Drench Resistance

What is Anthelmintic (Drench) Resistance?

Drench resistance occurs when 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

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?

  • The prevalence of drench-resistant worms in New Zealand appears to be on the increase.
  • Veterinarians from around New Zealand report that they are seeing an increase in the number of cases of combination resistance, particularly in sheep, but in addition, there are cases of combination resistance in cattle worms (D. Leathwick pers.com)
  • The last published report of New Zealand laboratory data from Gribbles Veterinary Pathology showed that 34% of farms had at least one parasite resistant to the standard Benzimidazole/Levamisole combination, and 11% of farms had resistance to triple combinations. See Update on anthelmintic resistance.
  • 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.

Managing drench resistance on farms

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.

  • 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

Have a plan

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

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

The drenching component of the plan should strive to minimise both the impact of worms on production and the 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 – utilising stock movement, grazing history, forage, weather, and stock type and classes.
  • Rotate the species and age of stock classes during the course of a year so that the worm population on the farm is continually mixed up. Utilise highly immune adult stock and alternative animal species to ‘clean up’ pasture larvae behind young stock.
  • Use stock class mapping as part of the planning process. This will help you to identify where a worm challenge is coming from and plan 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 Faecal Egg Count Reduction Test (FECRT) should be done to establish a base line and then to monitor the resistance status of the farm. Regular ‘drench checks’ can be undertaken in between to verify that the products in use during a season continue to be effective.
  • The available evidence suggests that combination drenches are able to slow the development of a drench resistance problem on a property – when used prior to resistance developing and in conjunction with other known risk mitigation tools.
  • Good drenching practice is essential. Animals should all receive optimal doses of the most efficacious drench of the chosen action-family/families – body weights should 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 decisions should be based on feed levels, body condition, age of animals, pregnancy status and parasite monitoring data (where possible).
  • Hoggets and two tooths often need to be monitored and treated differently to mature ewes. Heifers calving at two years old may need different parasite management from adult cows on some farms.


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 with R1 cattle in New Zealand, as well as work with lambs in Scotland and Ireland, has shown that Targeted Selective Treatment (TST) drenching programmes can be a way of reducing total drench inputs and maintaining levels of production, while slowing down the development of anthelmintic resistance. More work is required on refining these programmes for New Zealand grazing systems.

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 the 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.

Drenching practices to reduce or delay worm resistance to anthelmintics

  • 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 young stock 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.