Hi!
Full mites treatment consists of three rounds of ivermectin or selamectin as their active ingredient at the appropriate strength and the product specific interval in order to get rid of them. The first round should take care of any mites on the body, the second of any mites emerging from the eggs (mange mites burrow theirs in the skin and hay mites fix them to hairs especially at the bum end); the third round is there to catch any stragglers and prevent a return. In severe cases more rounds may be necessary.
Here is more information on guinea pig specific parasites:
New piggy problems: URI - ringworm - skin parasites
Mange mite eggs sitting in the skin cannot be killed off. they are usually kept in check by a fully working immune system but can make an appearance if the immune system is for some reasoned lowered or overloaded (illness, stress, bereavement; babies, pregnancy and old age).
Hay mites, as their name says, come these days mainly with industrially harvested and imported hay from international hay brands. As the ground is churned up a lot more than in more traditional smaller scale methods, more stuff gets into the hay. If you are dealing with hay mites, then changing the hay provider is usually helpful.
A Comprehensive Hay Guide for Guinea Pigs (incl. providers in several countries)
Could you please clarify which product you have been prescribed and whether your vet has diagnosed mange mites or hay mites?
