Sustainable Deworming
There has been a relatively big shift in deworming treatment strategies in recent years, due to increasing levels of worm resistance to the chemicals we have been using for decades to control them. Because of this there is a lot of confusion among horse owners – understandable, as many new recommendations turn the old rules upside down!
Intestinal parasites are not directly related to horse nutrition, however deworming is part of preventative equine health management, and so it is a topic I try to keep up to date with so I can properly advise clients if necessary. As the issue of resistance will ultimately affect everyone, I think it is important to communicate the scientific and veterinary research to as many people as possible, in as many ways as possible! So this article is a summary of an excellent review on the topic (Nielsen, Martin Krarup. "Sustainable equine parasite control: perspectives and research needs." Veterinary Parasitology 185.1 (2012): 32-44) – it’s several years old now, but many people are still unaware of this information!
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Traditionally, equine parasites have been controlled by identifying and targeting the most problematic species. In the 1960s deworming programs targeted S. vulgaris, the large strongyle, which was at the time the worm of most concern (and they can cause serious damage and even death). This was the birth of frequent deworming – every 8 weeks, as the drugs available were not effective against all life cycle stages. This was very successful, and by the 1980s this worm was almost non-existent in well managed horses.
But we still had worm problems! The focus through the 1980s and 1990s now shifted to cyathostomins, the small strongyles, when it was realised that the mass emergence of encysted small strongyle larvae which were not affected by early dewormers, could have life-threatening consequences. New drugs were developed, principally the 5-day fenbendazole course, and a little later moxidectin, to target the encysted stages, and again, the problem seemed solved – we could control both large and small strongyles.
Tapeworm was historically not considered a major problem, and very few drugs prior to 2003 controlled it. It is thought that the intensive use of popular dewormers that had no effect on tapeworm caused a relative increase in this worm, although this has been resolved by combining tapeworm-effective praziquantel with “ectin” drugs.
But we still had worm problems! The focus through the 1980s and 1990s now shifted to cyathostomins, the small strongyles, when it was realised that the mass emergence of encysted small strongyle larvae which were not affected by early dewormers, could have life-threatening consequences. New drugs were developed, principally the 5-day fenbendazole course, and a little later moxidectin, to target the encysted stages, and again, the problem seemed solved – we could control both large and small strongyles.
Tapeworm was historically not considered a major problem, and very few drugs prior to 2003 controlled it. It is thought that the intensive use of popular dewormers that had no effect on tapeworm caused a relative increase in this worm, although this has been resolved by combining tapeworm-effective praziquantel with “ectin” drugs.
Today our opponent our opponent is not so straight forward – no longer a specific species or two that can be studied, targeted and eliminated, the frequent treatment regimens of the past have left us with a significant problem – generalised drug resistance. The dewormers we have long relied on, simply put, no longer work. We are now at risk of losing control over internal parasites altogether, including the dangerous large strongyle, as resistance increased. No drugs with new modes of action against parasites have been developed since ivermection in 1983, so it is extremely important to preserve the effectiveness of what we have, since it is unlikely that new pharmaceutical solutions will be coming!
So what now? The focus has been widened – we need to consider ALL intestinal parasites, horses are never infected with just one, and accept that not only is eradication is no longer possible, but that “parasitism is a normal and natural state for horses” (Nielsen). The aim is to manage worm burdens such that disease and loss of performance are avoided, and to preserve the effectiveness of the current dewormers.
So what now? The focus has been widened – we need to consider ALL intestinal parasites, horses are never infected with just one, and accept that not only is eradication is no longer possible, but that “parasitism is a normal and natural state for horses” (Nielsen). The aim is to manage worm burdens such that disease and loss of performance are avoided, and to preserve the effectiveness of the current dewormers.
It is imperative to reduce exposure of worms to dewormers – this will slow resistance. This is achieved by performing faecal egg counts to find horses that have low, medium or high worm burdens, and worming accordingly. This preserves the “refugia”, worms that are not exposed to chemicals. Ideally faecal egg testing should be repeated 2 weeks after worming, to ensure that the product you have used is still effective – this is known as faecal egg count reduction testing (FECRT). Reducing worm transmission by pasture hygiene is also an important way to control worm loads without over-using dewormers.
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There are many excellent resources and articles available that explain in more detail how horse owners should be dealing with worms.
For a good explanation on exactly how resistance happens and how to slow it:
Horse Parasite Control Guidelines
And Ann Nyland’s book Horse and Donkey Worms and Worming is a must-read on how to deal with the current biggest menace – encysted small strongyles.
For a good explanation on exactly how resistance happens and how to slow it:
Horse Parasite Control Guidelines
And Ann Nyland’s book Horse and Donkey Worms and Worming is a must-read on how to deal with the current biggest menace – encysted small strongyles.