Drench Resistance

What is Anthelmintic (Drench) Resistance?

Drench resistance is present when previously susceptible worm populations in an animal
survive after a correctly applied dose of drench has been used.

The resistant worms survive and carry on to breed, passing their resistant genes to their

So over time the resistant worms make up an increasing proportion of the worm population
on the farm.

Worms can be resistant to one or more drench (action) family.

Should farmers be concerned?

  • Farmers need to balance the risk of drench resistance and yet manage worms so that production and animal welfare is not compromised.
  • A short term production gain may result in the longer term cost of drench resistance.

Prevalence of drench resistant worms in New Zealand
The New Zealand Veterinary Journal December 2006 issue presented the results of two drench resistance surveys, one for sheep and one for beef cattle. See Resources

Parasite management plan

  • Have an animal health plan for your farm
  • Provide good nutrition
  • Manage the level of larval challenge
  • Construct a worm management plan – include refugia and quarantine for new stock

In order to optimise animal production in the face of parasitism, each farmer should have a parasite management plan, as part of an overall animal health plan.

The parasite management plan, and its application, will require a thorough knowledge of the parasites present on the farm, their biology, and methods that can be used to manage them. Remember that each farm is different and plans need to be farm-specific.

The drenching component of the plan should strive to minimise both the impact of worms on production and selection for drench resistant worms.

All factors that impact on parasite management and animal production are inter-related and need to be considered together when developing and implementing the plan.

Don’t be afraid to ask for advice.


  • Poor nutrition or under feeding increases susceptibility to internal parasites.
  • All classes of stock should be provided with sufficient feed to enable them to attain specified targets.
  • Forward planning is essential; feed budgeting, regular monitoring of live-weights and body condition should be a routine operation.

Manage larval challenge

  • Production benefits arise from lower levels of challenge in young stock in particular.
  • Plan to create low levels of larval challenge – use stock movement, grazing history, forage, weather, and stock type and classes.
  • Rotate the type and all stock classes during the course of a year so that the worm population on the farm is continually mixed up.
  • Use stock class mapping as part of the planning process. This will enable you to identify where a challenge is coming from and anticipate how to deal with it.


  • You should always know which drenches are effective on your farm. Drench testing should be carried out regularly, with frequency depending on drench resistance risk factors on your farm – and your drench chosen on the basis of the results.
  • A FECRT test should be done to establish a base line and then to monitor resistance status of the farm.
  • The available evidence suggests that combination drenches have the ability to slow the development of a drench resistance problem on a property when used prior to resistance developing.
  • Good drenching practice is essential. Animals should all receive optimal doses of the most efficacious drench of the chosen action-family/families – body weights need to be known and you need to be confident, by regular checking, that your drenching gun is delivering the correct dose.
  • Drenching strategies will need to be tailored to suit individual farms and stock classes, so seek advice.

Drenching adult stock

  • The aim should be to achieve a balance between maximising animal production and minimising selection pressure on the worm population. An adult ewe or cow in good condition that is on satisfactory feed does not need to be routinely drenched. Drenching should be based on feed levels, age of animals, pregnancy status, parasite burdens, and other factors.
  • Hoggets and two tooths often need to be treated differently to mature ewes.


Refugia involves ensuring that some drench susceptible worms are maintained in the worm population to breed.

The aim is to allow enough susceptible worms to dilute the resistant ones without significantly compromising animal productivity.

Ways to generate refugia

  • By not drenching all the animals in a mob every time a drench is used.
  • Put un-drenched adults on pasture previously grazed by drenched young stock.
  • Drench the mob and return to same infective pasture for a week or two before they go onto ‘clean’ pasture.
  • Mix adult (un-drenched) stock with young stock.
  • Drench intervals be kept to 28 days or more.

Recent work from Scotland has shown a production Targeted Selective Treatment (TST) and a targeted drenching programme were able to reduce drenching frequency and maintain levels of production, while slowing down the development of anthelmintic resistance.

Quarantine procedure when introducing new stock to the farm

New stock coming onto a farm may bring drench resistant worms.

A quarantine policy must be part of any worm management plan.

The quarantine policy should be applied to all animals entering the farm no matter how long they are staying.

  • Unless there is evidence to the contrary assume bought in stock have worms.
  • On arrival at the farm, drench stock with the most effective drench (consult your veterinarian) and hold them in a quarantine area for 24 hours. Do not put them on “clean” pasture but on pasture that is likely to have susceptible infective larvae on it.
  • Ten days after arrival of the stock check via faecal egg-counts, that the drenching was effective.



Read more about quarantine treatment.

Using stock that are resistant to worms or resilient in the face of challenge

Sheep breeders have made progress with selection for resistance or resilience which give genetic options for the long term management of parasites. There are breeding indices for these traits to help with individual ram selection. https://www.sil.co.nz/

Other practices to reduce or delay worm resistance to drenches

Avoid using an ineffective drench.

Weigh animals so that they are not under dosed.

Avoid drenching young stock onto ‘clean’ pasture/low challenge grazing unless you have a plan to introduce/maintain refugia.

Avoid drenching more frequently than every 28 days unless there is a special need. i.e. animal welfare.

For sheep, avoid whole flock treatment pre-lambing with a long-acting anthelmintic product.

Use combination drenches even if drench resistance has not been identified on your farm.

Avoid using long-acting formulations that can result in an extended period of sub-lethal dosing.

Have a quarantine plan for new animals.