If you see a wild animal with scattered patches of missing hair, a thin or hairless tail, itching, and especially with squinted eyes, it has mange. The odor is unique if you are close enough. Mange is a mite that burrows under the skin and bites, causing irritation and the hair to fall out. It is usually fatal. In the winter the animal can freeze to death. In the warmer weather it can get secondary infections from the scratching and can starve as it eventually can’t see to hunt. When they die they are usually mostly hairless and emaciated. Normally the animal is avoided by its own family because of spreading the mange.
There are several types of mange, but mostly wildlife rehabilitators see Sarcoptic mange. Mange is easy to treat with the right medication, right where the animal is by placing the medication in food. Vigilant people have been very successful treating a wild animal with mange. Ivermectin paste is an antiparasite effective for treating Sarcoptic mange. People give ivermectin to their horses once a month for prevention. This can be bought at a feed store, from your veterinarian in an injectable/oral form, or online. One syringe treats a 1320 pound horse, so dosing is tricky with a smaller animal like a 10 to 15 pound fox. More is not better. The animal can be overdosed and become lethargic, which is a death sentence in the wild.
The first dose kills the mites and the second dose two weeks later kills the mites that hatch from the eggs. Dosing more often that that does not help and can contribute to overdosing. A third dose is optimal two weeks later to be sure all the mites are killed. A veterinarian knowledgeable about treating wildlife can assist you with this. The animal then will have to take its chance avoiding where it got mange in the first place.
This shows the progression of mange early stage This shows advance stage