Parasitic helminths (worms) are common pathogens of equids across the world. In some cases, heavy infections with these parasites can lead to severe disease. Resistance to the currently available anthelmintics (wormers) is a major issue, particularly in two key pathogens: the cyathostomins (small redworms) and Parascaris equorum (roundworm). Drug resistance in these worms has been demonstrated to all classes of anthelmintic currently available and, in some cases, multi-class resistance has been reported (Nielsen, 2022).
It would seem that no new equine anthelmintics are coming to market, leaving a situation where ever-increasing levels of multi-drug resistance will limit effective parasite control. It is therefore essential that steps be taken to reduce the further spread of anthelmintic resistance.
Central to mitigating the impact of resistance, is the effective dissemination and implementation of evidence-based approaches to parasite control. These focus on moving the equine sector away from interval treatment-based control programmes, where horses are treated regularly with wormers regardless of infection status and with no consideration of the resident worm population's drug sensitivity, to approaches which exploit knowledge of parasite epidemiology to break transmission cycles and employ diagnostics to identify worms present and levels of infection within, or excreted by, individuals.
Challenges in implementing effective evidence-based parasite control in the equine sector
Over the years, there has been a wide range of sources for parasite control advice. Throughout the 1980s and 1990s, the interval treatment dogma prevailed, largely led by marketing campaigns from pharmaceutical companies and based on epidemiological studies under-taken in the 1960s. These methods of control are straightforward to apply, with all horses dosed at intervals extrapolated from trials performed when each anthelmintic product was first registered. Since the initial reports of benzimidazole resistance (Drudge and Lyons, 1965), parasitologists have advised caution when using interval treatments because these generate high selection pressure for resistance. Gradually, these concerns led to a change in control recommendations, with strategies developed to reduce treatment frequency by removing pre-parasitic stages from paddocks through regular dung removal (Herd, 1986) and using faecal egg count tests to target treatments based on worm egg shedding (Uhlinger, 1993).
It is important to assess levels of strongyle egg excretion in individuals as gastrointestinal worm distributions within populations are not even and, usually, relatively few horses excrete the bulk of contamination onto pasture. This pattern (Figure 1), the negative binomial distribution, means that most (>80%) eggs put onto pasture are shed by a relatively small proportion (<20%) of the population (Relf et al, 2013; Nielsen et al. 2018). By using faecal egg count tests to guide treatments to only those horses excreting above a selected faecal egg count threshold (usually, 200 eggs per gram of dung), substantial reductions in anthelmintic treatment frequency can be achieved.
In addition to faecal egg count-led treatments, infection in the environment can be reduced by removing dung (‘poo picking’) at least once a week. This is because strongyle eggs in dung take a certain time to hatch, releasing larvae, which then go through two ‘moults’ before they move onto pasture as infective larvae (Figure 2). In optimal conditions in the UK (when it is warm and wet in summer), this takes 1–2 weeks (Ramsey et al, 2004).
Research studies have demonstrated that removing dung from paddocks at least once a week can have a significant effect on pasture contamination with larvae (Herd, 1986), or on subsequent faecal egg count levels in equids that graze paddocks from which dung is removed (Corbett et al, 2014). By employing dung removal and diagnostic tests to indicate which horses require treatment, substantial reductions in anthelmintic use can be attained to reduce selection pressure for resistance.
Progressively, as more knowledge has been garnered on cyathostomin epidemiology, the ubiquity of anthelmintic resistance in cyathostomin and Parascaris spp. (Kaplan, 2004; Matthews, 2008, 2014; Nielsen, 2022), the real value in faecal egg count testing (Lester et al, 2013; Lester and Matthews, 2014), and the development of new diagnostics (Lightbody et al, 2016), evidence-based parasite control recommendations have continued to evolve.
Unfortunately, the advice available has not always been well aligned and, in some cases, has been contradictory. This has led to poor uptake of evidence-based control by end-users, further hindered by the fact that these approaches involve more effort than simply treating regularly with wormers, a situation not helped by the ready availability of these medicines from online retailers, as well as internet misinformation. The issue is confounded by horse owners often not viewing anthelmintic resistance as a problem that directly affects their horse(s). This perception has meant there has been poor uptake of faecal egg count reduction testing (Easton et al, 2016), which is recommended for assessing anthelmintic effectiveness and in many equine populations across the UK, drug sensitivity is unknown and it is likely that wormers are being used that have a limited effect on the cyathostomin or Parascaris spp. populations present.
These issues emphasise the needs for a single, go-to entity that acts as a non-biased information source for equine parasite control in the UK. This is being addressed by the development of CANTER (Controlling ANTiparasitic resistance in Equines Responsibly), a cross-industry initiative that aims to create readily-accessible resources to provide consistent information on equine parasite control, with the practical implications of the advice delivered considered at every stage.
Aims of the CANTER initiative
CANTER aims to highlight the problem of resistance and its implications to anthelmintics prescribers (veterinary surgeons, suitably qualified persons and veterinary pharmacists) and horse owners.
Following two workshops attended by delegates from over 40 stakeholder organisations, and led by the Veterinary Medicines Directorate, this pan-industry group was officially launched in March 2023. The group includes all relevant stakeholders: prescribers (veterinary surgeons, Suitably Qualified Persons and veterinary pharmacists), parasitologists, horse owners, charities, levy boards, guidance/policy makers, retailers, diagnostics providers, and pharmaceutical representatives. It intends to develop and disseminate clear evidence-based recommendations for worm control that will reduce reliance on anthelmintics. Its guidelines will include information on management measures that will decrease levels of environmental infection, including appropriate stocking densities and guidelines for procedures such as dung removal. They will include up-to-date knowledge on the diagnostic tests available in the UK that can be used to inform the need to treat individuals, and the appropriate anthelmintic to select when treatment is required.
Up-to-date information will be provided on current methods that inform anthelmintic use and choice, including faecal egg count tests, Anoplocephala spp. saliva and blood tests, a cyathostomin blood test, lungworm faecal sedimentation, and the pinworm tape test. Applying faecal egg count reduction tests to assess efficacy is considered the gold standard method to establish drug sensitivity in the field; CANTER outputs will strongly recommend efficacy testing, to increase implementation by end-users. The group will use evidence to emphasise the potential cost, and other, benefits of using diagnostics to inform treatment choices, to encourage prescribers to promote testing over anthelmintic sales, and will explore how this can be addressed with online sales where prescriber/end-user interactions are more limited.
CANTER guidelines are being drafted and peer-reviewed throughout 2023 and will be publicly available later this year via the group's website and its wider network. Guideline sections are co-authored by parasitology and social science experts, together with stakeholders involved in implementing parasite control, including veterinarians, diagnostic laboratories, equine trade and suitably qualified person accreditation organisations, pharmacutical companies, welfare charities and equine societies
To expedite knowledge transfer to the sector, CANTER has a dedicated communications team, with representatives from across the sector. So far, this team has constructed a website, and executed an official launch to promote a call to action to publicise the initiative and its aspirations. This team are developing user-friendly tools summarising the main objectives of best practice control to kickstart engagement between prescribers and horse owners, including a parasite risk profile (Figure 3), widely shared to encourage discussions on parasite control within the sector.
CANTER is unique in that it includes representation from all relevant stakeholders and end-users. By involving these participants, it is hoped that the approach will address the challenge of poor uptake of best practice measures in a sector where knowledge and implementation of evidence-based parasite control is disparate within and between end-user groups (Easton et al, 2016; Tzelos 2019), in part, because of the fragmented nature and spectrum of premises within the sector.
It is anticipated that guidelines developed with input from across the sector will improve prescribing practices and encourage responsible use of anthelmintics by providing accessible knowledge of parasite epidemiology and the value of tests in supporting treatment decisions.