Hog mange can be caused by two different parasitic mites: Demodex phylloides and Sarcoptes scabiei var. suis. The Sarcoptes scabiei (Sarcoptic) form is the most common in the United States, and is also known as common mange. The Demodex phylloides (demodextic) form is also known as the red mange, demodectic mange, and follicular mange.
The first signs of hog mange appear as irritation around the eyes, ears, and snout. This spreads to the legs (because of the pig scratching with said appendages) and then to the rest of the body. The irritated area will be covered in small, red, swollen spots, each spot being where a mite has burrowed into the skin. There may also be blister formation, which hardens and oozes serum, hardening into yellow crumbs on the skin. The skin may also become rough and leathery due to excessive scratching. This being mange, hair loss will becoming along about this time. Severe cases can cause death.
The mites are so small that they require magnification for identification; adult mites are only about 1/50th of an inch long. They are whitish in colour, and look like little tiny ping-pong balls.
Spraying or dipping with pesticide -- being very careful which pesticide you use. All of your pigs will have to be treated, and some pesticides are not safe for young pigs (piglets younger than 3 months are especially at risk). Sows should be treated at least 40 days before farrowing, and boars should be treated before breeding. Mites live underneath the skin, so they are hard to kill -- many treatments may be needed, and you may have to try multiple pesticides before finding the one that finally works. Always follow the directions given when using a pesticide on an animal (or anywhere else, for that matter).
The mites spend all their life on pigs. The mites burrow into the upper layer of the skin, digging tunnels and laying eggs along the way. Each female will lay about 10-25 eggs during her egg-laying period, after which she dies in her burrow. The eggs hatch in 3-10 days, and the larvae migrate to the surface of the skin. They go through a few moltings before they reach adulthood. Once they reach maturity, they mate and burrow back into the skin to start the cycle over again. The life cycle takes about 15 days.
For the most part the mange spreads though pigs touching, but a mite can live for up to two weeks off of a host body, so may be spread through infested premises. Mites can infect humans, although it is not as dangerous for them, and an infestation will not usually last for more than 30 days. It is best to wash both yourself and your clothes after handling infected pigs.
I don't know as much about this type of mange; here's what I do know.
Demodex phylloides is even smaller than Sarcoptes scabiei, a grown adult being 1/100 of an inch. They are vermiform, and don't usually cause any trouble. They live in the hair follicles and oil glands of most pigs, reproducing inoffensively. Occasionally there will be a population explosion, and the pig will come down with mange. Apparently, this outbreak has something to do with the natural resistance of the individual pig to the mite. I cannot find a reliable source on how contagious this type of mange is; if you need more information you should contact your local veterinarian. Treatment is the same as for sarcoptic mange.