Worm threat is growing
Sheep 2002 saw persistent rain. But unable to get on
with fieldwork, visitors flocked to the event held
in Malvern, Worcs. Marianne Curtis and Wales
correspondent Robert Davies were there to bring you
the latest technical innovations and advice
WORMER resistance is a serious issue and unless sheep producers take sensible precautions now it will increase and threaten economic production, warns Neil Sargison of Edinburghs Royal Dick Vet School large animal practice.
Addressing a packed seminar at Sheep 2002, Mr Sargison said: "Over the last two to three years I have seen increasing numbers of clinical worm outbreaks and lambs failing to thrive despite regular use of wormers."
Resistance to white drenches had been found in 80% of flocks checked within the schools large animal practice, he said. "This represents a substantial increase over the last 10 years."
Mr Sargison told producers of a flock where resistance to all three wormer groups – benzimidazoles, levamisoles and ivermectins – had been confirmed.
"Economic sheep production will be difficult to sustain on this farm. Although uncommon, this case shows resistance to all three wormer groups is a definite possibility in UKflocks."
But Mr Sargison was able to recommend strategies for reducing the likelihood of wormer resistance. "Dose to the weight of the heaviest animal in the group. I examined faecal samples from a group of Suffolk shearling ewes which had been dosed assuming a weight of 100kg. The average weight was 110kg and the heaviest 130kg.
"Although the flock turned out not to be resistant to the wormer used, worm eggs were present in faeces 10 days after drenching, indicating ewes had been underdosed. Underdosing selects for resistance."
Clean grazing could also select for resistant worms when used inappropriately, he warned. "When sheep are wormed and placed on dirty pasture, resistant worms will only make up a small proportion of the total worm population.
"However, when worming sheep on to clean pasture, they will form 100%. When worming on to clean pasture ensure the wormer is 100% effective. Moxidectin or a full dose combination of two wormer groups is the closest we have."
Moxidectin should also be the wormer of choice for worming ewes after lambing and for bought in ewes before mixing with the main flock, said Mr Sargison.
"Ewes should be wormed after lambing rather than at housing. This prevents them picking up over-wintered larvae when they begin grazing, multiplying them and increasing pasture contamination for lambs." *