Drench Resistance

Drench resistance means that the worms inside the animal have developed a resistance to the drench and it is no able to kill the worms. Young animals will fail to thrive in spite of routine drenching or the drench might appear to make an improvement for a very short time only. Drench resistance is extremely common on NZ sheep and cattle properties and is especially common in goats. There are ways to slow the development of drench resistance and with the adoption of grazing management practices and strategic use of drenches, drench resistance can even be reversed. This requires long term commitment.

• Cause

  Combination of a high level of parasites and frequent use of drenches. A population of parasites is no longer able to be killed by one or more drench family. (See GI parasites)
  Resistance can occur in any parasite to any drench family but the most common are haemonchus resistance to ML and BZ in sheep and Cooperia resistance to ML in cattle (all farms in NZ) and Ostertagia resistance to ML in deer.

• Species affected

  All species that are treated with parasite control products including humans and pets

  Resistance can be transferred between sheep and goats but is less likely to be transferred between other species expect for a few parasites.

• What it looks like

  Parasitism, poor production and disease in spite of drenching. This may lead to the assumption that something else is causing the illthrift.

• Diagnosis

  Faecal egg count reduction test is an effective test for sheep, young cattle and goats

  Worm reduction test. Diagnosing drench resistance in deer is difficult due to low levels and unreliable egg output.

  Faecal egg count reduction test
  

  1. Best done in February in lambs
     Identify animals that will be used prior to the test. 15 for each drench family being tested + 15 control animals that will not be drenched.

  2. Collect faecal samples and send in for FEC, be sure to accurately identify each animal to its treatment group or control and label samples with animal ID and treatment group.

  3. Drench with the correct dose rate of each product. Ensure the drench is within its expiry, has been correctly stored and drench guns are delivering accurately

  4. Collect faecal samples 7 and 14 days after drenching. Ensure each sample is labelled with the animal ID and drench group. Submit samples for testing on the day they are collected.

  5. Alternatively just collect samples at 10 days. The advantage of 7 and 14 day samples is identifying early stages of resistance which often results in egg laying suppression for the first week.

  6. Cultures of samples pre and post drenching will be done to identify which worms are most resistant to the drench
     

  Post Drench Check

  A post drench check can give an indication as to whether the drench is effective, however it does not tell you whether resistance is present because you don’t know the pre-drenching levels.
  Collect fresh faecal samples from 10 animals 10 days after drenching and submit for FEC.

• Treatment

  Once drench resistance is established it is difficult to reverse.
  Animal welfare is paramount. If possible use a novel drench *** or drench combination to treat animals that are infected with worms to the level of causing animal welfare risk
  Treat all young animals that will be sent to the works and have growth reduction due to worms with a novel and effective drench and remove them from the property as soon as possible.
  Identify paddocks where drench resistant worms have been deposited. Do not drench while grazing these paddocks or within 28 days of grazing these areas.
  
  ***Do not use the novel drench as a routine drench - this will only promote development of resistance to this novel product.

• Risk factors

  • Routine drenching of all stock. Drenching adult stock, 3-weekly drenching of young stock

  • Using single active products
    Using pour on or long acting products

  • Under-drenching (underestimating live weight or inaccurate drench guns)

  • Ineffective quarantine drenching. Bringing animals on with drench resistance and not adequately killing these resistant worms before the animals enter the grazing platform

  • Drenching and moving onto clean pasture

• Prevention options

  Minimise the use of drenches

  Only drench animals when required based on faecal egg counts, body condition and growth rates, age and time of year.

  The need to drench can be reduced by adopting some of the following practices.

  Reduce exposure of animals to worms

  1. maintain pasture height above 1200kgDM/ha

  2. Use crops such as brassicas, beet, pure swards of lucerne, clover, chicory. Note that mixed grass, clover, herbs may not adequately reduce exposure to worms.

  3. Intergrazing with other species. Eg Sheep and cattle and horses don’t share many worms. The best way to integrate stock is to graze with one species, let the pasture recover and then graze with an alternative species. E.g sheep followed by cattle, deer or horses.

  4. Rotational grazing with long periods (week to months) before coming back to the same pastures

  5. Using alternative feed sources (hay, baleage, grains, palm kernel, tree fodder crops)

  6. Housing animals and cut and carry feeding

  Maximise animals immune development.

  Fully fed stock are better able to develop natural immunity to worms and will require less drenching

  Check mineral status and correct deficiencies, especially copper, selenium, cobalt, iodine and zinc.

  Ensure animals are getting adequate protein for their stage of life

  Ways to reduce the number of drenches

  When reducing the use of drenching, careful monitoring of stock is important. This is especially critical during the autumn and in young animals.

  Extending the drench interval to 4, 6 or 8 weeks between drenches. (Care with haemonchus in lambs and lungworm in deer if doing this). Talk to your vet about the combined risk factors to determine a suitable drenching interval and be prepared to change this based on weather and other risk factors.

  1. Reduce the number of drenches given to young stock - Start later, finish sooner if possible based on risk.

  2. Use a fully effective drench each time.

  3. Late weaning or running ewes and lambs or hinds and fawns on crop pre and post weaning can delay the start of drenching.

  4. Winter crops can reduce the need to drench over winter. Summer crops might include lucerne, chicory, red clover, forage brassica, plantain.

  5. Don’t drench adult stock or spot drench them only.

  6. Breeding for natural immunity to parasites (CARLA rams/bulls/stags) or select low FEC, high growth rate replacement stock.
     
     Maintain a population of susceptible worms (Refugia)
     

  1. Don’t routine drench adult stock.

  2. Use undrenched ewes to follow drenched lambs on pasture

  3. Leave some undrenched animals in the mob

  4. Have an undrenched mob graze the quarantine paddocks and put new animals into this paddock after quarantine drenching

• Further Reading and references

Drench Resistance Checklist

• Arrange to talk with your vet or animal health technician about a plan for monitoring and preventing drench resistance

• Choose from the prevention options and turn these into actions