We all have stories like that. Mine, very similar to yours, but the farm was over 1,000 acres of predominantly moorland and we had more than 500 sheep!
. My biz partner was an experienced farmer, but gathering that many semi-feral hefted Swaledales on that amount of that terrain was new to her too... well, we managed, but it certainly taxed us!
It was our hoggs that got infested, we never did find out why. Except that the first winter can be very stressful even for such hardy sheep as those, and the bugs take advantage if the sheep are stressed. Subsequent winters we kept the hoggs on easier ground, and gave them a little cake right through their first winter. We never had the problem again.
If it’s scab you have every right to be absolutely livid with the owner of the ram - and the farm he was on last.
Crovect is not effective against scab. Nor is it effective against sucking lice, nor keds. It will control - but not eradicate - chewing lice (aka biting lice), and ticks, but
only if correctly applied for the target parasite, which in my experience, older farmers often don’t do. They use the applicator they like and spray it on the way they like, and it won’t always give adequate protection for the problem you have at that moment. Sigh.
I’m unable to fathom how you’ve managed to Crovect them yesterday but can’t get them penned? How did you Crovect them?
Any treatment for any of these contagious beasties is only as good as your ability to gather and treat
every single sheep, to not allow a treated sheep to come into contact with an untreated one
nor with any fence, wall or tree or hedge an untreated one could have rubbed against in the last week or two, or indeed after you treat your sheep...
So if you share a boundary with another sheep or cattle keeper, you need to ‘fess up and all agree to treat all your animals on the same day, or one of you move your animals to a non-contiguous field.
When we had our traumas with the Swaley hoggs, we used Dectomax first but it didn’t work and in the end Crovect did. So if you can’t get a robust diagnosis, you need two clean fields you can use. One to move them to after the first treatment and the second in case the first treatment doesn’t work!