10 September 2026
Anthelmintic resistance is not a new threat, and neither are the principles of responsible parasite control. With diagnostic tools readily available and no steady pipeline of new actives, large animal vets must lead the move from routine blanket treatment to evidence led parasite control.
The warnings over increasing anthelmintic resistance on UK farms, and globally, is not new. The problem has been recognised for almost 70 years.
Anthelmintic resistance was documented in the scientific literature as early as 1957, when phenothiazine resistance was reported in sheep1,2, and resistance has now been reported across every major class of anthelmintic used in livestock globally, including the newer classes introduced to address parasites resistant to older products3,4,5,6.
This does not mean every active has failed, or that resistance is present on every farm, but it demonstrates that no chemical class can be regarded as immune from resistance development.
Responsible parasite control is not a new concept either. Sustainable Control of Parasites in Sheep (SCOPS) was formed in 2003, with its first guidelines produced in 2004. Its cattle counterpart, Control of Worms Sustainably (COWS), launched guidance in May 20107.
Yet, more than a decade later, routine parasite control on many UK cattle farms still relies on seasonal or calendar based treatment, without first establishing whether treatment is necessary, or checking afterwards whether it worked.
Routine blanket treatment of whole groups of cattle remains common, even though targeted and selective approaches are far from new ideas. The principle underpinning these approaches, maintaining susceptible parasites in refugia, has been discussed in veterinary research for more than 40 years8. Targeted selective treatment was formally described in the scientific literature 20 years ago9 and first investigated specifically in cattle in 200910.
In the UK, COWS has been promoting sustainable, evidence led cattle parasite control to vets, advisers and farmers since 20107. Yet targeted treatment, where a whole group or herd is treated only when monitoring or risk assessment indicates a need, and targeted selective treatment, where only the individual animals most likely to benefit from treatment are dosed, have still not become routine practice on many UK farms11.
Our understanding has moved on, diagnostic options have improved, but practice on farm has not always kept pace.
This raises an uncomfortable question for the large animal veterinary profession: if diagnostics are central to sustainable parasite control, why are they not routinely embedded within more herd health plans?
It is tempting to view resistance as a problem that pharmaceutical innovation will eventually solve. But, the realities of research and development suggest otherwise.
Gustavo Sabatini, global technical manager, ruminant antiparasitics at Boehringer Ingelheim, explains that developing a new veterinary medicine can take 10 to 15 years. Candidate compounds face substantial attrition during development, with potential products failing because of efficacy, safety, formulation stability, manufacturing scale up, intellectual property or regulatory requirements.
“We cannot expect a new product to be launched every five or 10 years,” he says. “Farmers, vets and industry all need to take care of the parasiticides currently available.”
The gap between new chemical classes illustrates the point. Mr Sabatini notes there has been no steady stream of new modes of action. Even where promising compounds exist, there is no quick route from discovery to a licensed product.
Preserving efficacy therefore depends on reducing unnecessary selection pressure now. That does not mean withholding treatment from animals that need it. It means using clinical assessment, grazing and treatment history, production data and diagnostics to identify the right animals, product, dose and timing, alongside non chemical control measures.
Without that evidence, a blanket treatment may be unnecessary. Worse, it may create false reassurance if poor administration or resistance means the expected efficacy was not achieved.
Rosevean Veterinary Practice has shown how diagnostics can become part of routine herd health work rather than an optional test offered in response to a problem.
The practice initially piloted faecal egg count monitoring with two or three farms after investing in automated technology that made processing samples easier and more standardised. It then developed a fully managed service for dairy, beef and calf rearing clients.
Senior clinical director Matthew Berriman says the decision to manage the process was important. A veterinary technician liaises with each farmer, records turnout dates, collects samples correctly and keeps testing to the required schedule. The vet interprets results in their wider clinical and farm context and advises on treatment. Where treatment is recommended, a correctly timed post treatment check can then be arranged, if required.
“There is a lot around timing and sampling technique that’s important to making it work,” he says. “We run it as a fully managed service, so farmers don’t need to think about it. We know the sample is taken correctly and at the appropriate time.”
That continuity has generated information the farm and practice would not otherwise have had. Mr Berriman reports that the practice has identified inadequate feacal egg reductions following some treatments, and that’s opened up conversations about administration technique, possible resistance, and the need to consider an alternative anthelmintic class.
While inadequate feacal egg reductions don’t indicate how common resistance may be, it does highlight the importance of checking that treatments have worked. Without efficacy testing, poor treatment performance may go undetected, allowing production losses to continue while further selection pressure is applied.
Farmers using the service have generally felt that cattle grew and looked better, Mr Berriman says. Regular contact has also led to investigation of conditions that can resemble or compound parasitism, including coccidiosis and mineral deficiencies.
Testing is not an isolated laboratory transaction. Its value comes from asking the right questions, sampling at the right time, interpreting the answer in context, and acting on it. Expecting farmers to submit samples whenever they remember is unlikely to deliver that consistently.
Mr Berriman believes its wider value to the practice comes from more consistent involvement in grazing animal health. Rather than having little contact with some clients through the summer, the team discusses results with them every three or four weeks.
He says the veterinary technician is essential to making the model workable. The technician runs the sampling protocol, while the vet retains responsibility for interpretation, diagnosis and treatment advice. It also gives vets, including recent graduates, a defined route into herd health advisory work.
No single test answers every question. The appropriate combination depends on the parasite, age and class of stock, grazing history, clinical presentation and purpose of testing.
Faecal egg counts (FECs) are most useful for monitoring gastrointestinal nematodes in first season grazing calves and youngstock. Regular testing during the first half of the grazing season is more informative than a single sample and should be considered alongside growth rates, nutrition, grazing management and clinical signs. FECs have limitations once cattle have developed immunity, and they do not diagnose lungworm.
Faecal egg count reduction tests (FECRTs) compare egg counts before and after treatment to assess efficacy. A reduction below the expected threshold can prompt investigation of dosing or administration problems, and possible resistance. The timing for testing varies with the active used.
Bulk milk ELISA testing can indicate herd level exposure to Ostertagia ostertagi in adult dairy cows by measuring antibodies. It is a monitoring tool, not a means of selecting individual cows for treatment, and results must be interpreted against treatment timing, grazing management, parasite history and milk production.
Serum antibody ELISAs can provide evidence of exposure to Ostertagia ostertagi, Dictyocaulus viviparus and Fasciola hepatica. In first season grazing calves, testing for O ostertagi late in or after the grazing season can help assess cumulative gutworm exposure and review the control programme. Liver fluke antibodies can be detected before infection becomes patent, helping identify when exposure has occurred and informing treatment timing. As antibodies can persist after successful treatment, results must be interpreted alongside clinical signs, grazing history and other diagnostic findings.
Blood pepsinogen testing, undertaken shortly after housing in first season grazing cattle, can help assess abomasal damage caused by Ostertagia ostertagi and evaluate how effectively the grazing season control programme protected youngstock. It is useful for reviewing the programme, rather than deciding whether to treat at housing.
Lungworm diagnosis often begins with clinical signs and requires prompt veterinary assessment. Bronchoalveolar lavage or larval examination of faeces or may support diagnosis, but larvae may not appear in dung until infection is established. A negative result must therefore be interpreted cautiously and should never delay necessary intervention in a rapidly deteriorating animal or group.
Vets have led substantial changes in antimicrobial stewardship. Anthelmintic stewardship requires the same professional ownership. The science, guidance and diagnostic tools are available. The next step is to make parasite monitoring and treatment efficacy review a routine, actively managed part of herd health planning.
If change is to happen on farm, vets have to lead it.
1. Drudge JH, Leland SE Jr and Wyant ZN (1957). Strain variation in the response of sheep nematodes to the action of phenothiazine. I. Studies of mixed infections in experimental animals, Am J Vet Res 18(66): 133-141.
2. Leland SE Jr, Drudge JH, Wyant ZN and Elam GW (1957). Strain variation in the response of sheep nematodes to the action of phenothiazine. III. Field observations, American Journal of Veterinary Research 18(69): 851-860.
3. Kaplan RM (2004). Drug resistance in nematodes of veterinary importance: a status report, Trends in Parasitology 20(10): 477-481.
4. Kotze AC and Hunt PW (2023). The current status and outlook for insecticide, acaricide and anthelmintic resistances across the Australian ruminant livestock industries: assessing the threat these resistances pose to the livestock sector, Australian Veterinary Journal 101(9): 321-333.
5. Mederos AE, Ramos Z and Banchero GE (2014). First report of monepantel Haemonchus contortus resistance on sheep farms in Uruguay, Parasites and Vectors 7: 598.
6. Sales N and Love S (2016). Resistance of Haemonchus sp. to monepantel and reduced efficacy of a derquantel/abamectin combination confirmed in sheep in NSW, Australia, Veterinary Parasitology 228:193–196.
7. Taylor MA (2012). SCOPS and COWS: “Worming it out of UK farmers”, Veterinary Parasitology 186(1-2): 65-69.
8. Martin PJ, Le Jambre LF and Claxton JH (1981). The impact of refugia on the development of thiabendazole resistance in Haemonchus contortus, International Journal for Parasitology 11(1): 35-41.
9. Van Wyk JA, Hoste H, Kaplan RM and Besier RB (2006). Targeted selective treatment for worm management: how do we sell rational programs to farmers? Veterinary Parasitology 139(4): 336-346.
10. Höglund J, Morrison DA, Charlier J, Dimander SO and Larsson A (2009). Assessing the feasibility of targeted selective treatments for gastrointestinal nematodes in first season grazing cattle based on mid season daily weight gains, Veterinary Parasitology 164(1): 80-88.
11. Vande Velde F, Charlier J and Claerebout E (2018). Farmer behaviour and gastrointestinal nematodes in ruminant livestock: uptake of sustainable control approaches, Frontiers in Veterinary Science 5: 255.
An educational service from Boehringer Ingelheim Animal Health UK Ltd. Further information available from Boehringer Ingelheim Animal Health UK Ltd, RG12 8YS, UK. ©2026. All rights reserved. Date of preparation: Aug 2026. MSP-0053-2026. Use Medicines Responsibly.