Equine helminth control practices require urgent and sustained change. Anthelmintic resistance is rapidly growing in the UK and overseas. Several sector groups have produced updated guidance for control in the UK, including the British Equine Veterinary Association (BEVA) ‘ProtectMEToo’ guidelines (BEVA, 2024) and the Controlling AN-Tiparasitic resistance in Equines Responsibly (CANTER) guidelines (CANTER, 2024). The latter was established in 2022 and is a voluntary pan-industry group, currently chaired by the Veterinary Medicines Directorate. In the UK, authorised equine anthelmintics are classified as prescription-only medicines that can be prescribed by veterinary surgeons, veterinary pharmacists and suitably qualified persons, and CANTER is aiming to produce best practice guidelines relevant to each of these groups. The BEVA guidelines focus on equine veterinary surgeons. In addition to addressing anthelmintic resistance, both guidelines consider important ecotoxicity concerns of anthelmintics (Haseler et al, 2023).
Equine helminths
The most common equine helminths are the cyathostomins (small redworms, small strongyles) and, historically, Strongylus spp. (large redworms, large strongyles). Other pathogens include the pinworm, Oxyuris equi (a persistent issue in some populations causing perineal pruritus) and the cestode (tapeworm), Anoplocephala perfoliata. Ascarids (Parascaris spp., large roundworms) are common, important parasites of foals. Infection occurs up to around 9 months of age. This species can cause poor condition and respiratory disease, and less commonly, intestinal impaction, which carries a poor prognosis, even with surgery. Eggs shed in faeces are very resistant and can persist for months to years in the environment; therefore, pasture hygiene is vital to prevent a build up of eggs on pastures grazed by broodmares and foals.
Virtually all grazing horses are exposed to cyathostomins. Although most adult horses control infection well and maintain low burdens, a small proportion (around 20–30%) harbour around 80% of a population's total cyathostomin burden (Flanagan et al, 2013; Relf et al, 2013) and can contaminate pasture with significant numbers of eggs. Control methods should thus minimise pasture contamination by such high egg shedders (by testing and treating), while maintaining a non-pathogenic low level of infection in others grazing the same pasture. Cyathostomins have a period of larval development in the large intestinal wall, where thousands of larvae can accumulate, especially in young horses (horses up to about 5 years old are more susceptible to developing high burdens; Nielsen et al, 2007). In some youngsters, this may result in acute larval cyathostominosis, characterised by severe weight loss, hypoalbuminaemia and diarrhoea; the disease caries a high mortality rate despite treatment (Walshe et al, 2021; Lawson et al, 2023). Other diseases associated with high cyathostomin burdens include colic and weight loss, with or without diarrhoea.
A. perfoliata is the most common equine tapeworm (Nielsen, 2016) and can infect horses of all ages. Infection occurs when horses ingest infected oribatid mites whilst grazing. Tapeworm infestations can cause erosions and inflammation of the intestinal mucosa (especially around the ileo-caecal junction) and result in colic, including spasmodic colic, impactions and intussusceptions (Nielsen, 2016).
Equine anthelmintics
In the UK there are three classes of anthelmintics licensed for use against equine nematodes, and two against tapeworm (Table 1). Praziquantel is only available in authorised combination products (with ivermectin or moxidectin), but these should only be prescribed if there is evidence of a need to treat for nematode infection. Praziquantel as a single formulation is available as an extemporaneous (‘special’) product that can be prescribed by a veterinary surgeon.
Table 1. Anthelmintics licensed for used in horses in the UK and the status of resistance reported globally in the key helminth species
| Anthelmintic class–compound | Cyathostomin | Anoplocephala perfoliata | Strongylus vulgaris | Parascaris spp. |
|---|---|---|---|---|
| Benzimidazole – fenbendazole | Resistance common | Not licensed | Not yet detected | Resistance emerging |
| Tetrahydropyrimidine – pyrantel embonate (tapeworm: double dose) | Resistance common | Resistance emerging | Not yet detected | Resistance emerging |
| Macrocyclic lactone – ivermectin | Resistance emerging | Not licensed | Not yet detected | Resistance common |
| Macrocyclic lactone - moxidectin | Resistance emerging | Not licensed | Not yet detected | Resistance common |
| Isoquinoline-pyrazine - praziquantel | Not licensed | Resistance emerging | Not licensed | Not licensed |
Anthelmintic resistance
Anthelmintic resistance is common in cyathostomins (Table 1), with reports of resistance to all classes of authorised anthelmintics (Nielsen, 2022). Based on faecal egg count reduction test studies, resistance to benzimidazoles is widespread in the UK (Lester et al, 2013) and there is concern about increasing resistance to pyrantel embonate. Evidence of reduced effectiveness of macrocyclic lactones (as demonstrated by reduced egg reappearance periods after treatment) is commonplace (Mair et al, 2024), with resistance demonstrated by reduced faecal egg count reduction tests 2 weeks post-treatment identified in the UK (Bull et al, 2023). Anthelmintic resistance is also a major issue in Parascaris spp. and is emerging in tapeworms (Table 1). With no new equine anthelmintics likely to be developed soon, every possible method should be used to reduce the use of the currently effective anthelmintics, because overuse promotes resistance.
Helminth control programmes
The primary aim is to minimise levels of infective worm stages on pasture to mitigate the risk of parasite-associated disease in individuals grazing that pasture. Previously, when anthelmintics were initially developed in the 1960's, regular (interval) treatments were recommended; this was very effective in reducing the levels of infection. However, interval treatments encourage resistance, and this approach is no longer effective and must be avoided. Control measures should focus on evidence-based assessments to determine which horses require and, importantly, which do not require, anthelmintic treatment. This targeted policy of anthelmintic use is not new, having been recommended for many years (Proudman and Matthews, 2000). However, at the same time, regular treatment with larvicidal doses of anthelmintics was advised in the autumn or winter to target cyathostomin larvae (including encysted larvae) that may have accumulated over the grazing period (Rendle et al, 2019). Limited sales and diagnostic test data suggest that a diagnostic-led approach to anthelmintic use is not commonly used (Rendle et al, 2021). Furthermore, the results of one horse owner survey indicated that around 50% of respondents routinely administered an anthelmintic (most commonly a macrocyclic lactone) once a year regardless of faecal egg count results (Mair et al, 2023). Anthelmintics authorised to treat encysted cyathostomin larvae include fenbendazole (for five consecutive days) and moxidectin. The practice of administering a routine larvicidal anthelmintic dose (usually moxidectin) is now considered inappropriate and is no longer recommended in horses that are well managed and at low risk of disease. Identifying low risk horses by means of a risk assessment forms the basis of current control guidelines.
Diagnostic tests
Tests are an essential part of the diagnostic-led approach to control.
Faecal egg counts tests
Faecal egg count tests can be used for:
- Monitoring the level of nematode egg shedding in the faeces – this provides an estimation of the magnitude of egg shedding (usually of strongyles, primarily cyathostomins) but gives no indication of the total worm burden of the horse as male worms and larval stages do not produce eggs
- Testing efficacy of anthelmintics (via faecal egg count reduction tests)
- Detecting ascarid eggs in foals, weanlings and yearlings
The faecal egg count is not a reliable technique for identifying tapeworm eggs. It should also be recognised that faecal egg count tests do not provide any information about the role of parasites in a diseased individual or the risk of parasite-related disease. Sample collection, storage and handling methods can all affect faecal egg count results (Lester and Matthews, 2014); a version of the McMaster technique is the most used method.
Faecal egg count reduction tests
Faecal egg count reduction tests measure efficacy of a specific anthelmintic against a parasite population in a single, or preferably, group of horses grazing the same pasture. Two faecal egg count tests are performed on the individual or group – one on the day of anthelmintic administration and a second 14 days later (Kaplan et al, 2023). The mean reduction in faecal egg count is calculated and assessed against faecal egg counts reductions reported when each anthelmintic was first licensed.
Small redworm blood test
The small redworm blood test is a serum enzyme-linked immunoassay that measures antibodies to host adult and larval cyathostomins. The test detects cyathostomin protein-specific serum IgG(T) and is capable of detecting the presence or absence of total cyathostomin burdens (larval and adult stages) in the 1000–10 000 total cyathostomin count range (Lightbody et al, 2024). Current CANTER and BEVA guidelines recommend caution in its use, but the test can be useful for horses in low infection-risk situations where strongyle egg shedding is low (<200 eggs per gram) to help confirm that anthelmintic therapy is not indicated, instead of traditional blanket treatments which are often applied to all horses at certain times of year (Matthews et al, 2014).
Tapeworm ELISA
ELISA tests have been validated for measuring specific IgG(T) antibody in serum or saliva samples to Anoplocephala spp. antigens (Lightbody et al, 2016). These tests are useful for determining the need for anti-cestode treatments on a given yard or in an individual horse.
Egg reappearance periods
The egg reappearance period is a measure of how long an anthelmintic treatment suppresses worm egg shedding, thereby providing an assessment of effectiveness. Reductions in the egg reappearance period compared to when the anthelmintic was first introduced are likely to be an early indication of developing resistance.
Pasture management
The most effective way to control parasite-associated disease is to limit the intake of infective larvae from pasture, or in the case of tapeworm, to limit the ingestion of the intermediate oribatid mite on pasture. Details of pasture management recommendations are beyond the scope of this article; the key considerations are sum-marised in Table 2 (Herd, 1986; Mfitilodze and Hutchinson, 1987; Tzelos et al, 2017; Haseler et al, 2023).
Table 2. Management considerations relating to equine helminth control in the UK
| Management strategy | Recommendations |
|---|---|
| Regular removal of faeces | Twice a week during the grazing period (eg March–October)Once a week in winter (eg November–February) |
| Lowering stocking density | One horse to at least 0.4–0.6 hectares (1–1.5 acres) |
| Resting pastures | End of one grazing period to middle of next grazing period |
| Grazing pastures with other species | Co-graze with sheep or cattle (consider liver fluke) |
| Pasture hygiene post anthelmintics | Where feasible, remove all faeces contaminated by anthelmintics for 7–8 days following treatment |
| Pasture hygiene to protect dung beetle populations | Remove older faecal piles (>3–4 days-old) and leave more recent faecal piles until the next collection |
| Muck heaps | Position muck heap at least 3 metres outside of grazing areaPosition muck heap at least 10 metres away from water coursesCover muck heap (eg with a tarpaulin) to reduce the risk of rainwater run-offCompost muck for at least 6 months before spreading on fields |
Risk assessment
Current recommendations advise careful consideration to establish a balance between the potential harms to horses from the promotion of resistance and to the environment through ecotoxic effects, with the risk of contaminating pasture with many eggs, which lead to increased risk of disease. The risk of helminth-associated disease in adult horses is generally low, and most well-managed healthy horses may require no or limited anthelmintic treatments; however, the situation in foals and young horses is different. A risk assessment-based approach is recommended to limit the use of drugs only to horses that need treatment and avoiding ad-hoc treatment without evidence of need.
Several risk assessment tools are available, but most have a similar approach. An example of a risk assessment table (based on the CANTER guidelines) is shown in Table 3. To perform an appropriate risk assessment for adult horses, routine testing should be undertaken, including:
- Strongyle faecal egg counts 3–4 times over the grazing period. Counts less than 200 eggs per gram are considered low, and do not justify treatment. Counts ≥200 eggs per gram in an individual are considered moderate and may justify treatment for that horse. However, in low-risk situations, the threshold for treatment can be increased, for example, to ≥500 eggs per gram
- Ideally, annual faecal egg count reduction tests in horses with faecal egg counts of ≥200 eggs per gram, especially in situations where anthelmintic resistance is suspected
- Tapeworm enzyme-linked immunoassay (serum or saliva) should be performed once a year (twice a year where tapeworm disease is suspected or identified in the past, or where the majority of horses have scores above the treatment threshold). Only horses with a score above the low threshold result should be treated for tapeworms
- A small redworm blood test may be considered in horses deemed to be at low risk of cyathostomin infection to convince owners that such horses do not require routine autumn or winter larvicidal treatments.
Table 3. Risk assessment example for applying to adult horses
| Risk factor | Low risk | High risk |
|---|---|---|
| Clinical history | No history of helminth-associated disease on the premises or in the individual | History of helminth-associated disease (cyathostomins or tapeworms) on the premises or in the individual |
| Age profile | 5–15 yearsNo youngstock | 1–5 yearsYoungstock presentGeriatric horses and horses with pituitary pars intermedia dysfunction |
| Stocking density | Low stocking density: >2 acres per horse | High stocking density: <2 acres per horse |
| Test results (individual horse on pasture) | Repeated low faecal egg count, low tapeworm antibody + low small redworm antibody results | Moderate–high faecal egg count and tapeworm antibody resultsAnthelmintic resistance identified by faecal egg count reduction test |
| Test results (more than one horse co-grazing) | Repeated low whole herd faecal egg count, low tapeworm antibody + low small redworm antibody results | Moderate–high whole-herd or individuals faecal egg count and moderate to high tapeworm antibody resultsAnthelmintic resistance detected in at least one horse in the group |
| Pasture management | Closed herdFaeces removal more than once a weekQuarantine procedures in place | Movements of horses in and out of the herdFaecal removal less than once a week or not performedHarrowing performedNo quarantine procedures |
Foals, yearlings and young adults
Anthelmintic resistance in ascarids is common (Table 1). However, resistance patterns are variable, so faecal egg count reduction tests should be performed to establish efficacy on site. Foals are more susceptible to helminths and associated disease, so routine treatment is often required to treat ascarids. Strongyloides westeri can be associated with gastrointestinal disease in foals, although this is uncommon (Reinemeyer and Nielsen, 2017). Routine treatment of mares in late pregnancy is not recommended.
At 2–3 months of age, foals should be administered fenbendazole to treat ascarids. Targeted treatments using faecal egg count results are not recommended at this age because ascarid larvae can be pathogenic and the prepatent period is around 10 weeks (CANTER, 2024). Between 4 and 9 months, treatment should be based on faecal egg count results. Fenbendazole should be used if ascarid eggs are detected, and ivermectin or moxidectin if strongyle eggs are detected (moxidectin should not be used in foals <4 months old). Tapeworm ELISAs should be used from 6 months to determine the need to administer anti-cestode anthelmintics. Saliva tests should only be used in weaned foals since antibodies in mare's milk can contaminate saliva (Rendle et al, 2019). Tapeworm infections can be particularly problematic on some premises and may increase the risk of colic and intussusceptions. Pyrantel used at double the anti-nematode dose can be administered to foals >4 weeks old. From 9 months, anthelmintic treatments should be based on faecal egg count tests. Weanlings and yearlings are particularly susceptible to accumulating high helminth burdens, and an autumn or early winter cyathostomin larvicidal treatment (eg moxidectin) should be considered at this time. Like-wise, young horses (1–4 years old) are susceptible to accumulating high burdens while grazing (especially where there is poor pasture management) and are at higher risk of developing acute larval cyathostominosis than adult horses; an annual larvicidal treatment is recommended in late autumn or early winter in such horses (CANTER, 2024). In all cases, an annual faecal egg count reduction test should be performed to assess effectiveness of the deworming products being administered.
Conclusions
Equine helminth control should rely on a targeted and diagnosticled approach to limit the further development of anthelmintic resistance and minimise the environmental effects of anti-parasitics. Recent survey results confirm that many horse owners are not adopting this approach, identifying a need for prescribers and others to encourage behaviour change.
KEY POINTS
- Equine helminth control should have a targeted and diagnosticled approach.
- Anthelmintic resistance is a growing problem globally, and resistance to all of the authorised classes of anthelmintics is being increasingly recognised among cyathostomins, ascarids and tapeworms.
- Over-use of anthelmintics promotes the development of resistance.
- Pasture management is a vital component of parasite control and should be prioritised in order to reduce the risk of parasite transmission when grazing.
- Routine administration of an anthelmintic at the end of the grazing system is not recommended and should be determined by a risk assessment.
- Regular testing, using faecal egg counts, tapeworm ELISAs and where appropriate, faecal egg count reduction tests and small redworm blood test should be used.
- The approach to worming foals and young horses should be different than that of adult horses.