I am very surprised by the vet's reply to Ivermectin. But, to each his own. But Ivermectin is safe for mares during pregnancy (used it consistently here for many years on all the pregnant mares), and takes care of lice with a retreatment in 2 weeks. But, if you're happy with your vet, that's fine. Just do what they advise so as not to aggravate any future need you might have for needing their assistance.