Resistance by sheep worms to drenches in WA continues to increase. There are sheep worms on virtually all farms that are resistant to white (benzimidazole or BZ) and clear (levamisole or LV) drenches and BZ/LV combination drenches (containing a white and clear drench) only remain fully effective on less than 20% of properties. More than 80% of properties also have brown stomach worms (Teladorsagia (Ostertagia)) resistant to the macrocyclic lactone group of drenches, the MLs (active ingredients include ivermectin, abamectin and moxidectin).
Although the recently released new drench type (monepantel) will be very useful, it is essential that we maintain the effectiveness of all drench groups and prevent the development of resistance to new types for as long as possible. One strategy that farmers can use to help reduce the risk of introducing new strains of resistant worms on their property is effective quarantine drenching of introduced sheep.
To prevent the introduction of new strains of resistant worms onto a farm, purchased sheep or those returning from agistment should be given a quarantine treatment before loading or on arrival. The current minimum recommendation is to treat with three or four different drench types, including monepantel (Zolvix). The sheep should then be released into a wormy paddock to dilute any remaining 'super-resistant' worms.
What should I do?
All introduced sheep should be treated immediately before or after they enter the property.
Treatment with products from four different drench groups is recommended. If the drench resistance status of the source of the sheep is unknown the current minimum recommendation is:
- Monepantel (Zolvix) plus abamectin (an 'ML', or macrocyclic lactone), white drench (benzimidazole) and a 'clear' drench (levamisole).
Giving these three types is most easily done by using a triple combination drench (the ML should be abamectin), such as Hat-Trick, Pyrimide, Q–Drench, Triguard or Napfix. (The drenches can be given individually and abamectin could be substituted by moxidectin — Cydectin, Moximax, Moxitak, SheepGuard).
Zolvix (monepantel) is an important choice as this product, from the latest amino-acetonitrile derivatives (AAD) group of anthelmintics, is the only option guaranteed to have no resistance in WA. Giving this product at the same time as other products, including one of the most potent macrocyclic lactones, will minimise the risk of any resistant worms making it on to the property.
Drenched sheep should be released into a wormy paddock to help dilute any surviving 'super-resistant' worms amongst the resident population of worms already on the property. If a wormy paddock is not available then other options should be discussed with your local vet or sheep adviser.
For more information talk to your local vet or sheep adviser or contact your local WormWise contact at the department.
General drenching tips
Correct drenching technique is essential to ensure drenches are effective:
- Under-dosing is a major cause of drench resistance so it is critical that sheep are given the correct dose. A few of the larger animals should be weighed so that the dose for the heaviest sheep in the flock can be calculated. For drench groups with a greater risk of toxicity (especially naphthalophos), flocks with a wide range in liveweight should be drafted into smaller, more uniform groups and each group given its appropriate dose to prevent overdosing.
- Check drench guns regularly to ensure they deliver the correct dose.
- Place the drench gun over the sheep’s tongue during treatment, rather than in the front of the mouth. This helps to ensure that all of the drench is delivered to the rumen for maximum effect.
- To help improve the effectiveness of the white, ML and closantel drenches, feed can be withheld from sheep for up to 24 hours before treatment. This slows gut content flow through the sheep, keeping the drench in the gut for a greater time and increasing its uptake. Sheep should not be fasted before treatments with levamisole or naphthalophos as this could increase the risk of toxicity with these products. (Note: sheep kept off feed should have access to water.)
Faecal worm egg count monitoring
Faecal WECs are the most effective way of checking the burden of worms in sheep. Between 10 (minimum) and 20 (ideal) individual faecal samples from each mob to be checked should be submitted. Contact the WEC service provider before sample collection to obtain specific sample submission requirements. WEC monitoring can be used to confirm a parasitic problem, to indicate whether drenching is needed or not, whether drenching has been effective and also the level of parasite contamination being deposited in a paddock.
Breeding worm resistant sheep
Resistance to worms in sheep is heritable and breeding worm resistant sheep is one of the best long-term solutions to assist worm control and combat drench resistance.
As with all genetic programs, selecting within a flock can take several years to achieve obvious improvements. However, growing numbers of farmers, including ram breeders, have been selecting sheep for worm resistance for some time and have achieved good results.
Paddock and grazing management
Paddock and grazing management can assist worm control by limiting worm contamination on paddocks and ensuring good sheep nutrition to maximise immunity to worms.
‘Clean’ pastures, that do not harbour a significant number of infective worm larvae, are a valuable asset in a worm control program. Reduced exposure of sheep to worms on paddocks can reduce the pick-up of worm larvae and therefore worm burdens. This can reduce the impact of worms on the sheep and reduce the frequency of treatments needed.
In general, worm larvae survive on dry pasture in summer for less than one month and four months or more on green pastures after the season’s break. The following list describes contamination levels of paddocks, starting with the cleanest (least contaminated):
- stubbles or crops
- pasture that has been de-stocked (for more than four months in winter or one month in summer)
- pasture grazed only by cattle
- pasture grazed only by adult dry sheep.
Paddocks grazed by pregnant or lactating ewes or by lambs or weaners are often highly contaminated. Problem paddocks should be stocked with animals with better immunity to worms such as older dry sheep or cattle (few worm species will establish in both sheep and cattle) to minimise the impact of the contamination.
Sheep develop a natural immunity to worms after they have been exposed to larvae for some months and are usually relatively immune by the time they reach about 12 months of age.
Good immunity to worms results in fewer larvae establishing to develop into adult worms, adult worms being rejected and passed out of the sheep and adult worms producing fewer eggs.
Scour worm immunity may fluctuate but it remains throughout the animal’s life, whereas immunity to barbers pole worm is lost several months after exposure to worms or larvae ceases. Immunity of ewes is reduced for up to two months after lambing, resulting in an increased egg output and more severe effects of worms on the ewes. Lambing paddocks can therefore become heavily contaminated with worm larvae and lambs should always be moved from the lambing paddock after weaning.
Good general sheep nutrition is an important component of an overall worm control program as sheep in poor condition have a reduced ability to deal with the added stresses of a worm burden. To minimise worm impact, weaners should be well grown and reach liveweights of at least 25kg by the beginning of their first summer. Older sheep should be maintained above condition score two. Mineral and trace element deficiencies can also add to the stresses on sheep and local advice on the need for supplements is recommended.