This is the info I have found about them.
Hay Mites
Tiny mites that live off dandruff, grease and skin debris. They are located in the coat, mainly on the rump.
Although not a variety of louse, these mites are commonly known under the name 'static lice', as opposed to 'running lice'. The scientific name is Chirodiscoides cavia. A single hay mite is hardly visible to the naked eye, but in the case of heavier infestation where they are found in abundance, they are easily seen, especially on the rump. The colour is greyish and thus more visible in dark and white pig, where they can completely destroy the overall appearance. They tend to sit on the mid-section of the hair straw, and, unless looked at very closely for some time, they do not appear to move.
There is some dispute about where hay mites come from. Some claim that they come from hay (hence the name), while others regard them as specific guinea pig parasites and claim they should be treated as such. They are not intrusive and live on hair straws with only little contact with the skin, the vast majority of pigs are unaffected by the presense of hay mites.
Treatment is quite difficult. Hay mites are not susceptible to many of the common antiparasitic preparations, and resistance develops readily. However, it is possible to reduce the numbers considerably, especially if a combination of preparations is used. This may lead to over-dosage, as one preparation may potentiate the effect of the other and it is important to consider whether the cure is worse than the disease.
However, one health problem may occur. Some pigs seem to become sensitised to mites such as dust mites and hay mites, and become pruritic. Such pigs must be kept as free from dust and static lice as possible, and glucocorticoid treatments may sometimes prove useful.
Kat