Mange is skin disease caused by mites. Two kinds affect dogs and they behave so differently that treating them as one condition is a mistake.

Sarcoptic mange

Caused by Sarcoptes scabiei, the mite that causes scabies. It burrows into the skin, and the reaction to it is intense.

The defining feature is severe itching. Dogs with sarcoptic mange scratch relentlessly, often to the point of being unable to settle at all. This is one of the itchiest conditions in veterinary medicine.

Where it appears: ear margins, elbows, hocks, chest and belly, spreading from there. Crusts, scaling, hair loss and skin wounded by scratching.

A useful clinical sign is the ear-scratch reflex: rubbing the ear tip makes the dog scratch with its hind leg.

It is contagious. It spreads between dogs by direct contact and through shared bedding, and it is caught from foxes. It also transmits to people, causing an itchy red rash, most often on the arms and abdomen. The human infection is self limiting once the dog is treated, but it is uncomfortable.

Diagnosis is by skin scrapings, though the mites are notoriously hard to find; scrapings are often negative in genuine cases. Where the picture strongly suggests it, treatment is given on that basis rather than waiting for a positive result.

Treatment is with specific antiparasitic medication, together with treating all in-contact dogs and washing bedding hot.

Demodectic mange

Caused by Demodex mites, which live normally in the hair follicles of most healthy dogs without causing any problem.

Disease occurs when the immune system fails to keep them in check, so demodex is a marker of something else.

The defining feature is that it is usually not very itchy, unless the skin has become secondarily infected. That is the quickest way to tell it apart from sarcoptic mange.

Localised form: a few small patches of hair loss, often around the eyes and muzzle, in puppies. Many of these resolve on their own as the immune system matures.

Generalised form: widespread hair loss, redness, scaling and secondary bacterial infection. In an adult dog developing this for the first time, an underlying cause needs looking for: hormonal disease, immunosuppressive medication or an internal illness.

It is not contagious to other dogs or to people.

Diagnosis is by deep skin scrapings, where the mites are usually found readily.

Which one is it

Intensely itchy, contagious, people in the house scratching too: think sarcoptic.

Not very itchy, patchy hair loss, a puppy or a dog with another illness: think demodectic.

Neither should be guessed. Both need scrapings, and treating the wrong one wastes weeks in an animal that is suffering.

When to come in

Any dog scratching persistently, any patchy hair loss, crusting at the ear margins or elbows, and promptly if anyone in the household has developed an itchy rash.