If the lambs are younger than 3 weeks and they have diarrhoea than cannot be cocci, but likely an infection (due to lack of colostrum). Cocci have an incubation time of three weeks, so you are most likely to see it in lambs 5 weeks and older, if there is a history of them in the specific field they are in. If bad, then you can have recurrence in just over three weeks time. In goat kids at least, there is some resistance to Veccoxan, so I have been using Baycox (in their milk as they are bottled). I have never had to treat my lambs for it.
If you suspect cocci, getting a sample tested is not much use, as only a few specific species of Coccidia cause disease, and lambs (and goat kids) can have really high counts without any disease. So quick treatment if you suspect coccidiosis is often best, as they can go down with it very quickly.
If you have small numbers of lambs/kids to treat, Baycox is the cheaper option, as it lasts for years...