned knee.
Skin cytology involves pressing a glass slide, a piece of clear tape, or a cotton swab against the affected area to pick up cells, bacteria, or yeast. This helps identify secondary infections that may be making the hair loss worse. For ringworm, a fungal culture is often needed, which can take a week or more to come back.
If these initial tests don’t reveal a clear answer, the next step is often blood work to check thyroid and cortisol levels. In more complex cases, a skin biopsy may be recommended. The vet takes a small sample of full-thickness skin (about the size of a pencil eraser) using a punch tool, closes the site with a few stitches, and sends it to a pathologist. Biopsies are especially useful for identifying autoimmune conditions, hair follicle disorders, deep infections, and skin cancers.
What Treatment and Recovery Look Like
Treatment depends entirely on the cause. Flea allergy dermatitis resolves with consistent flea prevention. Environmental allergies may need longer-term management with medications that calm the immune response. Mange is treated with antiparasitic medications, and most dogs recover fully once the mites are eliminated.
Ringworm typically clears with antifungal treatment over several weeks, though you’ll need to clean your home thoroughly to prevent reinfection. Hormonal conditions require ongoing medication but respond well once levels are properly managed. Hair regrowth after hormonal treatment can take weeks to months because the follicles need time to restart their growth cycle.
Alopecia areata, the autoimmune form, is more unpredictable. In a study of 14 dogs with this condition, immune-suppressing medication led to partial regrowth in most dogs and complete regrowth in some, but relapse was common when the medication was reduced. About 14% of dogs experienced spontaneous remission without treatment. It’s generally considered a chronic, relapsing condition.
Signs That Need Prompt Attention
Some accompanying symptoms suggest the problem is more serious or progressing. Rapid spreading of bald patches, skin that’s hot to the touch or oozing discharge, a foul odor from the skin, intense scratching or biting that disrupts sleep or eating, and any behavioral changes like lethargy or loss of appetite all warrant a quicker vet visit. Secondary bacterial infections can develop on damaged skin, turning a manageable problem into a more painful one. Physical causes like burns or friction from a poorly fitted collar can also damage hair follicles directly, so check for anything rubbing against your dog’s skin.