I have read that lice like to get into straw since it's hollow, I don't know how true that is , since I don't use it I like to use pine shaving the flake kind much more absorbent than straw or hay and I have also had ducks going on 10yrs this coming spring and they have never had lice or mites and it's mainly because they bathe so frequent it just a natural way to keep them off, so I doubt your has parasites just molting which can take several months to get through,. I will say even when we had a mite infestation last year in the chicken coop my ducks/geese did not have them. I do under stand about stressing though we want them to be healthy.