It is practically feasible to temporarily turn a grazing pasture into a hay field and recover the forage, which should greatly reduce the number of surviving infective larvae. However, evidence supporting this suggestion is lacking. Grazing contaminated pastures with ruminants may also assist in control (Eysker et al., 1986). Equine strongyle larvae are host-specific and cannot infect cattle, sheep, goats, or camelids. The only exception is the stomach worm, Trichostrongylus axei, which can infect both ruminants and equids, but this parasite rarely causes disease in either ruminants or horses. Ruminant liver flukes (Fasciola hepatica) can potentially infect horses as well, but this will be a rare and localized event and can only occur in areas enzootic for liver fluke.
The environmental control of strongyles using nematophagous fungi delivered to horses in feed has shown promising results (Healey et al., 2018), and a product is now available in North America. However, information is currently lacking on how to successfully integrate the product into a meaningful parasite control program.
8.3. Combination deworming Combination deworming has been shown to improve parasite control and reduce the development of resistance in both simulation and field studies performed on sheep, and is increasingly recommended in ruminants (Leathwick et al., 2012; 2015; Kaplan, 2020). However, there is scant data on this issue in horses. Equine anthelmintic products containing more than one active ingredient targeting the same parasite(s) are available in several countries in the Southern Hemisphere. Computer simulation studies have suggested that combining two new active ingredients, to which no resistance has developed, will effectively reduce the rate of resistance development (Leathwick et al., 2017). Furthermore, one study suggested value in combining a novel anthelmintic with an active ingredient for which anthelmintic resistance has already decreased the efficacy (Scare et al., 2020).
However, no new anthelmintic classes have been introduced for equine usage in the past 40 years, and no such products are expected within the foreseeable future. Thus, the evaluated scenarios described above do not exist and instead, combination deworming involves active ingredients which have been in use for decades, and anthelmintic resistance is often either already established or developing. One study evaluated a combination of oxibendazole and pyrantel pamoate against a population of cyathostomins resistant to both and found that the initially gained increase in efficacy was lost after a single administration of the combination (Scare et al., 2018), demonstrating that this approach was not sustainable.
Based on the above, we currently lack evidence to endorse combination deworming as a sustainable treatment approach for strongyle control. Veterinarians can attempt the combination of two or more active ingredients as an extra-label treatment approach on a case-by-case basis. However, it should be noted that in case of treatment of foals and weanlings, current drug efficacy profiles may result in the need for treatment with two different active ingredients in case the animals harbor both strongyles and ascarids. The off-label use of anthelmintics labelled for other species (e.g., ruminants), is inappropriate and may not be sustainable and is, thus, not recommended.
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