Sheep farmers who haven’t yet incorporated one of the newer group wormer actives into their worm control plans are being urged to do so now according to an industry open letter.

The warning comes amid fears that some farmers are continuing to rely solely on the older group 1, 2, and 3 wormers, which are likely to be leaving enough worms behind after treatment to hamper lamb growth rates.

Repeated use of the same wormer groups will also contribute to a growing level of wormer resistance, which will make the problem worse in future years.

Matt Colston, vet and ruminant technical consultant at Elanco Animal Health, says a recent study showed 77 per cent of farms with resistance to two or more of the older group 1,2 or 3 wormer classes .

“This presents a significant production threat, with resistant worms left behind after treatment able to reduce growth rates in lambs by up to 50 per cent, before any visible signs are present ,” explains Mr Colston.

On top of this, he says there are longer-term implications of growing wormer resistance.

“Wormer resistance is a problem that will already be increasing under the radar on most farms. Once established on a farm, a resistant worm population will not revert back to being susceptible, but if caught early, before there are obvious problems, developing resistance can be effectively managed. This would allow effective use of the older group 1, 2 and 3 wormers for many years to come,” he says.

The latest advice from SCOPS recommends that all sheep farmers use either an orange (4-AD, Zolvix) or purple (5-SI) wormer at two key points: as a one-off mid-late season dose for lambs, and for all incoming stock as part of quarantine treatments.

Mr Colston also reminds those buying in breeding stock or store lambs this autumn to use one of the newer wormer actives.

He added that Elanco’s sheep wormer checker provides advice on which treatments to use and when, to ensure a sustainable worm management plan is in place. Find the tool here: farmanimalhealth.co.uk/sheep/sheep-worms/sheep-wormer-checker