Sheep Worms 2016

Gribbles lab have done an update on anthelmintic resistance in sheep. They discovered that we have more of a problem in the north island than the south island (perhaps due to more frequent drenching or more intensive grazing?).
Overall, using a resistance definition of a reduction in egg count of less than 95% they found that:
  • 47% of parasites were resistant to benzimidazole
  • 45% resistant to levamisole
  • 56% resistant to ivermectin
  • 0 resistance to monepantel
  • 21% resistance to benzimidazole/levamisole combi
  • 6% resistance to triple combination
Bear in mind you may not notice the difference between a drench that is 100% effective and one at 90%  in the field, but the latter would be classed as a resistance. The Wormwise program is a great resource for reducing the onset of resistance. Some tips are:
  • Do faecal egg counts to see if your stock actually need worming. The less frequently you use a drench, the less risk of resistance. Rich grass can cause nutritional scours.
  • Co-grazing with other species helps reduce worm burden and minimize the need for drenching
  • Run older animals after youngstock for the same reason
  • Refugia – consider not drenching the top 10% of animals; their worms will not be exposed to drench and will compete with any potential resistant worms shed by drenched animals.
  • If concerned about resistance, contact us to organize a trial assessing pre-and post-drench worm levels.
  • Be accurate with dose rates – weigh some representative animals and drench to the heaviest, and double-check your drench gun is delivering an accurate dose by squirting 10 doses into a measuring jug.


The animal health question at the forefront of many people’s minds is “What product should I use to drench for worms?”
With the current public concern over worms developing resistance to drench, this should probably be re-phrased as “How should I approach a worm control strategy?”
“Anthelmintic resistance is now common and widespread. It has been reported in all of the main sheep worms…to at least one and increasingly to all three…drench groups” (Dr. P Mckenna, Gribbles pathologist, Vetscript November 2005).
Signs of a worm infestation include scouring (mucky tails and hocks), weight loss, poor coat, and production drop. The main problem worms are ostertagia, trichostrongylus, lungworm, cooperia, and additionally in sheep, barber’s pole (the lovely latin name haemonchus contortus) and nematodirus. They all follow the same basic life cycle: infectious larvae are eaten off pasture, develop into adults in the gastro-intestinal tract, and produce eggs which are excreted in the faeces back onto the grass. This normally takes about three weeks. The eggs then develop through the larval stages over two to three weeks depending on conditions.
Because of this cycle, reducing worm burden in the animal will reduce contamination of the pasture with eggs and vice versa.

mucky tails
Cattle and sheep naturally develop immunity to worms as they mature, but young animals are particularly susceptible to infestation, and even adults can be overcome if they are challenged by a really high worm burden. Generally, adult animals need less drenching than youngstock. However, ewes’ worm resistance declines over lambing, so it is often advisable to drench them around lambing time. They often also need treating in late summer to autumn, because at this period there is a peak in pasture larva levels. Some trials have shown increased milk production after treating dairy cattle at calving with a particular product. Stress, disease and poor nutrition will also knock an animal’s resistance, and affect their need for worming
Because most youngstock are going onto contaminated pasture, regular drenching will often be a necessity. The first drench is commonly given about three weeks after grass consumption begins, because of the three week life cycle. Thereafter, the frequency of drenching depends on the worm burden.
Oral drenches have no residual activity, so these may need to be administered monthly for the first year. Pour-on and injectable formulations generally have a residual effect, so application can be given at extended intervals (e.g. every six to eight weeks). Pour-ons are not available for sheep. Controlled release capsules for sheep are administered orally, and sit in the rumen to give a prolonged effect.
There are three groups of drenches: benzimadoles, levamisoles and abermectins. Development of resistance to one drench will often extend to several drenches of that group. Combination drenches are now available, but multiple resistance has also been documented!
In a given worm population, there will be some that are genetically resistant to a drench type. Application of the drench will kill the worms except for the resistant worms, so there will be more worms that are resistant in the next generation. Evolutionary selection via drenching.
How can we minimize this? We can identify a resistance problem by doing faecal egg counts (FEC) before and 10 days after treatment. A reduction in FEC of < 95% suggests resistance, or inadequate drenching. Larval cultures can also tell us which genera of worm are resistant, and to which anthelmintic, allowing us to choose the best product.
The more haphazardly drenches are used, the greater the selection pressure for resistant worms. Use them appropriately:
Using a drench group to which there is no resistance should reduce the number of worms that are resistant to other drenches, so changing your drench group every year may help.
Take multiple faecal samples to see if there is a need to drench.
Ensure you are giving a full dose of drench. Underdosing can allow the survival of partially resistant worms which then increase in proportion of the population. Weigh the animals, and dose to the heaviest in the mob. Calibrate your applicator gun to ensure accuracy (squirt a dose into a bowl and measure it in a syringe).
Non-chemical methods of reducing pasture burden are an integral part of a worming strategy. These include:
– Co-grazing or rotating different species. Most sheep worms do not survive in cattle or horses.
– Follow the youngstock with older animals. These have a better immunity to worms.
– Rotation of grazing will minimize paddock contamination, but it is unlikely to reduce the pasture burden as larvae can survive for many months.
– Lower stocking rate.
– Move onto clean pasture after drenching.
– Harrowing exposes the larvae to dessication by the sun during a long, hot period.
– Isolate new animals until a FEC shows they are not a source of resistant worms.
– Avoid grazing youngstock on high burden paddocks, e.g. those grazed by youngstock the previous season.
– Sow legumes (e.g. clover) – these limit the vertical migration of larvae, and thus their ingestion by grazing animals.
In the future, we may utilize such technologies as vaccines, biocontrol (e.g. fungi which eat larvae), biological anthelmintics (e.g. enzymes or antibodies that act on the worms), immunomodulation (to reduce the damage caused by the animal’s immune response), molecular biology, and breeding sheep lines with resistance (decreased FEC) or resilience (increased productivity despite worm burden).
Our vets can help you to formulate a strategy appropriate to your enterprise.


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