We've just been through this very thing- Weve now got it under control-
Closamectin injections- 2ml for lambs on day 1- also controls fluke. After 7 days inject again with 2ml Ivermectin. DO NOT use closamectin second time round- use Ivermectin only as there is no fluke guard in it. Its also an idea to treat with a pour on as you might find an infestation of maggots with flies being attracted t othe scabs and sores on the sheep- especially around the bum!!
Hope this helps.
Declan