Controlling worms in sheep


Gastrointestinal parasites occur in all sheep rearing areas of New Zealand. 

The primary species of concern are Barber’s pole worm (Haemonchus contortus), Black scour worm (Trichostrongylus spp.) Small brown stomach worm (Teladorsagia [Ostertagia] spp.) and Thin necked intestinal worm (Nematodirus). 

Secondary species include 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.

Economic impact 

Effective roundworm control is essential for the health and production of sheep on 

New Zealand farms.  

In a 2008 study in growing lambs, conducted by AgResearch(1), the results of using a fully effective drench, dosed monthly over a 6 month program versus a less than fully effective drench dosed monthly were compared. 

 In the animals treated with the fully effective product this provided: 

  • Increased weight gain 
  • Increase condition scores 
  • Decreased day to reach weight targets  

Importantly this resulted in an overall 14% increase in carcass values in the lambs treated with the fully effective drench. 

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.  

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.  

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. 

Controlling worms 

Sheep farmers implement worm control using anthelmintics (e.g. ‘drenches’) in combination with grazing management.  

Neew Zealand sheep farmers are likely to implement strategic worm control programs incorporating: 

  • Following a 5 drench lamb preventative drenching program 
  • strategic timing of treatments (e.g.  knock out, exit, quarantine) 
  • the use of combination drenches wherever possible  
  • the use of long-acting drenches and capsules 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. 

Knockout drench 

The accepted cornerstone of roundworm control programs on New Zealand sheep farms is the preventative lamb drenching program, whereby lambs are drenched every 28 days. This treatment interval is thought to minimise the effect of roundworms on growing lambs whilst also reducing levels of pasture contamination. 

It is important to include at least one ‘Knockout’ drench treatment as part of any routine 

preventative drenching program. This will help to control any resistant parasites that may have survived the preceding drenches, minimise subclinical parasitism and reduce the resistant parasite population available on pasture. 

To be effective as a ‘Knockout’ treatment the drench must contain an active ingredient that is proven to be highly effective against worms resistant to the other product used in the drench program. 

Computer modelling has indicated that using a drench such as Zolvix Plus as the fifth drench in a preventative drenching program has the most benefit in terms of slowing the development of resistance to the other products used in the program.(5) 

Quarantine drench

Knowing the drench resistance status of your property by conducting a FECRT and then actively managing parasites to maintain or improve resistance status is essential. 

The prevalence of properties with resistance to the widely used triple drenches was estimated at 11%(2) in 2018 and is thought to be considerably higher now. Triple drenches can therefore no longer be considered as effective quarantine treatments for bought in sheep. 

The importation of resistant parasites in bought in stock is recognized as posing one of the greatest risks to roundworm management.  

For sheep, Wormwise(3) recommendations are that as a minimum, either of the two novel drenches, Zolvix Plus or derquantel must be used. For more complete protection it is recommended that they be administered with a BZ + Levamisole combination product (delivered sequentially – not at the same time) however this may have meat with holding complications. Please refer to your veterinarian for further advice. 

It is important to understand that effective quarantining of bought in stock is not simply about which drench is used. Consideration must be given to holding treated animals in the yards for 24 hours after dosing, and then releasing them onto contaminated or “dirty” pasture to dilute the eggs of any resistant parasites which may have survived the quarantine treatment.  

Exit 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 compromised in terms of lactation, growth rates and clinical disease. Many New Zealand farmers rely on long acting injectable or slow-release capsule products to reduce the risks posed by roundworms around and after lambing.  

Long-acting treatments should be used in combination with a ‘primer’ or ‘exit’ drench at the start or end of the period of protection offered by the LA treatment. This will remove worms that survived the initial treatment or during the ‘tail’ period for injectable products and over the payout period for capsules– 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 worm egg counts during the payout period of the LA treatment is routinely recommended to check for effectiveness. An early exit drench is recommended if positive faecal egg counts are found at any time during the claimed protection period of the LA product or capsule. 

Which drench to choose? 

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

  • Benzimidazoles (BZ or white drenches), which were introduced to the New Zealand market in the 1960s. Examples include thibendazole, parbendazole, oxbendazole, mebendazole fenbendazole and albendazole.  
  • Levamisoles (LEVs or clear drenches), which were introduced to the New Zealand market in 1968. Examples include levamisole and morantel.  
  • Macrocyclic lactones (MLs or mectins), which were introduced to the New Zealand market in the mid 1980s. Examples include abamectin, ivermectin and moxidectin. 
  • Amino-Acetonitrile Derivatives (AADs or ‘orange’ drenches), which were introduced to the New Zealand market in 2009. Monepantel, one of the active ingredients in Zolvix Plus, is the only active ingredient in this class. 
  • Spiroindoles, which were introduced to the New Zealand market in 2010. Derquantel, one of the active ingredients in Startect®, is the only active ingredient in this class. 

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(2). Any reduction in drench efficacy has a direct impact on the health and productivity of your sheep throughout the year.  

Undetected drench resistance is estimated to cost the New Zealand sheep industry $48 mio per year and is thought to be getting worse(4).  

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 knockout, exit or quarantine drench.  

Zolvix Plus is the only combination drench that contains 25 g/L monepantel. Monepantel’s unique mode of action provides efficacy against a broad spectrum of internal parasites of cattle and sheep, including single, double and triple resistant strains.  

Zolvix Plus is the first choice for quarantine, exit and knockout drenching. Always read and follow label directions. 

Trusted solutions and advice from Elanco   

Elanco is an industry leader in sheep worm solutions with the product Zolvix Plus coupled with benchmark technical support and customer service. For any information or technical advice on managing sheep roundworms, contact Elanco 0800446121 

  1. Sutherland, I.A., Bailey, J., Shaw, R.J., 2010. The production costs of anthelmintic resistance in sheep managed within a monthly preventive drench program. Veterinary Parasitology 171, 300–304. 
  2.  McKenna, P.B. (2018). Update on anthelmintic resistance Vetscript 28(June):44–45. 
  3. Wormwise Technical Manual 2019 
  5. Leathwick, D.M. et al. (2009). Managing anthelmintic resistance: Modeling strategic use of a new anthelmintic class to slow the development of resistance to existing classes. NZVJ 57(4):181–192.  


Zolvix Plus Broad Spectrum Oral Anthelmintic for Sheep and Cattle is registered under the ACVM Act (1997), A011107. 

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