7 Mar 2019
David Rendle reviews a meeting of eight vets and researchers that convened to discuss ideal guidance for horse deworming.
David Rendle
Job Title
Less is known about this condition and its pathogenesis in the foal than in adult horses.
Eight vets and researchers with an interest in the control of equine parasites came together at a meeting to discuss best practice in equine deworming.
Decades of regular and often indiscriminate administration of anthelmintics have compromised efficacy of most, if not all, of these products licensed for use in horses.
The meeting and resultant guidelines were commissioned by UK-Vet Equine to provide veterinary surgeons with up-to-date information. The meeting was hosted by the Moredun Research Institute, with sponsorship from Virbac and additional support from The Horse Trust and VetPartners.
Appropriate stocking, pasture management and quarantine are critical to preserving the efficacy of anthelmintics. Faecal egg count (FEC) monitoring allows identification of groups that are shedding higher numbers of eggs and, therefore, paddocks that are more heavily contaminated.
Stocking density in these paddocks should be reduced and efforts to reduce pasture contamination increased. Good pasture management is effective in reducing exposure to parasites and is particularly important if stocking density is high.
A good level of awareness exists that interval dosing in line with egg reappearance periods is an obsolete concept. Occasional strategic preventive treatments are indicated in some circumstances, particularly in foals, but most treatments can be targeted based on evidence of risk of disease in the individual or the population and supported by diagnostic tests.
It is concerning that FEC reduction tests (FECRTs) are rarely performed in equine practice. The test is simple, but requires the use of a quantitative FEC method that has a low multiplication factor. FEC are performed prior to, and 10 to 14 days after, anthelmintic treatment.
Anthelmintic resistance (AHR) cannot be determined reliably unless results are available for at least 6, and preferably 10, horses.
Performing FECRT following treatment with fenbendazole can be an effective means of gaining engagement with the importance of monitoring for AHR. Some commercial providers offer a free follow-up FEC to encourage best practice.
Group FEC reduction (%) =
pre-treatment group mean FEC –
post-treatment group mean FEC × 100
Pre-treatment mean FEC
Less than 50% of adult horses in the UK are infected with adult tapeworms; small numbers of tapeworms are not considered pathogenic, and clinical disease associated with tapeworms in adult horses is rare, so the traditional approach to treating for tapeworms annually or six-monthly without diagnostic testing is obsolete.
Exposure to tapeworms can be determined reliably using either a serum or saliva antibody test. False-negative results are rare, and both tests reliably identify horses that may be at risk of tapeworm-associated disease.
Fenbendazole, pyrantel, ivermectin and moxidectin are all licensed for reducing patent strongyle numbers and reducing egg excretion during the grazing season.
Resistance to fenbendazole is ubiquitous and the prophylactic use of moxidectin ought to be avoided to preserve its efficacy.
Ivermectin and pyrantel are the preferred drugs for routine treatment in response to high FEC. Traditionally, it was advised to “dose and move” to reduce numbers of helminths transmitted to clean grazing. This serves to reduce refugia, thereby increasing selection pressure, and is contraindicated on most properties with reasonable management.
The need for a strategic treatment at the end of the grazing season aimed at larval cyathostomins will depend on the level of exposure and risk of infection. Most horses will receive moxidectin, but in well-managed horses this may be unnecessary.
An alternative, or no, treatment may be more appropriate. When specific treatment for tapeworms is required, the use of pyrantel should be avoided in favour of praziquantel, which has a narrower spectrum.
The licensed praziquantel-only product has been withdrawn in the UK; however, praziquantel paste is available in the UK via veterinary surgeons as a “special” formulation. While pyrantel is licensed for the treatment of tapeworms at 13.2mg/kg of pyrantel base (“double dose”), a 6.6mg/kg (“single dose”) has been demonstrated to reduce tapeworm numbers by 60% to 100%, so horses that have received a single dose for tapeworms are unlikely to require specific tapeworm treatment.
If treatment is being administered for tapeworms then the double dose should be used.
FECs and FECRTs should be used in foals to inform decision making. However, pre-patent infection is an important cause of disease and strategic treatments also need to be administered, particularly if stocking densities are high. Parascaris equorum is an important cause of disease, and resistance is common.
Migrating larval stages cause respiratory disease in young foals, and patent infections are a common cause of (potentially fatal) colic. Clinical signs in foals are typically seen from late summer or early autumn.
Strongyloides westeri is rarely a cause of disease and preventive treatment targeted against this parasite is not warranted unless a history of disease exists on the property. Strategic deworming doses will be necessary in foals and are discussed in greater detail in the article.
It is well accepted in human psychology that human decision-making is not rational. We know from surveys of horse owners that they are interested in deworming, but these surveys also corroborate more general human behaviour studies in confirming social norms are a more powerful influence than professional advice; horse owners are more likely to do what everyone else on the yard does than what the vet advises them to do.
Most owners indicate they engage with targeted worming, yet, when questioned on what they actually do, the responses indicate most are still deworming at frequent intervals, with the results of FEC failing to guide the need for treatment.
While most barriers can be overcome, assistance may be necessary to support owners in applying appropriate deworming and management strategies for their horses.
It is recommended advisers aim to discuss issues around deworming to help and support owners to overcome such problems, rather than simply telling them which anthelmintic to use and the results of FECs and assuming they will be able to manage appropriately.
Discussion with owners should break deworming down into five key areas – for example, paddock maintenance, FEC, choosing an appropriate wormer, worming the horse and efficacy testing. Breaking the process down into stages may help both the advisor and owner to identify particular problem areas, and plan for effective strategies to appropriately treat the horse or horses. Behavioural science also shows people usually respond well to success stories.
A reduction in anthelmintic use is imperative to prevent increased morbidity and mortality as a result of parasitic disease in the future. Resistance to multiple anthelmintic classes on the same property now appears to be common, particularly on premises with large numbers of youngstock. Youngstock move widely as they mature, making it inevitable that AHR will spread.
The principles of sustainable anthelmintic use are simple; the factors that prevent their implementation are more complex. The success of any targeted worming plan hinges on changing human behaviour. A full article about the meeting is available at www.magonlinelibrary.com/journal/ukve
