Late winter, spring, early summer
During these seasons, immature fluke are less likely to be present.
You can use any flukicide treatments at these times – so try not to use triclabendazole. Save it for late summer/autumn/early winter when it is truly needed, to avoid resistance developing.
Instead, in most cases, two doses of nitroxynil 7-8 weeks apart or two doses of closantel 6 weeks apart will be effective, so this is recommended instead.
You should discuss the origin of the new sheep with your vet to decide which would be the most appropriate quarantine treatments.