External Parasites? And solutions

[cfwindsfitchburg]

I've got a few sheep that are losing wool in tufts and one that has a large bald patch on her neck. Checked her temp and it's regular. She'd eating hay & mineral. My thought is that it may be an external parasite like lice or mites. I dusted her neck with diatomaceous earth. Any thoughts on how to address the wool loss? Especially when the ewes are pregnant?

I've got a split flock of Tunis, Cheviot, and Tunis/cheviot cross sheep. I'm also talking to a sheep nutritionist and he's evaluating my feed for nutritional deficiencies, making suggested changes etc.