Anthelmintic Resistance: What the Research Says
Evidence from 30 peer-reviewed studies
1 Systematic Review
1 RCT
12 Cohort Study
1 Case Report
15 Expert Opinion
What Professionals Should Know
- •Time anthelmintic treatments strategically based on climate zone: avoid treatment in winter (Northern climates) or summer (warm/tropical climates) when refugia are smallest, as this increases resistance risk
- •Do not rely on seasonal weather changes to naturally clear pastures of strongyles between grazing seasons—implement integrated parasite management with grazing rotation and pasture management alongside targeted treatments
- •Maintain a pool of untreated horses and use selective treatment protocols rather than blanket deworming to preserve drug efficacy and reduce anthelmintic resistance development
- •Consider using threshold-based deworming for mares (treating only when faecal egg counts exceed 300 epg) as an effective alternative to routine prophylactic treatments, reducing anthelmintic use and selection pressure for resistance
- •In foals, less frequent deworming (2 treatments vs. monthly) may not adequately control ascarid populations—monthly or strategic deworming may still be warranted for young stock to manage parasite burdens and development
- •Monitoring bodyweight and general health indicators alongside parasite control can help identify whether reduced treatment intensity is sustainable on your farm without negative clinical outcomes
- •Nematode microbiota represents a potential novel target for parasite control strategies independent of traditional anthelmintic approaches, offering hope for managing anthelmintic-resistant populations
- •Understanding parasite-associated microbiota composition may help identify biological vulnerabilities in nematode survival and transmission that could be exploited therapeutically
- •The distinct microbiome signatures across nematode subfamilies and anatomical sites suggest control strategies may need to be tailored to specific parasite populations and infection locations
- •Worm burdens in UK horses appear to be rising steadily over the past 15 years—review your parasite control strategy with your vet, particularly if relying on older anthelmintic protocols
- •Be aware that different labs use different threshold cutoffs for reporting 'positive' results (ranging from >0 epg to ≥300 epg), which affects comparability—ask your lab what threshold they use and request serial monitoring from the same lab for consistency
- •Rising positive counts suggest potential anthelmintic resistance development; consider faecal egg count reduction tests (FECRT) and rotating drug classes rather than routine blanket treatments
- •Despite promising in vitro results, Acacia mearnsii extract at 25 g daily failed to reduce parasite burden in naturally infected horses and cannot be recommended as an alternative to conventional anthelmintics
- •The discrepancy between in vitro efficacy and in vivo failure suggests bioavailability or dosage issues that would need to be addressed before reconsidering this extract for field use
- •Continue to rely on conventional anthelmintic protocols while monitoring for resistance; consider alternative parasite management strategies such as pasture rotation and targeted selective treatment
- •Ivermectin efficacy may be declining in Thai horse populations; shortened egg reappearance period (6 weeks vs. expected longer intervals) warrants closer monitoring and consideration of alternative or rotational anthelmintic protocols
- •Fecal egg count reduction testing and molecular species identification are valuable tools to detect emerging resistance patterns before clinical failures occur, enabling proactive management strategy adjustments
- •Strongyle species respond differently to ivermectin, so targeted treatment protocols based on species composition may help preserve drug efficacy and reduce resistance development
- •Sainfoin supplementation may reduce strongyle burden in naturally infected horses, particularly before anthelmintic treatment; consider as a complementary strategy to synthetic drugs
- •Full replacement of grain with sainfoin pellets appeared more effective than partial replacement; trial sainfoin inclusion in farms with documented anthelmintic resistance
- •The effect appears to work through reducing larval viability rather than preventing infection; timing of sainfoin feeding relative to anthelmintic treatment may matter
- •Do not use fenbendazole for strongylid control in Czech operations—it has completely failed and may paradoxically increase parasite burdens in nearly 30% of cases.
- •Ivermectin and moxidectin remain highly effective options, though practitioners should monitor for emerging resistance and rotate between drug classes to slow further resistance development.
- •Implement targeted parasite control strategies beyond single-drug reliance: consider combination therapies, strategic timing of treatments, and regular fecal egg count monitoring to detect resistance early.
- •Current anthelmintic drugs (ivermectin, doramectin, fenbendazole) remain fully effective in Egyptian working horse populations, suggesting current deworming protocols are appropriate
- •Regular anthelmintic treatment within 12 months significantly reduces strongyle infection risk, supporting continued routine parasite control programs
- •Young male horses warrant closer monitoring for Parascaris equorum infections, which may require targeted treatment strategies
- •Species-level resistance patterns exist within cyathostomin infections—no single dewormer eliminates all species, so rotation strategies and targeted protocols based on local resistance patterns are necessary
- •Moxidectin appears most effective at reducing species diversity, but Pyrantel works faster at reducing egg shedding; choose based on clinical goals and local resistance profiles
- •Seven specific cyathostomin species are resistant to all three major anthelmintic classes tested; management should focus on pasture rotation, strategic deworming intervals, and monitoring fecal egg counts rather than assuming complete parasite elimination
- •Real-time PCR is more reliable than traditional larval culture for detecting S. vulgaris infections and should be considered for diagnostic confirmation, especially in low-prevalence regions
- •Even with historical frequent deworming, S. vulgaris has not been eliminated—targeted testing is warranted before changing deworming protocols
- •Serial fecal testing may be needed to confirm persistent infections, as single samples may miss some positive cases
- •Anthelmintic resistance is widespread in strongyles, so efficacy testing of your chosen products is critical before implementing a yard protocol
- •Understand the three available drug classes (benzimidazoles, tetrahydropyrimidines, macrocyclic lactones) and consider rotating between them to slow resistance development
- •With no new anthelmintics on the horizon, rational use of current medications and targeted grazing management are essential strategies for maintaining control of cyathostomins on your yard
- •Increase anthelmintic treatment frequency alone is ineffective in foals and yearlings; combine with non-chemical parasite control strategies and regular faecal egg count monitoring
- •Avoid fertilising pastures with horse manure to reduce Parascaris equorum transmission, particularly for foals
- •Implement quarantine treatment and faecal egg count reduction testing for new arrivals to monitor for anthelmintic resistance development on your farm
- •A history of frequent anthelmintic use paradoxically results in more low egg-shedding horses when treatment stops, suggesting selection for lower-shedding phenotypes or potential parasite population changes
- •Not all horses require regular anthelmintic treatment; identify and monitor low egg-shedders to reduce unnecessary drug use and potential resistance development
- •Faecal egg count monitoring should inform anthelmintic programmes rather than routine treatment, particularly in stables with established control programmes
- •Consider using saliva-based diagnostics for tapeworm detection as a more reliable alternative to faecal egg counts when deciding which horses need treatment
- •Implementing targeted selective treatment based on diagnostic testing helps reduce unnecessary anthelmintic use and mitigates resistance development
- •Adopt diagnostic-driven parasite management protocols rather than blanket treatment approaches
- •Macroalgae-based supplements show promise as natural alternatives to antimicrobial and anthelmintic drugs given emerging resistance concerns, but evidence in horses specifically remains limited
- •Seaweed's nutritional profile and bioactive compounds may support equine gastrointestinal health and welfare, though manufacturer claims require further substantiation through rigorous research
- •Until regulation and standardization improve, veterinarians should exercise caution with macroalgae supplements and recommend only those with documented safety profiles
- •Nanocomposite treatments represent a promising research direction for addressing anthelmintic resistance in strongyle control, though clinical efficacy and safety in living horses remain to be tested
- •These findings are currently laboratory-based only; field validation and regulatory approval would be required before any practical application in equine practice
- •Monitor this emerging technology as a potential future tool, but continue relying on current evidence-based parasite management strategies until clinical trials are completed
- •Ivermectin resistance in equine roundworms involves tissue-specific mechanisms, particularly in intestinal tissue; understanding these mechanisms may help develop new treatment strategies
- •The dose-dependent gene expression patterns and alternative drug uptake pathways identified suggest that alternative anthelmintics or combination therapies may be warranted in resistant populations
- •Future parasite control programs should consider that resistance involves complex transcriptional regulation rather than single-gene mechanisms, informing strategic rotation and selection of anthelmintics
- •Current parasite control recommendations lack strong evidence—work with your veterinarian to tailor protocols to individual horse risk rather than following routine blanket strategies
- •Anthelmintic resistance is a real threat; judicious use and selective treatment based on fecal egg count testing help preserve drug efficacy for when you truly need them
- •These new BEVA guidelines provide a framework for evidence-based decisions on parasite control, replacing outdated consensus where research is lacking
- •Most Australian equine veterinarians are not implementing evidence-based parasite management using FEC testing; request FEC-guided deworming protocols from your veterinarian rather than accepting routine interval-based treatment
- •There is a significant disconnect between veterinarian awareness of anthelmintic resistance as a problem and their actual diagnostic and treatment practices—ask your vet about AR status on your property and consider rotating drug classes
- •Visual weight estimation for anthelmintic dosing is common practice but unreliable; ensure your horse is weighed accurately on scales to optimize drug efficacy and reduce resistance development
- •Current parasite control on Australian stud farms relies too heavily on anthelmintics; adopt integrated strategies combining targeted treatment based on faecal egg counts with improved pasture management to reduce selection pressure for resistance
- •Macrocyclic lactone overuse is driving anthelmintic resistance development; work with your veterinarian to implement risk-based treatment protocols and monitor drug efficacy through faecal egg count reduction testing
- •Despite recognising anthelmintic resistance as important, many studs lack awareness of evidence-based surveillance methods; prioritise faecal egg count testing and non-chemotherapeutic practices (pasture rotation, manure removal, grazing management) as core elements of sustainable parasite control
- •Most trainers are overusing anthelmintics on fixed intervals rather than using evidence-based surveillance; work with your veterinarian to implement FEC testing to reduce unnecessary treatments and slow resistance development
- •Consult your veterinarian about deworming strategies—trainers who do are significantly more likely to adopt more targeted, sustainable parasite control practices
- •Consider switching from blanket interval dosing to risk-based management using FEC results to maintain treatment effectiveness and reduce costs
- •Surveillance-based parasite control is the current recommendation but adoption on US horse farms is unclear—assess your own farm's current protocol against current guidelines
- •Implementation barriers likely include both knowledge gaps and economic considerations; consulting with your veterinarian about cost-effective surveillance options may improve adoption
- •Understanding farm owner attitudes and willingness to invest in new parasite management strategies is essential for designing effective transition programs from routine deworming
- •High and indiscriminate use of moxidectin represents a significant risk factor for macrocyclic lactone resistance development—reconsider routine use and rotate drug classes
- •Implement faecal egg count testing to guide targeted treatment decisions and reduce unnecessary anthelmintic administration, which has been shown to lower annual treatment frequency
- •Ensure your veterinarian understands resistance risk factors and targeted dosing principles, as they are the primary influence on your parasite control strategy; clarify that 'targeted dosing' means treating only when FEC or clinical signs warrant it, not simply using appropriate doses
- •Move away from blanket interval anthelmintic dosing schedules and adopt targeted FWEC testing to identify which horses actually need treatment, reducing unnecessary drug exposure
- •Be aware that widespread anthelmintic resistance exists in cyathostomins and Parascaris equorum, so efficacy cannot be assumed—regular efficacy testing is necessary to ensure your chosen products remain effective
- •Implement selective treatment protocols where only horses with moderate to high egg counts are dosed, leaving some worm populations unexposed to selection pressure and helping preserve anthelmintic effectiveness across the population
- •Review your quarantine and treatment protocols for introduced horses—co-grazing without pre-treatment anthelmintics significantly increases resistance risk
- •Stop treating all animals indiscriminately; use faecal egg count testing to identify which individuals actually need treatment and target your anthelmintic use accordingly
- •Work with your veterinarian to develop a targeted parasite control strategy based on diagnostic testing rather than routine whole-group treatments, which accelerates resistance development
- •Many horse owners lack confidence in their worming protocols and would benefit from improved veterinary guidance on tailored anthelmintic programmes
- •Livery yards should reconsider one-size-fits-all anthelmintic policies, as nearly half of users are dissatisfied with imposed programmes
- •Veterinarians have an opportunity to increase their role as primary advisors on anthelmintic use, as currently under 50% of owners consult them for worming advice
- •Anthelmintic resistance is now a significant clinical problem — routine monitoring and resistance testing should inform treatment strategies rather than relying on standard protocols alone
- •Individual cyathostomin species vary in their pathogenic potential and resistance profiles, so species-level identification may become important for targeted control decisions
- •New serum antibody-based diagnostic tests offer opportunities to detect encysted larval burdens without relying solely on fecal egg counts, improving detection of clinical infections
- •Stop relying on fixed calendar-based deworming schedules; instead use faecal egg count testing to determine actual parasite burdens and treatment necessity
- •Perform faecal egg count reduction tests regularly on your horses to monitor whether your chosen anthelmintics are still working effectively on your property
- •Implement rotational grazing and strategic treatment timing that maintains some parasites in refugia (untreated horses/pastures) to slow resistance development, rather than treating all horses at once
- •Using piperonyl butoxide as a metabolic inhibitor with oxfendazole substantially improves anthelmintic efficacy in horses and may help combat resistant parasite populations
- •Combined OFZ + PB treatment eliminated mature nematodes and reduced larval stages more effectively, potentially extending the useful lifespan of benzimidazole drugs in your parasite control programme
- •Consider metabolic inhibitor combinations as a strategy when resistance to standard anthelmintics is suspected or when treating difficult-to-eliminate migrating larval stages
Key Research Findings
In Northern temperate climates, pasture refugia of strongyle free-living stages are smallest during winter, while in warm temperate and subtropical/tropical climates refugia are lowest during summer
Climatic influences significantly affect development and survival of free-living strongyle stages but cannot effectively 'clean' pastures from one grazing season to the next
Treatment should be avoided when pasture refugia are small to minimize selection pressure for anthelmintic resistance in worm populations
Refugia levels are the most important factor affecting the rate of resistance development to anthelmintic drugs in equine strongyle populations
Reduced-intensity deworming protocols (2 treatments vs. monthly) in foals resulted in no significant bodyweight differences between groups over 6 months
Foals receiving only 2 anthelmintic treatments (FA group) had significantly higher ascarid and strongylid egg counts than monthly-treated foals (FB group)
Mares dewormed on an as-needed basis (MB group, when egg counts exceeded 300 epg) showed no significant parasite burden differences compared to fixed-schedule protocols
Health incidents (colic and diarrhoea) were rare across all treatment groups, suggesting reduced treatment intensity did not compromise welfare over the study period
Equine gastrointestinal nematodes harbour distinct microbiomes dominated by Firmicutes, Proteobacteria, Bacteroidota, Verrucomicrobiota, and Actinobacteriota that differ significantly from host faecal microbiota (p=0.007)
Nematode microbiomes showed lower alpha diversity richness and distinct beta diversity community structures compared to host microbiota
Proteobacteria were consistently enriched in nematodes particularly in the caecum, with nematode-exclusive taxa including Fusobacterium, Mesorhizobium, and Mycoplasma identified
Microbial composition varied significantly by gastrointestinal site, nematode subfamily, and sex, indicating ecological specialization
Proportion of positive faecal worm egg counts in UK horses increased significantly from 2007-2009 baseline to 18.9% by 2022-2023 (p<0.001)
Lower FWECT thresholds (>0 to <100 epg) showed higher positivity rates compared to thresholds ≥100 epg, with 12-18% reductions at higher thresholds
No significant seasonal variation in positivity rates was detected across quarters
Evidence Base
Climatic influences on development and survival of free-living stages of equine strongyles: implications for worm control strategies and managing anthelmintic resistance.
Nielsen Martin K, Kaplan Ray M, Thamsborg Stig M et al. (2007) — Veterinary journal (London, England : 1997)
Systematic Review
Monitoring equine ascarid and cyathostomin parasites: Evaluating health parameters under different treatment regimens.
Nielsen Martin K, Gee Erica K, Hansen Alyse et al. (2021) — Equine veterinary journal
RCT
Beyond the host: Unveiling the independent microbiome of equine gastrointestinal nematodes.
Gentilini Fabio, Ogundipe Tolulope Grace, Turba Maria Elena et al. (2026) — PloS one
Cohort Study
Reasons to be fearful? Rising proportions of positive faecal worm egg counts among UK horses (2007-2023).
Whitlock, van Dijk, Hodgkinson et al. (2025) — Equine veterinary journal
Cohort Study
In vitro and in vivo effects of Acacia mearnsii De Wild extract for cyathostomin control in horses.
Silva G L S, Curcio U A, Boufleur J A et al. (2025) — Journal of equine veterinary science
Cohort Study
Patterns of Equine Small Strongyle Species Infection after Ivermectin Intervention in Thailand: Egg Reappearance Period and Nemabiome Metabarcoding Approach.
Hamad Mohamed H, Islam Sk Injamamul, Jitsamai Wanarit et al. (2024) — Animals : an open access journal from MDPI
Cohort Study
Inclusion of Sainfoin in the Diet Might Alter Strongyle Infection in Naturally Infected Horses.
Grimm Pauline, Laroche Noémie, Julliand Samy et al. (2022) — Animals : an open access journal from MDPI
Cohort Study
Total Failure of Fenbendazole to Control Strongylid Infections in Czech Horse Operations.
Nápravníková Jana, Várady Marián, Vadlejch Jaroslav (2022) — Frontiers in veterinary science
Cohort Study
Prevalence of gastrointestinal nematodes, parasite control practices and anthelmintic resistance patterns in a working horse population in Egypt.
Salem Shebl E, Abd El-Ghany Amany M, Hamad Mohamed H et al. (2021) — Equine veterinary journal
Cohort Study
The Use of Molecular Profiling to Track Equine Reinfection Rates of Cyathostomin Species Following Anthelmintic Administration.
Johnson Alexa C B, Biddle Amy S (2021) — Animals : an open access journal from MDPI
Cohort Study
Detection of Strongylus vulgaris in equine faecal samples by real-time PCR and larval culture - method comparison and occurrence assessment.
Kaspar A, Pfister K, Nielsen M K et al. (2017) — BMC veterinary research
Cohort Study
An investigation of anthelmintic efficacy against strongyles on equine yards in Scotland.
Stratford C H, Lester H E, Pickles K J et al. (2014) — Equine veterinary journal
Cohort Study
Endoparasite control management on horse farms--lessons from worm prevalence and questionnaire data.
Fritzen B, Rohn K, Schnieder T et al. (2010) — Equine veterinary journal
Cohort Study
Effects of previous control programmes on the proportion of horses shedding small numbers of strongyle-type eggs.
Lloyd S (2009) — The Veterinary record
Cohort Study
Use of a saliva-based diagnostic test to identify tapeworm infection in horses in the UK.
Lightbody K L, Matthews J B, Kemp-Symonds J G et al. (2018) — Equine veterinary journal
Case Report
A review of current and potential applications of macroalgae to equine veterinary science.
Warren P, Mc Crudden D, O'Reilly S et al. (2026) — Journal of equine veterinary science
Expert Opinion
In vitro evaluation of anthelmintic activity of biocompatibile carbon quantum dot nanocomposite against egg and larval stages of equine strongyles.
Fakheri Armin, Esmaeilnejad Bijan, Akbari Hamid et al. (2025) — BMC veterinary research
Expert Opinion
Gene co-expression network analysis reveal core responsive genes in Parascaris univalens tissues following ivermectin exposure.
Dube Faruk, Delhomme Nicolas, Martin Frida et al. (2024) — PloS one
Expert Opinion
BEVA primary care clinical guidelines: Equine parasite control.
Rendle David, Hughes Kristopher, Bowen Mark et al. (2024) — Equine veterinary journal
Expert Opinion
Assessment of worm control practices recommended by equine veterinarians in Australia.
Abbas Ghazanfar, Stevenson Mark A, Bauquier Jenni et al. (2023) — Frontiers in veterinary science
Expert Opinion
Show 10 more references
A questionnaire study of parasite control in Thoroughbred and Standardbred horses in Australia.
Wilkes Edwina J A, Heller Jane, Raidal Sharanne L et al. (2020) — Equine veterinary journal
Expert Opinion
Cross-sectional survey of parasite control practices on Thoroughbred and Standardbred training yards in New Zealand.
Rosanowski S M, Scott I, Sells P D et al. (2016) — Equine veterinary journal
Expert Opinion
Attitudes towards implementation of surveillance-based parasite control on Kentucky Thoroughbred farms - Current strategies, awareness and willingness-to-pay.
Robert M, Hu W, Nielsen M K et al. (2015) — Equine veterinary journal
Expert Opinion
A questionnaire study of equine gastrointestinal parasite control in Scotland.
Stratford C H, Lester H E, Morgan E R et al. (2014) — Equine veterinary journal
Expert Opinion
Faecal worm egg count analysis for targeting anthelmintic treatment in horses: points to consider.
Lester H E, Matthews J B (2014) — Equine veterinary journal
Expert Opinion
A questionnaire study on parasite control practices on UK breeding Thoroughbred studs.
Relf V E, Morgan E R, Hodgkinson J E et al. (2012) — Equine veterinary journal
Expert Opinion
Equine anthelmintics: survey of the patterns of use, beliefs and attitudes among horse owners in the UK.
Allison K, Taylor N M, Wilsmore A J et al. (2011) — The Veterinary record
Expert Opinion
Facing the threat of equine parasitic disease.
Matthews J B (2011) — Equine veterinary journal
Expert Opinion
Practical aspects of equine parasite control: a review based upon a workshop discussion consensus.
Nielsen M K, Fritzen B, Duncan J L et al. (2010) — Equine veterinary journal
Expert Opinion
Changes to oxfendazole chiral kinetics and anthelmintic efficacy induced by piperonyl butoxide in horses.
Sánchez Bruni S F, Fusé L A, Moreno L et al. (2005) — Equine veterinary journal
Expert Opinion