Surveys of horse owners have identified a poor level of understanding of appropriate equine de-worming practices. There is also a disconnect between owners and vets when it comes to obtaining advice on anthelmintic use (Relf et al, 2012; Stratford et al, 2014). Although an increasing number of owners are aware of the need to target anthelmintic treatments, a minority seem to put these principles into practice appropriately. Owners are often looking for advice on worming at the end of the grazing season, which provides an opportunity to ensure they are getting the right advice but also provides an opportunity to encourage diagnostic-led de-worming throughout the following grazing season.
Of all the equine endoparasites, cyathostomins are the most important cause of disease in adult horses and their control is therefore the focus of any equine de-worming strategy. Traditionally, autumn and winter de-worming has also included blanket treatments for tapeworms (Anoplocephala spp.) and bots (Gasterophilus spp.). In foals, ascarids (Parascaris) and threadworms (Strongyloides) also have to be considered.
When making decisions for clients on de-worming policies there can be a feeling of vulnerability. On the one hand there is pressure to dissuade owners from using anthelmintics, while on the other hand there is fear regarding being held responsible if any strategy goes wrong and clinical disease develops. It is becoming more widely acknolwedged that there has been too much attention put on the individual, at the expense of the population, and as the prevalence of resistance increases the relative threat to the population increases. Furthermore, adult horses (>4 years old) develop robust (but variable) immunity to cyathostomins and it is frequently forgetten that they can tolerate relatively large burdens of cyathostomins without developing clinical disease (American Association of Equine Practitioners Parasite Control Subcommittee, 2013). While the administration of ‘larvicidal’ anthelmintics in the autumn has become accepted, there is no evidence that this reduces the risk of clinical disease associated with mass emergence of cytathostimins in the winter or spring.
Is treatment for encysted cyathostomins required?
Treatment for larval cyathostomins should no longer be considered the default position, and should only be performed after consideration of the risk of parasite-associated disease. The level of risk will be determined by horse factors (such as age, immunity and results of diagnostic tests) and by environmental factors (such as stocking density, horse movements and pasture management). Factors that determine the level of risk are highlighted in Table 1 (Rendle et al, 2019). Classifying horses as low, medium or high risk can help in determining the required frequency of diagnostic testing and appropriate. The availability of a serological test to assess levels of exposure to cyathostomins prompted great excitement, but it remains difficult to translate the results of the test into practical recommendations as the association between antibody levels and risk of disease is yet to be elucidated. Anecdotally, serology results seem to suggest that horses are at higher risk than one would have anticipated based on the results of faecal worm egg counts and knowledge of management factors. There is a lack of validation of the serological test in low-risk populations and, for fear of over-treating horses that do not require larvicidal treatments, the author refrains from using serology where management is known to be good and faecal worm egg counts have provided reassurance that the level of pasture contamination is low.
Table 1. Risk factors for parasitic infection
| Factors contributing to low risk | Factors contributing to moderate risk | Factors contributing to high risk |
|---|---|---|
| Repeated negative faecal egg counts and tape-worm antibody levels | Low to moderate faecal egg counts and antibody levels | High faecal egg counts and antibody levels |
| Cohorts negative faecal egg counts and tape-worm antibody levels | Cohorts low faecal egg counts/tapeworm antibody levels | Cohorts high faecal egg counts and tapeworm antibody levels |
| 5–15 years old | >15 year-old | <5-years-old |
| Faecal collection >twice per week | Sporadic faecal collection | No faecal collection |
| Good pasture management | Moderate pasture management | Poor pasture management |
| Stable population | Occasional movement | Transient population |
| Low stocking density | Medium stocking density | High stocking density |
| No youngstock | Grazing with youngstock | |
| Effective quarantine | No quarantine | |
| No history of parasitic disease | History of parasitic disease | |
| No history of colic | History of colic | |
| Anthelmintic resistance identified on site by faecal egg count reduction tests |
When to treat for encysted cyathostomins
As the ambient temperature drops in the autumn the conditions for cyathostomin egg hatching and larval development on pasture become less favourable and grazing horses are exposed to a reduced risk of infection. The decrease in temperature is also associated with a reduction in egg production by adult intra-luminal cyathostomins (Poynter, 1954). The first frost is often cited as being the time to administer a larvicidal dose of anthelmintic in order to eliminate as many parasites as possible from within the host, at a time when there are minimal parasites developing on pasture. This was logical when the aim of anthelmintic administration was to eliminate parasites. However, now that the aims are to minimise selection pressure for anthelmintic resistance while preventing clinical disease, the routine administration of anthelmintics at a time when the majority of the endoparasite population within the ecosystem will be exposed is undesirable. A central concept in delaying the onset of anthelmintic resistance is that of ‘refugia’, the idea that it is beneficial to have parasites within an ecosystem, that are not exposed to anthelmintics. The presence of refugia reduces selection pressure and delays the onset of resistance. Therefore, aiming to treat when there are lower numbers of parasites outside the horse is likely misguided. The presumption that eggs and parasite numbers on the pasture will reduce over the winter is also likely to be misplaced, as most are capable of over-wintering.
What to administer as a larvicidal dose for cyathostomins
Moxidectin is the most effective treatment for eliminating larval cyathostomins. Administration of fenbendazole for 5 sequential days used to be an alternative, but resistance of cyathostomins to fenbendazole is virtually ubiquitous and this treatment should only be used if faecal egg count reduction tests have been performed to confirm the absence of resistance. Moxidectin is therefore preferred over the alternatives for the treatment of clinical disease caused by larval cyathostomins, or to prevent disease where the perceived risk is high. If pasture contamination has been controlled throughout the grazing season (and has been monitored by performing faecal worm egg counts on horses on the property), then levels of larval infection will be low and may not need to be reduced further. Ivermectin is effective against adult and luminal larval stages and may be sufficient as a prophylactic treatment, with the added benefit of the encysted stages serving as a source of refugia.
Treatment for tapeworms
Treatment for tapeworms with either praziquantel or a ‘double dose’ of pyrantel has historically been performed once or twice per year in the autumn and/or spring. Evidence for this approach is somewhat lacking. Praziquantel is often administered in combination with ivermectin or moxidectin in the autumn, to treat both roundworms and tapeworms in a single administration. The intermediate hosts for tapeworms are the oribatid mites, which are free living on pasture and are susceptible to extremes of temperature and hydration. Treatment for tapeworms was thus advised in the colder months, when mite numbers were low, to break the tapeworm life-cycle. Although anthelmintic resistance is of less concern in tapeworms than it is in cyathostomins, the principle of administering treatments to break the life cycle of a parasite (and thus eliminate it) is outdated and unlikely to be effective.
Infection with A. perfoliata has been associated with colic and specifically spasmodic colic, ileal impactions and ileocaecal lesions. However, compelling epidemiological evidence that tapeworms are a major cause of colic is lacking (Nielsen, 2016). Where multiple cases of colic have been reported, it has tended to be in young horses that have not received an anthelmintic effective against tapeworms for a number of years. Tapeworms produce small mucosal erosions at the site of attachment and when present in relatively high numbers, may cause clinically relevant inflammation and oedema. However, most horses infected with tapeworms have very few parasites, and these are unlikely to produce significant intestinal damage.
In some horses, treatment targeted at tapeworms may not be necessary at all. Horses that are well managed are unlikely to have clinically significant tapeworm burdens. Where the risk of clinical disease is small, a serological test or faecal analysis can be performed to determine levels of tapeworm infestation rather than administering routine treatment. Where treatment is considered necessary, the use of praziquantel over pyrantel is preferred as it is specific to tapeworms and does not place a selection pressure on other parasites. Registered praziquantel-only products are no longer available in the UK, although an extemporaneous preparation is available for use in accordance with the cascade and should be used in favour of combination products if there is no demonstrable need for cyathostomin treatment.
A further consideration in assessing the need for an autumn tapeworm dose is the efficacy of the lower dose of pyrantel against tapeworms. While a 13.2 mg/kg dose was chosen for product registration, the standard roundworm dose of 6.6 mg/kg eliminated over 80% of tapeworms in one study (Lyons et al, 1989). In the majority of horses in which tapeworm burdens are low, administration of a standard ‘single’ dose of pyrantel is likely to be more than adequate for preventing clinical disease. Where pyrantel is used for controlling roundworms through the grazing season (ideally suited to this purpose as its use reduces the selection pressure on moxidectin or ivermectin), and the risk of tapeworm-associated disease is considered low, there may be no need for a specific treatment targeted at tapeworms in the autumn.
Bots
Ivermectin and moxidectin are efficacious against Gasterophilus spp., but pyrantel, fenbendazole and mebendazole are not. However, bots are not considered to be significant pathogens in horses unless they are present in exceedingly high numbers and anthelmintics should not be administered with the sole purpose of controlling them. They are likely to be controlled while controlling other endoparasites. Fly control and egg removal should be performed where the risk of Gasterophilus is perceived to be high.
Large strongyles
In the UK, disease associated with the large strongyles is exceedingly rare currently as a result of these parasites' sensitivity to anthelmintics, coupled with decades of indiscriminate anthelmintic use. However, in Denmark, cases of colic associated with Strongylus vulgaris have started to recur following a dramatic reduction in anthelmintic use as a result of legislation prohibiting anthelmintic use without demonstrable need. Given the potentially fatal nature of disease associated with S. vulgaris and the absence of commercially available tests to detect the parasite, it is reasonable that a drug effective against S.vulgaris is administered once per year. Both ivermectin and moxidectin will be effective with the former being the preferred choice in order to reduce selection pressure against moxidectin.
Foals and youngstock
Immunity to parasites develops with time and exposure. Therefore, some exposure is desirable to promote the development of immunity. However, this has to be balanced carefully against the increased susceptibility to parasite-associated disease. Administration of anthelminthics is advisable in the autumn for any horses less than 5 or 6 years of age, to prevent disease associated with cyathostomins or ascarids. Disease associated with tapeworms is less of a concern in foals in their first grazing season. Faecal worm egg counts would ideally be performed to determine whether treatment needs to be targeted against ascarids, cyathostomins or both.
Conclusions
Autumn is a time when most horses have traditionally received some form of anthelmintic treatment by default. However, where possible, it is preferable to avoid blanket treatment to reduce the selection pressure and the development of resistance. For each horse of over 5 or 6 years of age, there are a number of questions that can be asked:
- Is treatment necessary if management has been good throughout the grazing season?
- Could serology be used to assess likely levels of infection?
- Is moxidectin truly necessary, or could its use be spared with the use of ivermectin?
- Is a specific treatment for tapeworms really required?
- What could be done to reduce the need for anthelmintics next year?
- Is there a risk of infection with large strongyles if no other treatment has been administered in the preceding year? Should ivermectin be administered?
Systems of monitoring and targeted de-worming are not only proven to reduce anthelmintic use, but also reduce the overall cost of endoparasite control (Lester et al, 2013).
KEY POINTS
- Blanket worming of all horses in the Autumn is no longer appropriate.
- Risk assessment for each horse should be performed.
- The risk of disease to the individual needs to be balanced against the wider threats of resistance and environmental damage.
- Treatment decisions should be informed by diagnostics where possible.
- Plans should be made now to reduce the use of anthelmintics in the next grazing season.