The 2 main skin problems that piggies get are mite ,lice or fungal infections.
Gps have mites on them all the time,in such low quantities that they do not cause a problem.If the gp gets stressed for any reason,even a change of diet,or going to holiday care can cause this in some gps.Some pigs are easily stressed ,others a lot calmer.
The only cure for mites if Ivermectin from a vet.This is available as an injection or topical form.I prefer to use the topicaL ,although it is a bit more expensive than the injectable. The dose has to be repeated in 10 days to kill any mites that may have hatched from eggs.
As mites are species specific and cannot live away from the host body,they cannot come in on hay or anything else.They cannot be seen with the naked eye.
The symptoms are sores,scabs,frantic scratching and in bad cases,fits.There may be some hair loss.
Lice can be seen on the hair shaft as white wriggly things like tiny maggots.They could be brought in on bad hay.
Ivermectin is not affective against them,so the gp will need a shampoo. A shampoo containing Neem will be affective.Make sure you get a product that does not contail parabens etc.There are pure brands available from health food shops.
Fungal infections are less common and again will need shampoos.Neem is affective against fungus as well as lice.
For sores or scabs caused by scratching,Green People do an excellent Organic product called Baby salve.This is soothing,healing and anti fungal.This product only contains pure ingredients.It is water repellant and anti bacterial so is a good barrier . It is also good for dry or sore pads,and protects pads against urine scald.