They need to be shampooed every week for 3 weeks to kill all the lice/mites that have hatched since the last treatment. I find that shampoo very effective, I dilute the 5 ml dose with 30 ml ish of water - pour half over the wet pig, lather, rinse, repeat shampoo with second half, lather, rinse and rinse again. I usually rub it up my arms too then rinse, just in case some of the little have escaped!
I shampoo every 6-8 weeks to prevent reoccurance or you can use a spot on treatment - but you must leave it 3 days after a bath before applying.
HTH
Suzy x