21 Sept 2015
David Rendle
Job Title
Figure 1. Worm egg counts and faecal egg count reduction tests should be central to the development and implementation of plans for worm control on all properties.
Questionnaire studies have identified a poor level of understanding of appropriate equine deworming practices among horse owners and a disconnect between owners and vets when it comes to obtaining advice on anthelmintic use (Relf et al, 2011; Stratford et al, 2014).
Although an increasing number of owners are aware of the issues of increasing anthelmintic resistance, and the need to target anthelmintic treatments, a minority 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 not only to ensure they are getting the right advice now, but also to encourage targeted worming next year.
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 deworming strategy.
Traditionally, autumn and winter deworming has also included blanket treatments for tapeworms (Anoplocephala species) and bots (Gasterophilus species). In foals, ascarids (Parascaris) and threadworms (Strongyloides) are further considerations.
The primary purpose of deworming strategies used to be to eliminate parasites. This strategy failed and has had the unintended consequence of selecting for parasites resistant to anthelmintics. The focus should, therefore, be on preventing clinical disease and maintaining good health in the context of an acceptance horses have always lived with, and will always live with, a burden of endoparasites. The presence of parasites may even confer some health benefits to the host.
When making decisions for clients on deworming policies, there can be a feeling of vulnerability – on the one hand we, as clinicians, are under pressure to dissuade owners from using anthelmintics, while on the other we will be held responsible if any strategy goes wrong and clinical disease develops. Fortunately, adult horses (older than four years old) develop robust (but variable) immunity to cyathostomins and can tolerate large burdens of cyathostomins (Nielsen et al, 2013) so there is a reasonable margin for error.
As the ambient temperature drops in 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 associated with a reduction in egg production by adult intraluminal cyathostomins (Poynter, 1954).
The first frost is often cited as a trigger for the administration of a larvicidal dose of anthelmintic to eliminate as many parasites as possible from within the host at a time when minimal parasites are developing on pasture.
This was logical when the aim of anthelmintic administration was to eliminate parasites. However, now the primary aim is to prevent development of clinical disease while minimising selection pressure for anthelmintic resistance, 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 it is beneficial to have parasites within an ecosystem that are not exposed to anthelmintics. This reduces selection pressure and delays the onset of resistance. Therefore, a better approach to addressing larval cyathostomin burdens would be to avoid worming after cyathostomins present on the pasture have been reduced in number by lower temperatures.
Administering anthelmintics when there is still the potential for reinfection from pasture will increase the burden of cyathostomins carried within the host over winter; however, as long as pasture contamination has been controlled (and preferably monitored using faecal worm egg counts; Figure 1) through the grazing season, the numbers ingested ought to be small and pose no risk to the host (Figure 2).
Furthermore, the survival of parasites over winter will serve to increase refugia the following year.
While luminal cyathostomins will be killed by ivermectin, moxidectin, pyrantel, fenbendazole and mebendazole, larval stages are less susceptible to anthelmintics (Figure 3). Moxidectin is the most effective and will eliminate around 60% to 90% of L3 and L4 larvae, while ivermectin has limited activity and other classes are ineffective (Xiao et al, 1994; Monohan et al, 1995).
Hypobiotic L3 larvae are even more challenging to eliminate and reports of efficacy for both ivermectin and moxidectin are highly variable, ranging from 10% to 90% (Eysker et al, 1992; Klei et al, 1993; Bairden et al, 2001) with moxidectin generally considered to be more effective. Moxidectin is therefore preferred over ivermectin in the treatment of clinical disease resulting from larval cyathostominosis, but does this mean it is the most appropriate treatment for routine, and essentially prophylactic, treatment in autumn?
Where pasture contamination and, therefore, risk of infection has been controlled throughout the grazing season, levels of larval infection will be low and may not need to be reduced further. Encysted larvae that overwinter within the horse provide a valuable source of refugia. Serological tests that provide an indication of levels of cyathostomin infestation are under investigation and will prove valuable in determining the need for larvicidal doses of anthelmintics.
Treatment for tapeworms with either praziquantel or a “double dose” of pyrantel is typically performed once or twice per year in autumn and/or spring. Praziquantel is often administered in combination with ivermectin or moxidectin in autumn to treat both roundworms and tapeworms in a single administration.
The intermediate hosts for tapeworms are the ori-batid mites, which are free-living on pasture. They are susceptible to extremes of temperature and hydration and therefore reduce in number over the winter.
Treatments aimed at tapeworms were advised in winter or in spring and autumn if the risk of infection was judged to be high. Although anthelmintic resistance is of less concern in tapeworms than it is in cyathostomins, the principle of administering treatments in an attempt to break the life cycle of a parasite (and thus eliminate it) is outdated, in the author’s opinion.
Infection with Anoplocephala 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 et al, 2015). 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 have repeatedly received annual or biannual treatments while being well managed are unlikely to have clinically significant tapeworm burdens. Where the risk of clinical disease is small, a serological test or faecal analysis should be performed to determine levels of tapeworm infestation rather than administering routine treatment.
Serological tests, while convenient, are less reliable at an individual horse level and tend to overestimate infestation (Nielsen et al, 2015), therefore modified egg counting techniques should be considered at an individual horse level.
Where treatment is considered necessary, use of praziquantel over pyrantel is appealing as it is specific to tapeworms and does not place a selection pressure on other parasites.
A praziquantel-only product is available 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 13.2mg/kg dose was chosen for licensing purposes, the standard roundworm dose of 6.6mg/kg eliminated more than 80% of tapeworms in one study (Lyons et al, 1989).
In the majority of horses in which tapeworm burdens are low, this may be more than adequate for preventing clinical disease. Where pyrantel is used one or more times for the purpose of controlling roundworms through the grazing season, and the risk of tapeworm-associated disease is considered low, there may be no need for a specific treatment targeted at tapeworms in autumn. Pyrantel is an appropriate choice for the control of cyathostomins through the grazing season as benzimidazole resistance is effectively ubiquitous and pyrantel use reduces the selection pressure on ivermectin and moxidectin.
Ivermectin and moxidectin are efficacious against Gasterophilus species; pyrantel, fenbendazole and mebendazole are not.
However, bots are not considered to be significant pathogens in horses and anthelmintics should not be administered with the sole purpose of controlling them. They are likely to be controlled in the course of controlling other endoparasites.
In the UK, disease associated with the large strongyles is now incredibly rare 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 a drug effective against S vulgaris is administered once per year. Ivermectin and moxidectin will be effective, with the former being the preferred choice to reduce selection pressure against moxidectin.
Immunity to parasites develops with time and exposure. Some exposure is therefore desirable to promote the development of immunity; however, this has to be carefully balanced against the increased susceptibility to parasite-associated disease. Administration of anthelmintics is therefore advisable in autumn for horses less than three or four years of age. Faecal worm egg counts would ideally be performed to determine whether treatment needs to be targeted against ascarids, cyathostomins or both.
Autumn is a time when most horses will require some form of anthelmintic treatment. However, where possible, it is preferable to avoid blanket treatment to reduce the selection pressure for resistance. For each horse of older than four or five years of age, a number of questions can be asked.
If it is difficult to answer these questions then there would be merit in encouraging monitoring with faecal analysis, and possible serology, through the following grazing season to inform future decision-making. Systems of monitoring and targeted worming are not only proven to reduce anthelmintic use, but also reduce the overall cost of endoparasite control (Lester et al, 2013).
