IN most parts of the world, only three anthelmintic drug classes are available for treatment of nematode parasites in horses. These include the benzimidazoles, the pyrimidines, and the macrocyclic lactones.

The newest anthelmintic class to be introduced for equine usage was the macrocyclic lactones with ivermectin being launched in the early 1980s. Thus, no new anthelmintic classes with new modes of action have been introduced for equine usage during the past 40 years, and no such new products are expected in a foreseeable future.

This emphasises the need to monitor efficacy of existing anthelmintics and work towards identifying the most sustainable parasite control strategies. It is important from an owner perspective to understand what each class is, the different brands that may contain the same active ingredient and the use of each (See Table 1).

How well do worming products actually work?

When we talk about efficacy of a product, we are referring to how well it works against a given parasite. This efficacy and in turn resistance to the products is evaluated in a few different ways.

The most common means is the fecal egg count (FEC) reductions test which evaluates the reduction in FEC from a before treatment sample to a post treatment sample taken two weeks later.

This can only be reliably used where the egg count pre-treatment was sufficiently high enough to show a reduction and it is also complicated in parasites in which egg shedding is not consistent.

Necropsy studies are frequently the most informative but ethically are difficult to justify. It is also important to note that no data is specifically available for Irish horse populations. Some work has been performed recently but is not yet published, and some farms have carried out their own efficacy studies. Therefore the information that is presented here is largely based on a review paper published this year that summarises available information from around the world.

Another important point is the distinction between label claim and current efficacy studies. The information provided with products is that from studies which were performed to gain the marketing authorisation (licence) for that product. The efficacy of (and resistance to) a product may change over time but this is not frequently reflected in the data sheets provided with the products.

Adult small strongyles

Benzimidazole resistance is widespread in equine strongylids around the world, as it has documented in every study evaluating this class published since 2000. Resistance to Pyrantal was documented in 92% of studies and 23% showed resistance to ML.

A 2020 study with over 110 yearlings, only the 59 imported from Ireland as weanlings displayed reduced ivermectin efficacy, while the US-born counterparts treated with the same products at the same time points displayed full efficacy on the FECRTs. This clearly suggested that the resistant cyathostomins were imported from Ireland and introduced to the US operation.

The study consistently documented reduced ivermectin efficacy in three different groups over several treatments, and also documented resistance to moxidectin among the imported yearlings.

Macrocyclic lactones (ivermectin and moxidectin) are the treatment of choice for adult small strongyles.

Larval stages of small strongyles

The concept of larvicidal efficacy deserves particular attention in the context of anthelmintic resistance. For cyathostomins, this term is used for anthelmintics with efficacy against encysted larval stages. This is particularly important in the Irish setting as it is larval cyathostomiasis that presents the greatest clinical threat to horses. The parasitic phase of the cyathostomin life cycle contains three distinct larval stages; the early third stage (EL3), the late third stage (LL3), and the mucosal fourth stage (ML4). Collectively, the LL3 and ML4 stages are referred to as developing larvae, whereas the EL3 can undergo arrested development for up to several years.

Two anthelmintic compounds are currently registered with efficacy against encysted cyathostomin larvae: moxidectin (single dose) and a five-day regimen of fenbendazole (See Table 2)

The fenbendazole regimen was originally reported to have high efficacy (>90%) against all larval stages, whereas moxidectin displayed lower and more variable efficacy levels with the percent reduction of EL3s ranging between 0 and 37% and the efficacy against LL3/ML4s in the 50–89% range. It should be mentioned that due to these data, moxidectin products do not have a label claim for efficacy against EL3s in North America (it does in Europe), whereas the fenbendazole regimen has label claims for all encysted larval stages

In recent years, two studies have investigated the larvicidal efficacy of the five-day fenbendazole regimen and three studies examined the same property of moxidectin. These studies were all terminal controlled efficacy studies based on mucosal digestion and enumeration of the encysted larval stages. The larvicidal efficacy of the five-day fenbendazole regimen against EL3s was 30–40%, which was a substantial reduction from historic levels, and, hence, confirmed resistance of all parasitic larval stages to fenbendazole.

In the first two studies evaluating moxidectin, larvicidal efficacy estimates at or above the higher end of the historic ranges (60–70%) were found. However, the most recent moxidectin study returned substantially lower efficacy estimates with 18% against EL3s.

Given the largely variable historic larvicidal efficacy estimates for moxidectin, it is not possible to determine if these three recent studies suggest a change from historic levels.

The take-home message

The important message here is that no product is 100% effective against EL3 and the choice of product recommended by your veterinarian should be based on historical farm factors. Both products are still widely used, and the choice of product will be influenced by farm factors and animal factors.

Moxidectin and fenbendazole (five-day regimen) are the treatments of choice for encysted cyathostomins. This recommendation may change or may become more dependent on diagnostic testing over the coming years.

Ascarids

Compared to the situation in cyathostomins, anthelmintic resistance in equine ascarids is a fairly new development with the first peer reviewed report occurring in 2002, although there is some evidence that early signs of resistance were identified in 1987.

Of 32 studies from 20 different countries and five continents evaluated efficacy against ascarids. Of these, 29 evaluated macrocyclic lactone efficacy and all reported evidence of resistance to this class. Four of 16 studies (25%) reported pyrimidine resistance, and three of 13 studies (23%) reported benzimidazole resistance.

Thus, it can be concluded that resistance to macrocyclic lactones is widespread around the world. Due to the limited number of studies that have evaluated pyrimidines and benzimadazoles, it is possible that resistance to these two anthelmintic classes may be more common in equine ascarids than suggested in the scientific literature. Researchers and clinicians around the world should be encouraged to monitor the efficacy of all these classes against Parascaris spp.

Some consideration should be given to the design of ascarid FECRT studies, as the biology is substantially different from the cyathostomins. Ascarid egg shedding is highly age dependent with foals and weanlings usually only shedding eggs for a short period of time before an age dependent immunity eliminates the parasites.

Typically, foals shed ascarid eggs between three and six months of age, with egg counts peaking at four-five months of age. Any efficacy study ideally needs to be carried out within this age category.

Benzimadazoles and pyrimidines are the treatment of choice for ascarids, but efficacy should ideally be evaluated on a farm level.

Tapeworms

There is no established method to evaluate treatment efficacy against tapeworms. Pyrimidines (pyrantel and praziquantel) are currently the treatment of choice for tapeworms. The recent introduction of diagnostic tests such as the saliva test are likely to lead to a decrease in blanket approach to tapeworm treatment.

Other control measures

By now, it should be clear to the reader that we are rapidly running out of options in terms of effective drug-based control of parasites and that other control measures should not only be considered but should take the forefront.

Co-grazing

Parasites are species specific. That means that equine parasites cannot infest cattle, sheep or other animals and vice versa. Bearing in mind that our strongylid infestations are picked up on pasture and ascarids can also be acquired on pasture, co grazing with other species can reduce the number of available eggs or larvae for consumption by the horse of they are eaten by the cattle or sheep.

Follow-on grazing

This refers to one species following another to ‘clean’ up the ground by removal of as many eggs/larvae as possible. This is usually done by having sheep graze equine pasture in the early autumn.

Faeces removal

It takes three-seven days for strongylid eggs to develop to infective larval stages. Therefore, if faeces is removed from the paddock before the infective larval stage has developed then there is nothing there to lead to parasitic disease. In many cases this is considered too labour intensive but the increasing availability of paddock sweepers is making this an easier and more affordable option.

On farms where removal could not be considered from all paddocks it should be given priority in some areas, e.g. foal paddocks. Many farms will have a number of small foal paddocks and removal of faeces from these paddocks can not only dramatically reduce the incidence of parasite associated disease and ill thrift in foals but can also decrease the incidence of other disease such as R.equi which is passed in large numbers in faeces.

Harrowing

Harrowing was thought to allow increased egg and larval destruction through dessication and heat destruction. However, the Irish climate does not have temperature extremes which allow for this and harrowing in the Irish setting merely facilitates the spread of the eggs/larvae across the paddock/field. Harrowing should be discouraged as a means of parasite control and if it must be performed as a means of paddock maintenance, then faeces removal prior to harrowing is required.