Fur Changing Colors in Spots: Dog Has Darker Areas on Hair

Just like humans, dogs can develop increased pigment in the skin due to age and UV exposure – but in many cases, altered coat color can indicate underlying disease.  So, how can you tell if your dog’s dark areas of coat are benign or something to be worried about? Let’s review some of the causes of fur becoming darker.

Overview of fur changing colors in spots of dogs

Dogs have variable pigment in their skin and fur, depending on their breed and coat color; the dark pigment in the skin is called melanin. While some conditions can cause pigment loss and pale areas of skin or fur, this article focuses on causes of darker areas of coat.

The main cause of dark, often slightly thickened patches of skin is hyperpigmentation. Primary hyperpigmentation is rare and breed-related; secondary hyperpigmentation is much more common, occurring due to inflammation and hormonal changes and affecting any breed. Hyperpigmentation should also be distinguished from other causes of dark areas, like bruising or pigmented tumors.

What are the causes of fur changing color or becoming darker?

Primary hyperpigmentation

This has been described almost exclusively in Dachshund dogs and is believed to be genetic. Widespread hyperpigmentation develops, usually in dogs less than 1 year old. This is usually cosmetic and doesn’t require treatment. In some cases inflammation can develop, requiring intervention, but there is no cure.

Secondary hyperpigmentation

This is a very common cause of increased pigmentation, often visible through short or sparse coats. The exact mechanism is unknown, but several factors are known to cause it.

Chronic inflammation – in the form of allergic skin disease, infection, parasites or wound healing – commonly causes hyperpigmentation. Darkened skin often appears as the initial redness and inflammation settle and can take weeks to months to resolve after the underlying disease has been treated. Conditions like atopy may cause recurrent inflammation and persistently darkened skin.

Hyperpigmentation can also occur secondary to friction, in areas like the armpits and groin, or over pressure points. This is more common in obese patients.

Hormonal conditions like Cushing’s syndrome (hyperadrenocorticism) and hypothyroidism commonly cause skin changes, including hair loss and hyperpigmentation. These typically occur in middle aged or older dogs, and also cause other symptoms like increased thirst and urination, lethargy or weight gain. These skin changes (and other symptoms) usually improve and potentially resolve over months once medication is started.

Finally, UV exposure is also known to contribute to hyperpigmentation. Hairless or sparsely furred areas can be affected, with pigmentation often darkening seasonally. It’s suspected that UV exposure also contributes to the hyperpigmentation frequently associated with conditions causing alopecia (hair loss), including:

• Pattern alopecia
• Canine recurrent (seasonal) flank alopecia
• Alopecia X 
• Hormonal causes of alopecia

Tumors

Some tumors, like melanomas, melanocytomas and adenomas can appear pigmented. While these can be benign, if you notice a pigmented lump it’s worth getting this checked by your vet to rule out a malignant tumor.

Dirt

Surprisingly, even dogs that are regularly bathed can accumulate tightly adherent dirt. As a vet, I’m frequently surprised by the amount of dirt we get off seemingly clean dogs when prepping for surgery! If your dog’s belly appears diffusely darker than normal or has black specks on the skin, it’s worth doing your best to soak, lather and gently clean these off.

Saliva staining can also cause darkened areas of fur – and may indicate underlying allergies, pain or stress, so it’s worth getting this checked out.

Finally, skin infections like hot spots produce discharge that can make the fur appear darker.

When should I be concerned if my dog’s fur color changes – and when should I see the vet?

The main thing to pay attention to is whether your dog shows any other symptoms, like itching, altered behavior, or a lump – if they do, it’s time to see a vet. Hyperpigmentation itself isn’t a problem – but it can indicate an infection, hormonal problem or tumor, for example. If there aren’t any other symptoms but you’re still concerned, it’s worth reaching out to a vet so they can perform a thorough examination.

What to do if this issue persists with my dog 

First, make sure your dog’s parasite treatments are up to date, and if they have a short or sparse coat take care to protect them from the sun. Check thoroughly for any signs of inflammation or lumps and keep an eye on their weight, water intake and behavior so you can provide a thorough history for the vet.

Treatment options and likely costs

Treatment depends on the cause – some hyperpigmentation requires no treatment. Parasites or infections may require a course of anti-inflammatory or anti-itch medication combined with antibiotics, antifungals or parasite treatments, which may be oral or topical. 

Hormonal conditions usually require a blood test for diagnosis, and dogs typically need lifelong oral medication, which can become expensive but can hugely improve their overall health and quality of life.

References

Bajwa J. (2022). Cutaneous hyperpigmentation in dogs. The Canadian veterinary journal = La revue veterinaire canadienne, 63(1), 85–88.

H. Jackson; R. Marsella (Eds.) (2012), BSAVA Manual of Canine and Feline Dermatology (3rd ed., pp. 130–151). British Small Animal Veterinary Association.