Scaly leg mites are a sarcoptic mite of poultry (Knemidocoptes mutans). They like to live under the scales of the chicken's legs. They burrow into the tissue beneath the scales causing the scales to become loose from the tissue.
Infected birds usually have thickened, crusty legs and feet. Some birds may become lame due to the pain or discomfort associated with infestation. When scales are lost the legs may be tender. There can be redness and inflammation seen in some infected individuals.
It is more often seen in older birds, perhaps because their scales are not as tight to the leg as those of younger birds making it easier for the mites to get beneath them. The mites do sometimes attack the combs and wattles of severely infected birds. The entire life cycle of the mite is carried out in the skin beneath the scales. The mites are transmitted through contact with infected birds.
Affected birds should be quarantined from the rest of the flock to prevent further infection. The area should be cleaned and sprayed with a product effective at treating mites, such as malathion or a pyrethroid compound, Sevin dust has also been proven effective.
Individual birds should be treated with oral or topical ivermectin. You can wash the legs with warm soap and water and using a toothbrush scrub away any exudate that has formed on the scales. Do not try to pick the scales off as that is damaging to the bird. You should allow the scales to fall off and regrow on their own. This will usually occur during the next molt, it can be up to a year before the legs look normal again.
You can also apply Vaseline to the legs, getting up under and in-between the scales as best as possible. This is supposed to help smother the mites. There are some remedies talked about that recommend the use of Creosote or Diesel. These chemicals are not safe for your birds and I do not recommend their use.
Just a little tidbit of information: Sarcoptic mites of dogs cause sarcoptic mange also known as Scabies to many people, the mite of the dog and that of the chicken are not the same mite though, so you don’t have to worry about you or your dog catching them from your chickens or vice versa.