How not to increase resistance to wormers on your farm

Anthelmintic resistance has been detected on some Irish cattle farms. It occurs when worms are able to withstand a normally effective dose of an anthelmintic.

At Beef 2016 Teagasc outlined what practices on farms increase the likelihood of anthelmintic resistance developing.

A number of factors can encourage the rate at which AR develops on farm. These include the
following:

1. Excessive frequency of anthelmintic treatment or incorrect volume of dose administered.

Every time a dose is given to an animal there is selection for resistance. is is because the only worms that survive in the animal post-treatment will be resistant ones. As a result, only eggs from resistant worms will be passed in the faeces from the animal for the subsequent few weeks as it takes approximately three weeks from the time infective larvae are picked up from pasture, develop into adult worms, and start producing eggs. erefore, in animals that are treated too frequently (every 3-4 weeks), adult worms susceptible to the dose won’t get a chance to establish themselves and produce eggs.

Under-dosing is another significant factor contributing to the development of resistance as this allows partially resistant worms to survive treatment. Under-dosing can be the result of faulty equipment, poor dosing technique (dosing into the mouth as opposed to over the back of the tongue) or administration of insufficient anthelmintic to the animal relative to its actual live weight.

2. Purchase of cattle carrying resistant worms.

Purchasing cattle carrying resistant worms can result in the introduction of resistant worms onto a farm especially where these animals are not correctly treated and managed on arrival.

3. Size of the refugia-based population.

Refugia refers to that portion of the worm population not exposed to anthelmintic treatment.
This can include unexposed worm eggs and larval stages on pasture, as well as worms in
untreated cattle

The size of the worm population in refugia is also very important and it can be affected by frequent dosing. Another consideration might be prevailing weather conditions, particularly dry conditions which do not aid the survival of free-living larval stages on pasture.

4. Speed at which animals are re-infected after dosing.

The speed at which animals are re-infected after dosing also dictates the rate of resistance development. It is dependent on factors such as the level of pasture contamination that animals experience following dosing, the type of dose given (long- or short-acting) or if the animal is relatively immune to re-infection.

The traditional ‘dose and move’ system where animals are typically dosed and moved straightaway to clean pasture is now regarded as highly selective for AR as the only worms that survive treatment will be resistant ones and these will form the basis of the worm population in the new pasture.