Outbreaks of nematodirus on sheep farms in south-west England and the Scottish Borders have prompted a warning to lamb producers against adopting a “wait-and-see” policy on pasture management and drenching.
The advice comes amid rising spring temperatures and the consequential risk of a mass hatching of overwintered parasites.
With two confirmed cases of nematodirus in very different parts of the UK, the Animal and Plant Health Agency (Apha) expects cases to increase in the coming week as temperatures rise.
Nematodirus, caused by the Nematodirus battus worm, can result in a high number of lamb deaths and is responsible for stunting the growth of many others.
The Sustainable Control of Parasites in Sheep (Scops) usually provides a nematodirus forecast to the UK sheep industry, but technical difficulties relating to the availability of Met Office data mean this is not currently available.
Scops said it was working hard to find a solution, but was unsure when the service will be up and running again.
But in response to the recent nematodirus outbreaks, it issued a warning that rapid action is needed to avoid infestations.
Scops sheep consultant Lesley Stubbings said because nematodirus can strike very quickly, farmers “can’t afford to have a ‘wait-and-see’ policy”.
Faecal egg counts are not a reliable indicator of risk, she added, because the damage is done by large numbers of immature larvae that are not producing eggs.
The main risk group is 2022-born lambs in fields where lambs grazed last spring.
Scops said lambs typically start eating significant amounts of grass at six to 12 weeks of age, although they may start at a younger age if ewes are under stress and not milking well.
The recommendation is to move at-risk lambs to low-risk pasture – grass that was not grazed by lambs last spring.
The alternative is to treat with an appropriate anthelmintic, in most cases with a white 1-BZ drench, but farmers are advised to speak to a vet or adviser and to take follow-up steps to ensure any treatment given is effective.
While roundworms that are resistant to white drenches are widespread within the UK, these wormers are still highly effective for nematodirus, said Ms Stubbings.
“But remember, it may be necessary to treat lambs more than once depending on the spread of ages in a group and subsequent weather conditions,” she said.
The lifecycle of Nematodirus battus differs from that of other parasitic worms because the development to an infective larvae takes place inside the egg, and infection passes from one year’s lamb crop to the next.
The eggs must undergo a period of cold weather, followed by temperatures of 10C or more, before they can hatch.
If these conditions occur over a short period of time, triggering a mass hatch, and it coincides with the time when lambs are starting to take in significant amounts of grass, the result can be devastating.