Care should be taken when using wormers and flukicides containing the active ingredient closantel, following reports of overdosing which can cause permanent blindness.

The warning comes from the Sustainable Control of Parasites in Sheep (SCOPS) following incidents across the UK in recent months.

Closantel, an important active in the control of internal parasites, is contained in Supaverm and Flukiver, Solantel, Closamectin and Closiver.

Speaking on behalf of SCOPS, independent sheep adviser Lesley Stubbings says animals should be dosed with closantel according to their weight.

“The safe use of animal medicines relies on all the instructions for use being followed as per the label – and we know a key issue is ensuring animals receive the correct dose."

She said that sheep farmers should dose with a wormer or flukicide to the heaviest in the group – with the advice having to be carefully adhered to when using closantel risk.

“Because of the risks posed by overdosing, SCOPS urges sheep farmers to split a group if the weight variation is large, grouping sheep into weight bands and altering the dose rate accordingly.

"That is safer than dosing to the heaviest animal and administering too much closantel to small animals.”

SCOPS also encourages sheep producers to report any adverse reaction to closantel in treated animals to the Veterinary Medicines Directorate.

There is a full list of products that can used to treat internal sheep parasites on the SCOPS website, which highlights the active for each. The website also provides links to the VMD product database and adverse reactions reporting process.

“This isn’t a recommendation to avoid the product – just a reminder of the risks of not weighing animals and giving them too much," said Ms Stubbings, who along with Dr Philip Skuce, of the Moredun Research Institute was also speaking at the first of this year's National Sheep Association (NSA) and Moredun webinars.

Polls conducted during the webinars found that producers can balance effective parasite control with environmental impact.

Ms Stubbings said: “Not only is good performance key to profitable sheep production, but also in minimising our environmental impact. The important message is that we can maintain and, in many cases, improve animal performance by following sustainable practices that reduce our reliance on parasiticides.”

Using data as evidence, the speakers were able to show a balance is achievable with reduced use of parasiticide, without compromising lamb growth. This evidence is central to both slowing the development of anthelmintic resistance and the impact of these medicines on the environment, while improving animal health and maintaining performance is vital to reducing greenhouse gas emissions. With almost 50% of the audience being farmers, the webinar demonstrated a clear understanding of the impacts and interest in improving worming practices in the farming community.

Dr Skuce added: “It was very encouraging to see the results of the online polls throughout the webinar, with farmers already adopting the ‘as little as possible, but as much as necessary’ approach, together with other non-chemical methods”.

Another poll taken suggested the top three options for achieving sustainable parasite control in sheep were grazing management, testing and monitoring and targeting animals. Most people felt the industry was very reliant on parasiticides in sheep but a unanimous vote showed there is huge scope to reduce chemical use while maintaining (or even improving) animal health and performance.

Phil Stocker, NSA chief executive added: “With the launch of the DEFRA Animal Health and Welfare Pathway that includes a compulsory worming treatment check within six months of applying, it shows policy makers are also keen to tackle anthelmintic resistance and the challenges it brings to the UK sheep sector. It’s increasingly important that responsibility in anthelmintic use is taken to the highest level, with other strategies employed such as genetic selection and grazing management used alongside strategic worming.”

The recording is available on the Moredun website under resources www.moredun.org.uk/resources/webinars or you can view on the NSA website at www.nationalsheep.org.uk/webinars.