A number of on-farm trials undertaken in recent years have shown a hugely worrying increase in the level of anthelmintic resistance and the rate that it is developing on farms.

In her presentation at the Teagasc lowland sheep conference in Killarney on Tuesday and in Kilkenny on Thursday, Dr Orla Keane spelled it out clearly that unless management practices surrounding the use of wormers changes fast, then sheep farmers are very soon going to be left with very few options to treat their sheep.

Positives first

Orla began with the positive news that no resistance has been detected for nematodirus and, as such, white drenches should continue to be used for this purpose.

Unfortunately, it's a much different story for other gut worms. Detailing a drench test that was carried out on a large number of farms between 2013 and 2015, she said: "The results showed that on 70% of the farms, the white doses (Benzimidazoles) were not working properly. On 48% of the farms the yellow doses (levamisoles) were not working properly and on 36% the clear doses (ivermections) were not working properly."

The situation got even worse when follow-up tests were carried out in 2018 and 2019.

"We followed this up in 2018 and 2019 with a smaller number of farms, but with a more specific, more detailed test. What we found was on every farm we tested the white doses were not working, on 17% of the farms the yellow doses weren’t working and on 56% ivermectins were not working."

In 2013, the resistance to ivermections was just over 30%, so to now get to over 50% is a worrying challenge for the industry.

Stemming the decline

Anthelmintic resistance is explained as the genetic ability of the worm to survive a dose that should kill it. If a dose fails to kill 95% of the worm population present, then there are resistance issues.

This can range from treatments still achieving a relatively good kill and being not far off the 95% mark to, in the worst cases, in the region of only 20% to 30% of the worm burden in an animal killed.

Some of the reasons highlighted as speeding up the rate of resistance establishing include treating animals when they do not need it, overuse of particular anthelmintic classes and under-dosing animals.

Orla highlighted four practices that can be carried out straight away on every farm to reduce the rate of resistance developing. These are all simple cost-effective measures that there should be no issue in adopting. They include:

  • Avoid dosing mature ewes unless there is a demonstrated need.
  • Use a Benzimidazole (white dose) only to treat nematodirus.
  • Implement strong biosecurity measures. Quarantine treat stock coming on to the farm using either of the two new active ingredients.
  • Regularly monitor faecal egg counts to appropriately time treatments.