We had similar in the odd ewe (among 500+) on moorland in Northumberland. The vet explained that similar symptoms for listeriosis, CCN and a third thing I can't remember, maybe it was just lack of Vitamin B. Anyway, we were recommended to give VitB jabs and antibiotics, as that combination would address all 3 possibilities, and we found it pretty effective.
If you haven't been giving a mineral drench then it's likely that, especially in a ewe of that age, a drench in autumn and another before lambing (if they're lambing, or in spring if not) would go down well.
You'd need to talk to your vet to get antibiotics and B vits, and it would be worth talking to them anyway. So many of these things are quite regional / topographical etc, and no-one has a better overview of local conditions and livestock needs in your area than a local farm vet.