No sheep here but goats.

I suspect in sheep, as in goats, resistance to wormers is common. Try switching wormers to a different class of wormer. In goats we dose ORALLY due to their quick metabolism. Segregate the ewe so she deposits the bulk of resistent worms she sheds any place other than your pastures.

It could also be a coccidiosis issue(damage), or possibly Johne's. I'd have her tested for Johne's ASAP as if she is positive you are risking the rest of your herd every day she is defacating on your pastures. You don't say how old this Katahdin is, if younger than 6 months I might have given a different answer.

UC Davis(WA), Cornell(NY), or PAVLAB(TX) will perform the tests. Submit 3cc serum (blood sample) for a quick result, or some pellets for a long wait but more accurate results.

Brendasue

One breed characteristic of Katahdins is supposed to be that they're naturally a bit parasite resistant. They've got a lot of St. Croix blood in them. I can't help but wonder if this ewe is just damaged, long term or permanently, from being fed grain rather than pasture and hay. I rotationally graze and don't worm mine, and I haven't had any problem.

Dan

If this sheep has been fed on grain it could be that a sudden change to rich pasture has upset her . pen off a part of your pasture and feed her hay introducing her gradually to your pasture.