All cats shed. The useful question is whether the hair is falling out evenly across a healthy coat, or coming out in patches from skin that is not healthy.

Normal shedding

Cats moult most heavily in spring and autumn, triggered mainly by day length. Indoor cats living under artificial light often shed moderately all year round instead of in two clear seasons.

Normal shedding is even. The coat thins overall, the skin underneath looks normal, and the cat is not scratching.

Long haired cats shed the same amount as short haired ones; it is simply more visible and more likely to mat.

Shedding that is a problem

Concerning signs are bald patches, thinning that is worse in some places than others, redness, scabs, scaling or black specks in the coat, and a cat that is scratching, over-grooming or chewing at itself.

Also concerning: hair that comes out in clumps, a dull greasy coat, and a cat that has simply stopped grooming.

The common causes

Fleas. Still the single most common cause of skin disease in cats. Many cats are allergic to flea saliva, so a small number of fleas causes a large reaction, typically along the back and at the base of the tail. You often cannot find the fleas, because a grooming cat removes them.

Allergy. To something in the food, or environmental, causing itching around the head, neck and belly.

Over-grooming from stress. Cats groom to self soothe. A stressed cat can create a symmetrical bald strip along the belly and inner thighs with no skin disease underneath at all. Owners rarely see the grooming happening, because cats do it in private.

Ringworm, a fungal infection, which causes circular patches of hair loss and transmits to people.

Systemic disease. Hyperthyroidism and kidney disease both change the coat, and a cat that feels unwell stops grooming, so the coat becomes dull and matted.

Poor nutrition. An unbalanced diet shows in the coat before it shows anywhere else.

What helps

Brush regularly. Daily for long haired cats, a few times a week for short haired ones. It removes dead hair before the cat swallows it, reduces hairballs and lets you check the skin.

Keep flea treatment current all year, and treat every animal in the household. Feed a complete diet appropriate to the cat’s age. Provide scratching posts, high places and routine, since stress shows in the coat.

Never use a dog flea product on a cat.

When to come in

Bald patches. Red, scabby or scaly skin. Constant scratching or chewing. Circular lesions, particularly if anyone in the household has a rash. Sudden coat change in an older cat. Or a cat that has stopped grooming itself.