Has your farm got a worm resistance problem?

Resistance to anthelmintics (wormers) has recently been detected on dairy calf-to-beef farms in Ireland, according to research presented at Teagasc’s National Beef Conference.

Speaking at the event, Teagasc’s Dr. Orla Keane referred to anthelmintic resistance as the “ability of worms to survive a dose that should kill them”.

“Grazing cattle are naturally exposed to gut worms which can cause disease including scour; they can also cause a reduction in feed intake and growth-rates – particularly in young cattle.

“The greatest risk period for infection is during autumn, in the second half of the grazing season,” she added.

The control of such worm burdens has generally been achieved by the administration of broad-spectrum anthelmintics.

There are currently three classes licenced in Ireland for the control of gut worms in cattle:
  • Benzimidazole (white wormer);
  • Levamisole (yellow wormer);
  • Macrocyclic lactones (clear wormer).

On-farm study

Dr. Keane outlined that on all farms tested, ivermectin failed. This indicates that worms were resistant to ivermectin (clear wormer), as the treatment administered failed to reduce the faecal egg count by more than 95%.

Resistance to benzimidazole was found on 71% of farms, while 25% of farms were found to have levamisole (yellow wormer) resistance.

The two species of worm resistant to benzimidazole and ivermectin were identified as being Cooperia and Ostertagia.

In addition, animal performance was effected. A lower average daily gain (ADG) was recorded in calves that received an ineffective dose when compared to those that were successfully treated.

Dr. Keane added that calves don’t eat enough to perform optimally due to a reduced feed intake, as a result of the worm burdens.


Touching on the control of worms, she said: “If you have resistance, you want to stop it getting worse and dosing only when the animals need it will help control the resistance problem. If calves are thriving, they probably don’t need to be dosed.”

In order to reduce resistance, under-performing calves should be dosed with the appropriate product at the correct rate. A faeces sample taken from multiple animals in the herd can be used to identify a worm burden.

“A faecal egg count of greater than 200 eggs/gram may indicate a need to dose,” she said.