Patchy hair loss in cats almost always points to one of a handful of causes: parasites, fungal infection, allergies, over-grooming, or less commonly, an underlying medical condition. The pattern, location, and appearance of the bald spots often reveal the culprit, which is why paying attention to where the hair is missing matters as much as the hair loss itself.
Flea Allergy Dermatitis
Fleas are the single most common reason cats lose hair in patches. What makes this tricky is that the problem isn’t really the fleas themselves. It’s an allergic reaction to proteins in flea saliva. When a flea bites, it injects saliva containing histamine-like compounds and enzymes that trigger a hypersensitivity reaction in the cat’s skin. A cat with this allergy can lose significant amounts of fur from just a few bites.
The hair loss follows a distinctive pattern. It concentrates along the back, the base of the tail, the back of the thighs, and sometimes the head and neck. You’ll often see small crusty bumps, raw spots from scratching, or thinned fur in these areas. Some cats don’t scratch visibly. Instead, they overgroom, licking the fur away so efficiently that owners never see a single flea. If the bald patches are on your cat’s lower back or hind legs, fleas should be the first suspect even if you’ve never spotted one.
Ringworm
Despite the name, ringworm is a fungal infection, not a worm. It’s the most common fungal skin disease in cats, and it produces the kind of distinct, round bald patches that send most people to a search engine. The classic appearance is a circular area of hair loss with flaky, scaly skin and sometimes a reddish ring around the edge where the skin is healing in the center.
In many cats, particularly healthy adults, ringworm is self-limiting. The cat loses some hair, develops scaling, and eventually clears the infection on its own. But in kittens and cats with weakened immune systems, it can spread from a single patch to multiple spots across the body or even become generalized. Ringworm is also contagious to other pets and to humans, so it’s worth identifying quickly. Cats pick it up from other infected animals, contaminated environments, or in some cases from soil. Outdoor cats in rural areas are especially prone to a soil-dwelling species of the fungus.
Food and Environmental Allergies
Allergies are a major driver of patchy hair loss, and they come in two forms that look slightly different on a cat’s body.
Food allergies tend to cause itching concentrated around the head and neck. You may notice your cat scratching its ears and face aggressively, developing small crusty bumps, or losing hair symmetrically on both sides of the body. Some cats develop raised, reddened, swollen patches of skin called eosinophilic plaques.
Environmental allergies (the feline version of hay fever) cause excessive scratching and licking that leads to hair loss, scaling, crusts, and inflamed skin. The distribution varies more from cat to cat, making environmental allergies harder to pin down based on location alone. Dust mites, pollen, and mold are common triggers. The key difference from food allergies is that environmental allergies often follow a seasonal pattern, at least initially, while food allergies persist year-round.
Over-Grooming and Stress
Some cats lick, chew, or pull their own fur out without any visible skin disease underneath. This is called self-inflicted alopecia, and it’s one of the more frustrating causes for owners because the skin looks completely normal beneath the missing fur. The hair loss tends to appear symmetrically on the belly, inner thighs, forelegs, and groin, areas the cat can easily reach with its tongue.
Veterinarians can confirm self-inflicted hair loss by examining the remaining hairs under a microscope. If the tips of the hair shafts are broken or frayed rather than tapered to a natural point, the cat has been damaging its own coat. This distinction matters because it separates hair that’s falling out from a disease process versus hair that’s being licked or chewed off.
Stress and anxiety (a new pet, a move, changes in routine) are sometimes the trigger, but vets typically rule out all medical causes first. Many cats diagnosed with “psychogenic” over-grooming actually turn out to have a subtle allergy or low-grade skin irritation driving the behavior.
Mites and Mange
Mange in cats is caused by a burrowing mite that produces intense itching. The hair loss and crusty skin lesions start on the ears, head, and neck, then spread outward across the body if left untreated. The itching is severe, noticeably worse than what you’d see with most other skin conditions, and the crusting can become thick and widespread. Mange is less common than fleas or ringworm, but it’s highly contagious between cats and requires treatment to resolve.
Hormonal and Internal Diseases
When hair loss isn’t accompanied by itching or scratching, an internal condition may be involved. Hyperthyroidism, one of the most common diseases in older cats, can cause the coat to look unkempt, matted, or greasy, though the hair loss tends to be diffuse rather than in neat patches.
A rarer but more distinctive pattern appears with certain internal tumors. Cats with pancreatic or bile duct cancers can develop a condition called paraneoplastic alopecia, where the hair falls out symmetrically and progressively, and the exposed skin looks unusually shiny and thin. The hair pulls out with almost no resistance. Affected cats often also show softened or thickened footpads, weight loss, and lethargy. This condition is uncommon, but its appearance is striking enough that it warrants mentioning: if your cat’s bald skin looks glossy and paper-thin, that’s a reason to see the vet promptly.
Injection Site Reactions
A small, localized patch of hair loss can develop at the site of a recent vaccination or injection. These reactions are not uncommon in cats and typically appear within days of the shot. You may notice swelling, redness, or irritation at the spot along with the missing fur. In most cases, the hair grows back on its own. If the area remains swollen or grows larger over weeks, a vet should evaluate it.
How Vets Figure Out the Cause
Because so many conditions produce patchy hair loss, your vet will typically work through a sequence of tests rather than jumping to a single diagnosis. The process usually starts with the simplest and most common possibilities. A flea comb checks for flea dirt (tiny black specks of digested blood). Skin scrapings examined under a microscope look for mites. A fungal culture, where hairs from the edge of a bald patch are placed on a growth medium, confirms or rules out ringworm.
If parasites and infections come back negative, the next step is often cytology, where cells from the skin surface are examined for signs of bacterial or yeast overgrowth. A diet trial lasting several weeks can test for food allergies. When none of these reveal an answer, a skin biopsy provides a deeper look at what’s happening in the tissue itself. Blood and urine tests enter the picture if a hormonal or systemic disease is suspected. The process can take time, but each step narrows the list considerably.