Controlling Lice on Sheep

Controlling worms in sheep


Gastrointestinal parasites occur in all sheep rearing areas of Australia, with the prevalence and severity of infections greatest in higher rainfall areas (>450 mm per year). The primary species of concern are Barber’s pole worm (Haemonchus contortus), Black scour worm (Trichostrongylus spp.) and Small brown stomach worm (Teladorsagia [Ostertagia] spp.). Secondary species include Thin necked intestinal worm (Nematodirus), Small intestinal worm (Cooperia), Large mouthed bowel worm (Chabertia) and Nodule worm/large bowel worm (Oesophagostomum). 

Young sheep or those under physiological stress (e.g. pregnant and lactating ewes) are most susceptible to infection. Left untreated, clinical infections in young sheep can cause death. Even sub-clinical infections can significantly reduce liveweight gains and wool growth in both young and adult sheep via reduced food intake, poor nutrient utilisation and redistribution of protein for tissue repair. Some production losses occur even when relatively immune sheep are exposed to infection.1 

Economic impact 

A Meat & Livestock report published in 2015 found gastrointestinal parasites cost the Australian sheep and wool industry nearly $436 million in lost production and treatment costs each year.2 This report found that worm burdens in sheep cost an average of $28.29/head in high rainfall regions and $12.34/head in the sheep/wheat zone.2 Record prices for lamb and sheep and strong prices for wool means the current cost of gastrointestinal parasites is probably much higher, even allowing for a reduction in the size of the national flock.

Life cycle

Worm species pass through three stages: 

Host stage: Consumed infective L3 larvae mature into adults and reproduce. Egg laying commences about 18 days after ingestion. Mature worms can live within the sheep’s gut for many months. The sheep’s immune system can expel worms or suppress egg laying.1 

Dung stage: Dung containing worm eggs is passed from the sheep to the pasture. Eggs develop through the L1, L2 and L3 stages. This process takes between 4 and 10 days, depending on temperature. L1 and L2 larvae feed on bacteria in the dung.1  

Pasture stage: Infective L3 larvae wriggle out of the dung onto the ground and pasture, where they are consumed by sheep during grazing. Infective L3 larvae are reasonably tolerant of temperature. L3 larvae do not feed and uneaten larvae typically die three to six months after hatching once their energy reserves are used up.1 

Controlling worms 

Most sheep and lamb producers implement some form of worm control using anthelmintics (i.e. ‘drenches’) in combination with grazing management and genetic selection.1   

Producers in higher rainfall regions, where worm pressure and resistance are more prevalent, are more likely to implement strategic worm control programs incorporating: 

  • strategic timing of treatments (e.g. weaning, summer, pre-lambing, quarantine) 
  • rotating between drenches with different modes of action 
  • the use of combination drenches wherever possible  
  • the use of long-acting single active drenches in combination with ‘primer’ or ‘exit’ treatments 
  • monitoring drench efficacy using worm egg counts (pre- and post-drench) 
  • monitoring drench resistance using faecal egg count reduction tests (FECRT), including larval differentiation. 

Weaning drench 

Lambs at weaning are particularly susceptible to parasites and disease. Their immune system is still developing. They have little natural immunity to worms and the stress associated with the weaning process can make them even more vulnerable. This can be further compromised by nutritional stress, either through transition to a new diet or declining feed quality.3 

Worm burdens can cause significant reduction in feed intake4, growth rates and the time taken to reach market specifications.5 In turn, worm burdens can also decrease wool production6, and carcase dressing percentage and meat yield in lambs.7 

In moderate to high rainfall regions, lambs should always be drenched at weaning and then again at the first summer drench. For those born later in the season, the weaning drench may coincide with the first summer drench. 

First summer drench

The first summer drench is the cornerstone of strategic worm control in south-eastern Australia. Administered when pastures are drying off between mid-October and December, it helps to minimise pasture contamination, the carry-over of worm populations over summer and create ‘low risk’ pastures for the next lambing.8 Effective worm control in ewes during this time can also optimise post-weaning recovery.  

In Western Australia, young stock typically receive a summer drench, while ewes are recommended to be drenched in autumn (often described as ‘summer-autumn’ drenching). 

The timing of the first summer drench will vary depending on the local conditions. A second summer drench should only be given if required. Worm egg count monitoring should be undertaken from about six weeks after the first summer drench. Administer a second summer drench, if necessary, in accordance with regional drench decision guidelines.9 

Pre-lambing drench 

Effective worm control at lambing is a critical component of farm management. Low worm burdens can quickly escalate as the ewe’s natural immunity drops. Pasture contamination can rapidly increase and both ewes and lambs can be dramatically compromised in terms of lactation, growth rates and clinical disease. Reaching for the drench gun at marking is often too late to reduce the impact on production and profitability. Monitoring and preparation before lambing is the key. 

 If drenching is required, a short-acting drench known to be effective on your property is recommended – ideally a drench (or combination of drenches) that will reduce WEC by at least 98%.10 

 Long-acting treatments should be used in combination with 'primer' and 'exit' drenches at the start and end of the period of protection offered by the LA treatment.9 This will remove worms that survived the initial treatment or during the ‘tail’ period as drench activity declined – and stop ongoing pasture contamination with worm eggs that will give rise to an increasingly resistant population. The ‘primer’ or ‘exit’ drench should be a short-acting drench from a different chemical class to that of the LA treatment. 

Monitoring WECs during the payout period of the LA treatment is routinely recommended to check for effectiveness.9 An early exit drench is recommended if WEC exceeds 100 epg at any time during the claimed protection period of the LA product.9 

Quarantine drenching

The introduction of any new sheep to a property risks the introduction of resistant worms. All introduced sheep, whether purchased or returning from agistment or another property, should be 'quarantine drenched using four unrelated drench groups.10  

 This mix must include at least one ‘novel’ active ingredient (i.e. monepantel or derquantel). It is not recommended to manually mix products, but instead to treat first with one product and then with a second product by going down the race a second time. 

Which drench to choose? 

A wide range of oral, injectable and long-acting capsule drenches are available to control gastrointestinal roundworms. These drenches can contain one, two, three or even four active ingredients from the following classes of drenches: 11 

Broad spectrum Mid spectrum Narrow spectrum 

Amino-acetonitrile derivatives 



(BZ or ‘white’ drenches) 

(e.g oxfendazole, albendazole) 


(LEV or ‘clear’ drenches) 

Macrocyclic lactones  

(MLs or ‘mectins’) 


Organophosphates (OPs) (naphthalophos) 





The economic impact of drench resistance 

Unfortunately, there is widespread single, double and triple resistance to all older active ingredients, including BZ, LEV and ML drenches.12 Any reduction in drench efficacy has a direct impact on the health and productivity of your sheep throughout the year.   

Even mild or moderate levels of drench resistance can reduce potential income by 2–10%.13 Assuming a conservative gross margin of about $150/ewe (excluding feed costs),14 it is estimated that a 15% reduction in drench efficacy costs more than $3/ewe in lost production, while a 35% reduction in drench efficacy costs more than $15/ewe in lost production.  

Zolvix™ Plus/Zolvix 

Zolvix Plus (25 g/L monepantel and 2 g/L abamectin) from Elanco is ideal for use in strategic drenching programs as a summer (WA: autumn), pre-lambing, weaning or quarantine drench.  

Zolvix Plus is the only combination drench that delivers the power of monepantel. Its unique mode of action provides >99.9% efficacy against a broad spectrum of internal parasites, including single, double and triple resistant strains.15,16 Its 21-day Export Slaughter Interval is the lowest of any combination drench containing abamectin. 

Zolvix (25 g/L monepantel) should be used in those situations where a highly effective single active ingredient is required (e.g. as a rotation partner for ML drenches, as an exit drench or as part of a recommended quarantine program). 

Always read and follow the label directions.

Trusted solutions and advice from Elanco 

Elanco is an industry leader in sheep worm solutions with the products Zolvix Plus and Zolvix coupled with benchmark technical support and customer service. For any information or technical advice on managing sheep parasites contact Elanco 1800 995 709 or email   

Resistance may develop to any chemical. Ask your local veterinary practitioner or animal health advisor for recommended parasite management practices for your area to reduce development of resistance. It is advisable that a resistance test be conducted before any parasite treatment is used. Use in accordance with the registered label directions and regional drench decision guidelines (


2 Lane, J. et al. (2015). Priority list of endemic diseases for the red meat industries. Meat & Livestock Australia. Report B.AHE.0010.  


4 Kahn, L.P. et al. (1999). Enhancing immunity to nematode parasites in pregnant and lactating sheep through nutrition and genetic selection. Recent Advances in Animal Nutrition in Australia, 12–14 July, 1999; University of New England, Armidale, NSW.  

5 Carmichael, I. (2009) Parasite control in southern prime lamb production systems. Meat & Livestock Australia Report AHW 045.  

6 Kelly, G.A. (2010). Integrated parasite management for sheep reduces the effect of gastrointestinal nematodes on the Northern Tablelands of New South Wales. Animal Production Science, 50(12):1043–1052.  

7 Kirk, B. (2016). Internal Parasitism and Production in Prime Lamb Flocks. Masters of Veterinary Science Thesis, Faculty of Veterinary and Agricultural Sciences, The University of Melbourne.  

8 Anderson, N. (1983). The availability of Trichostrongylid larvae to grazing sheep after seasonal contamination of pastures. Aust J Agric Res 34:583-92.  




12 Playford, M.C. et al. (2014). Prevalence and severity of anthelmintic resistance in ovine gastrointestinal nematodes in Australia (2009-2012). Aust Vet J 92(12):464–471.  

13 Besier, R.B., et al. (1996). Drench resistance – a large economic cost. J. Agric. West. Aust. 37:60–63.  


15 Refer to registered label.  

16 Hosking, B.C. et al. (2010). A pooled analysis of the efficacy of monepantel, an amino-acetonitrile derivative against gastrointestinal nematodes of sheep. Parasitol Res 106:529-532. 

Zolvix Plus contains 25 g/L monepantel and 2 g/L abamectin. Zolvix contains 25g/L monepantel. ®Registered trademark. Elanco, Zolvix™, and the diagonal bar logo are trademarks of Elanco or its affiliates. ©2022 Elanco or its affiliates. EAH22015. PM-AU-22-0114