Large redworms (Strongylus vulgaris) are no longer as common, while roundworms (Parascaris equorum) are the main problem in young horses. Next, traditional worming methods have also led to a constant evolution of the worm population, so that today the main pathogenic worms in adult horses are small redworms* (Cyathostomin) and tapeworms (Anoplocephala perfoliata).

It is important however to be aware that resistance to wormers is highly variable from yard to yard, even within the same region, and therefore one cannot conclude that a given population of horses is resistant without specific and appropriate testing. However, these traditional practices also led directly to worms becoming resistant to chemical wormers (anthelmintic resistance), and also led to a change in the pathogenic worms found in horses today.īecause, firstly, it should be noted that the traditional interval and treatment of all horses creates strong selection pressure on worms for resistance to anthelmintic drugs (wormers) - and once resistance is present in a worm population, this does not appear to be reversible. These traditional practices were extremely effective in controlling the most common parasite at the time, Strongylus vulgaris, which can cause verminous arteritis, for example. Previously, the trend was to try to completely eliminate gastrointestinal parasites in horses, which involved treating all horses (in a herd) at regular intervals (e.g every eight weeks), with a seasonal rotation in the choice of wormer. Over the past two decades, our knowledge and understanding of internal parasites in horses has evolved considerably.

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