Worm dosing on sheep farms only effective half the time

Worm dosing on Irish sheep farms was found to be ineffective almost half the time in 2014.

New figures from the Sheep Adoption and Technology Programme (STAP) in 2014 has shown that in 48% of cases the dosing was not effective, according to Teagasc’s Dr Orla Keane who was detailing the figures at Sheep 2015 in Athenry.

Among the tasks sheep farmers can undertake in STAP is to test the efficacy of the drenching with a faecal sample test.

With the option farmers test the commonly used drenches in Ireland (white, yellow and clear).

Farmers take a sample just before they dosed their lambs. Then re-test 7 – 14 days later depending on the type of drench.

Dr Keane says the idea is that on the second sample there should be no eggs present and the worms should all have been killed.

However, this was not the case on many of the farmers who carried out the test last year.

“When we looked at the data from the STAP in 2014 we found that 48% of treatments failed. ie there was eggs post dosing, she said.

Dr Keane also noted that there was a significant difference in the three classes of drugs.

She said the white drenches (Benzimidazoles) where the least effective at only 29% while the yellow drenches the (Levamisoles) they were effective in 56% of cases. The ivermectin or the clear drenches were effective were the most effective in 76% of cases.

“But in all cases there was evidence that the drenching didn’t work last year.

“This is quite concerning,” she said.

While Dr Keane pointed out that the drench test is not diagnostic for anthelmintic resistance she said that it is indicative.

“We do believe for a majority of reasons that these drench failures do represent anthelmintic resistance,” she said.

According to Dr Keane a variety of factors give rise to resistance.

“To some extent there will always exist a small number of worms in the population that are naturally resistant.

“What you want to do is to try and avoid selecting for those,” she said.

According to Dr Keane inappropriate dosing is a big factor in the increase in resistance. She said factors such as not calibrating the dosing gun, under dosing, dosing too often, dosing unnecessary will all give rise to anthelmintic resistance.

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