What this means
Darkening of the skin usually develops gradually, over weeks or months. It is easiest to see on bald or thinly haired areas such as the belly, groin, armpits, flanks and neck. The skin turns grey, brown, bluish or black and sometimes looks thicker, wrinkled and rough, a little like elephant hide.
It is typically a mark of long-standing irritation or inflammation of the skin. The skin responds by producing more pigment, so the darkening can stay even after the original cause has passed. That is why it matters to find out what started it: itching, friction, inflammation, infection or an internal hormonal change.
Look at whether there is hair over the darkened area. Bald, darkened skin, especially if symmetrical on both sides, may point to a hormonal or hair-growth disorder. If your dog scratches or licks the area, the cause may be a skin disease or allergy. Sometimes the darkening is noticed together with smell, greasiness or flaking.
It matters whether weight, energy, appetite and your dog's tendency to feel the cold are changing as well. In hormonal disorders these signs often go together with the skin changes. Write down when you first noticed the darkening and show it to your vet.
Possible causes
These are things that can cause this sign. It is not a diagnosis, and they are not equally likely for your dog.
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Long-standing itching and friction
When skin is kept damp by licking or irritated by scratching for a long time, it darkens and thickens. It is often darkest where the dog can reach with teeth or paw.
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Too little thyroid hormone
When the thyroid makes too little hormone, skin may darken and thicken and the coat may thin. The dog often becomes less active, puts on weight more easily and feels the cold more.
About the condition: Hypothyroidism -
Hormonal coat-growth problem
In some dogs the coat falls out symmetrically and the bald skin darkens. The dog often feels well, but a vet needs to establish the cause.
About the condition: Alopecia X (black skin disease) -
Allergy or skin inflammation
Recurring skin inflammation leaves darker, thickened areas, especially on the belly, groin and in the armpits.
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Bacterial or yeast infection
Infection adds to the irritation, and the skin becomes dark, greasy or flaky and may smell unpleasant.
About the condition: Skin infections -
Friction in skin folds
In skin folds and where skin rubs on skin, darker, thickened areas may form over time. A vet should check that there is no inflammation.
What else to watch for
These observations help your vet build the picture faster. Note down what you saw and when.
- Which part of the body has darkened and whether it is symmetrical
- Whether there is hair over the area or the skin is bald
- Whether the skin is thickened, wrinkled or rough
- Whether your dog scratches or licks the area
- Whether there is smell, greasiness or flaking
- Whether weight, energy and tolerance of cold have changed
When to book a routine visit
Book a visit if the skin has darkened or thickened, particularly if the coat is thinning too, your dog is scratching, the skin smells, or weight, energy or appetite are changing.
Related conditions
Conditions whose entries mention this sign. The list is somewhere to read on, not a way to reach a diagnosis.
- Alopecia X (black skin disease) Skin and coat Alopecia X is a condition in which the hair growth cycle is disrupted: the coat gradually thins until bald patches appear on the body and the exposed skin darkens. It neither hurts nor itches, so it affects the dog's looks more than its wellbeing. Hormonal disorders can look very similar, though, so changes in a balding dog's coat are worth showing to a vet.
- Hypothyroidism Hormones and metabolism Hypothyroidism is a condition in which the thyroid gland produces too little of the hormones that regulate metabolism. Because these hormones affect almost the whole body, a shortage gradually slows many of its processes. The changes come on slowly, so they are easy to mistake for ageing or simple laziness.
- Skin problems Skin and coat This is an umbrella term for a range of skin disorders, from allergies and parasites to infections, hormonal diseases or growths in the skin. The skin also reflects a dog's overall health, so changes in it sometimes point to a problem elsewhere in the body, and constant itching or soreness takes a real toll on a dog's comfort. What lies behind particular changes is for a vet to work out.
Body systems: Skin and coat Hormones and metabolism