Hi,
I've never heard it said that mihes can't be eradicated before a break out, certainly in the UK, many guinea owners routinely give Ivomec as a preventative. Again, I've never heard of mites becoming resistant to Ivomec, but I can understand your logic.
I personally do not administer Ivomec routinely as such. I do give it to new guineas who arrive, but only to established ones if they do develop a problem...which thankfully is extremely rare. I, again, personally do not like to give drugs to my animals if I feel they are not needed. But this is just my approach and what works for me and my piggies
If I do have a guinea pig in with mites, then I will only routinely treat the ones it has been in contact with. All new guineas are quarantined for 3 weeks.
In mild cases mites may be treated and eradicated with the use of shampoos. The shampoos in themselves are a good preventative. Though with a case of skin mites, Ivomec is always (to me) worth giving.
Have you seen the "Gorgeous Guineas" site in the UK that sells aromatherapy shampoos to help with skin problems?
If you do a search, it is easy to find, or someone will add a link for you
Hope this helps and remember these are just my thoughts, I'm sure others will be along with their own!
Barbara