Recently published research has found that resistance to older drenches and their combinations is now both widespread and severe, rendering many of these products all but ineffective for the control of worms in sheep.
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The study, entitled Anthelmintic resistance in sheep nematodes in Australia: A compilation of recent test results, was published in the peer-reviewed Australian Veterinary Journal in April.
The study was conducted by Adjunct Associate Professor Brown Besier, one of the country’s foremost authorities in veterinary parasitology and anthelmintic resistance, and Elanco technical services veterinarian, Dr Nick Rolls.
It examined the efficacy of seven different drench groups against the three most economically-significant worm species in Australia: Barber’s pole worm (haemonchus contortus), brown stomach worm (teladorsagia circumcincta) and black scour worm (trichostrongylus spp.).
Faecal egg count reduction tests (FECRT) were conducted in 80 large flocks across New South Wales, Victoria, South Australia and Western Australia between 2021 and 2024.
In each test, groups of 12 young sheep, typically weaners, were treated with one of nine different drenches.
The sheep had either never been previously drenched or not drenched in the previous six weeks as a minimum.
They included the older active ingredients from the benzimidazole, levamisole and macrocyclic lactone (mectin) chemical groups; a three-way combination of same; and the newer active ingredients, monepantel and derquantel (in combination with abamectin).
The efficacy of two narrow-mid spectrum drenches, closantel and naphthalophos, was also assessed in those regions where haemonchus contortus is prevalent.
Individual faecal samples were collected from 10 sheep in each group 10–14 days after treatment, with worm egg counts compared to the untreated control group to determine overall efficacy.
Individual worm species were identified using larval culture and L3 differentiation.
The results were then analysed to calculate the severity of resistance to each of the three worm species.
Resistance is recognised as occurring when a drench is unable to reduce the worm burden by more than 95 per cent, with appropriate statistical considerations.
Elanco brand manager for sheep, Dave Rathbone, says the study confirms drench resistance is widespread and getting worse.
“Previous national studies conducted in 1994 and 2014 found widespread resistance to the older drench groups, but this new study shows that the situation is now much worse,” he says.
“The level of resistance to these older drench groups is severe.
“Resistance to albendazole, levamisole, abamectin and moxidectin was detected in least one of the major worm species in 96 per cent, 88 per cent, 91 per cent and 84 per cent of the flocks tested, respectively.
“However, there was wide variation in efficacy against individual worm species.
Albendazole had a mean efficacy of just 35 per cent against T.circumcincta but 65 per cent against H.contortus and 67 per cent against Trichostrongylus spp.
Levamisole had a mean efficacy of 49 per cent against T. circumcincta but 92 per cent against H.contortus and 69 per cent against Trichostrongylus spp.
Abamectin, still one of the most widely used drenches in Australia, achieved a mean efficacy of 60 per cent against T.circumcincta, 26 per cent against H.contortus and 89 per cent against Trichostrongylus spp.
Moxidectin is most potent of the mectins, but even so, resistance was detected in 79 per cent of H.contortus tests, with a mean efficacy of 67 per cent.
“Put simply, producers simply cannot rely on these drench groups for broad-spectrum control of the major worm species,” Dave says.
Likewise, resistance to three-way combinations of a benzimidazole, levamisole and abamectin was detected in 66 per cent of properties tested, more than twice that reported in 2014.
Mean efficacy was 80 per cent against T.circumcincta, 91 per cent against Trichostrongylus spp. and 99 per cent against H.contortus.
“The speed of the deterioration in the efficacy of the tested three-way combinations is alarming,” Dave says.
“These drenches can no longer be regarded as bullet-proof protection against resistance.
“Even mild or moderate levels of drench resistance can result in a significant economic loss.
“It would be fair to say that the vast majority of Australian sheep farmers who are using single, double or three-way combinations of older active ingredients are losing money to worms without realising it.”
The study also identified a marked increase in the incidence of resistance to the newer active ingredients, monepantel and derquantel, with 23 per cent of tested flocks showing evidence of resistance to Zolvix (monepantel) and another 23 per cent showing evidence of resistance to Startect (derquantel and abamectin).
Monepantel was released as Zolvix, a single active formulation in 2010.
Resistance is recognised as occurring when a drench is unable to reduce the worm burden by more than 95 per cent, with appropriate statistical considerations.
Since 2016, it has been marketed as Zolvix Plus, a co-formulation of monepantel and abamectin.
Startect, a co-formulation of derquantel and abamectin, was released in 2014.
“While resistance was detected to monepantel and derquantel with abamectin, both delivered high efficacy of between 97 and 99 per cent across the three major worm species,” Dave says.
Research has found resistance to older drenches and their combinations is now widespread and severe, rendering many of these products all but ineffective for the control of worms in sheep.
“It is a sad reality that unless producers adopt strategic drenching programs using effective chemistry and regular drench testing, these newer active ingredients will probably follow the same trajectory.”
“Currently only 6 per cent of Australian sheep producers conduct routine drench efficacy tests, so most have no idea if their current drench is working or not. 1
“It is impossible to make good decisions if you don’t know the efficacy of your drench.
“Conducting a FECRT every two or three years is recommended to ensure optimal sheep worm control.”
Always read and follow label directions.
Good agricultural practice is essential for optimal worm control.
Reference: 1. Besier, R.B. and Rolls, N.M. Anthelmintic resistance in sheep nematodes in Australia: a compilation of recent test results. Aust Vet J. 2026;1–7. Zolvix Plus contains 25 g/L monepantel and 2 g/L abamectin. Registered trademark. Zolvix, Elanco and the diagonal bar logo are trademarks of Elanco or its affiliates. 2025 Elanco or its affiliates. EAH26153. PM-AU-26-0467.
