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Strike a Balance between Roundworm Treatment and Resistance

09 March 2015

UK – Sheep farms planning grazing and weaning strategies to beat the larval challenge this spring should target specific ewes for roundworms, say vets at the National Animal Disease Information Service (NADIS).

This winter has continued to be wet and mild compared to average years, leading to some concern for parasites.

However, rather than treat the whole flock for roundworms, livestock veterinarian Peers Davies, as part of the NADIS parasite outlook, advised farmers to “strike a balance” between treating enough to limit the burden in ewes and leaving as many untreated to keep a susceptible worm population.

“We want to avoid lambs being exposed to high worm larval numbers but we also want to delay the development of resistance as a whole to the population on any farm,” said Mr Davies.

“This means leaving ewes carrying twins or singles if they’re in good body condition, only treating young ewes, thin ewes and ewes carrying triplets as these are the ewes that carry the majority of eggs onto pasture and will benefit from worm treatment.”

Choosing a product depends on the pasture being grazed, added Mr Davies. Many farms have infected pastures, in which case a persistent product, rather than a short acting product is advisable.

Explaining why, Mr Davies said: “It avoids the immediate reinfection of the ewe, thus giving you a longer period when the ewe is not producing eggs and contaminating the pasture.”

But he reiterated that these products increase the risk of resistance development.

Mr Davies recommends product choice and weaning strategy be discussed in detail with veterinary surgeon to arrive at the most appropriate strategy.

This can help decide which pastures can be prioritised and are going to be most suitable for lambs from mid-summer onwards.

Mr Davies underlined the challenge from Nematodirus roundworm, blaming a cold snap of winter for priming the eggs to hatch over a short period of time this spring, meaning a short hatching period.

“We can predict to a degree the timing and severity of this peak, based on weather conditions and provide a detailed regional forecast in the next parasite forecast,” said Mr Davies.

“This will allow farmers to time their anthelmintic drenches more appropriately, particularly the most appropriate drench.”

He said larval challenge is best avoided by grazing young lambs and mothers on different pasture to last year if possible, giving last year’s eggs passed a chance to die.

However, if drenching for Nematodirus, he suggested Benzamidazole drenches as the most appropriate.

Photo courtesy of NADIS

Michael Priestley

Michael Priestley
News Team - Editor

Mainly production and market stories on ruminants sector. Works closely with sustainability consultants at FAI Farms.

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