Revisiting how we understand equine parasite control

When it comes to intestinal parasite control, caretakers often look to de-wormers as a first line of defense. However, you may be doing more harm than good. Take a look at why parasites, like bacteria, aren’t all bad.

It’s a common misconception that all parasites are bad – but this couldn’t be further from the truth. In fact, it’s normal for horses to harbor intestinal parasites the same way they harbor bacteria. The intestinal tract is full of bacteria and some are potentially harmful, yet we don’t administer penicillin to all our horses with fixed intervals. Why not? Because there are several potential unwanted side effects to such a strategy – most notably antimicrobial resistance and disruption to natural bacterial flora.

Worm parasites should be thought of in a similar manner. They are normal inhabitants of the gut and are going to be there no matter what we do. Decades of intensive de-worming has only led to lots of drug-resistant parasites around the world, and that’s a significant problem.

There are situations in which de-worming a horse with an appropriate anthelmintic product is warranted, and we want to make sure we have safe and effective products should such situations arise. Unfortunately, we are slowly running out of treatment options, since it’s a lengthy process for the pharmaceutical industry to test and develop safe and effective new products with completely new modes of action. We have not seen any new classes of de-wormers developed for horses since the early 1980s.

Why de-worm?

At this point you might be asking yourself – why de-worm at all? The truth of the matter is that intestinal parasites can cause disease in individual horses that are exposed to an unusually high infection pressure (a high number of infective eggs or larvae) and/or are particularly susceptible to parasite infection due to young or old age, concurrent disease or other predisposing factors. We want to make sure we keep a low infection pressure and may choose to de-worm horses at certain ages or certain times of the year based on the parasite and climatic conditions.

What can be done to mitigate parasite infections?

To help lessen your horse’s risk of parasite infection, good management of his health, paddocks and pastures is key. Making sure pastures are not overgrazed is essential, as all major parasites are acquired while grazing on pasture. High stocking density on overgrazed pastures means high parasite infection pressure.

Good pasture management, which has been shown to reduce parasite infections, includes pasture rotation and mixed/alternate grazing with ruminants such as sheep or cattle. The specifics on how well this might work and how it should be set up is highly weather- and climate-dependent. In some countries, it is quite normal to have sheep “clean up” paddocks after horses. The horses graze first, and then the sheep take over and graze everything down. Finally, the paddock is rested until the grass has come back up, and horses can be turned out again. This can haven a huge impact on reducing pasture parasite infectivity – especially for foals and young horses.

What are the most important parasites to think about?

  • In foals, the most important parasite is the ascarid (Parascaris), which infects virtually every foal during its first six months of age. Ascarids can cause stunted growth, weight loss, and most importantly, small intestinal impactions in the young foal.


  • In other age groups, strongyle and tapeworm parasites are most important. Among the strongyles is the bloodworm, which has become rare in most managed horse populations, but has been found to re-emerge in horses that never or rarely receive anthelmintic treatment. The bloodworm is the most harmful of all equine worm parasites as it migrates through the blood vessels and causes substantial damage. This can lead to severe and sometimes fatal disease in the horse.
  • Tapeworms are very common in most horse establishments, except for the more arid and dry climates. They can cause certain types of colic in horses with high burdens. Not all de-wormers have activity against tapeworms.

Are there any effective natural de-wormers?

The short answer is no, unfortunately. There are lots of natural products, but very few with any documented effects against parasites. I have been involved with testing some of these products and have yet to find any that reduce parasite burdens. Colleagues elsewhere in the world have similar experiences. And no, diatomaceous earth does not work. But as a famous parasitologist once said: “Somewhere out there is an excellent de-wormer that just hasn’t been discovered yet.” So we keep an open mind and continue the search.

Using a natural product for parasite control should never be a goal in itself. The goal should be to use safe and effective products of good quality. The term “chemical de-wormer” is misleading, since everything is chemistry – even a natural de-wormer — would only work if the plant, root, fruit, flower, oil, etc. contains a chemical with activity against the parasite.

The term “natural de-wormer”, therefore, is equally misleading. Ivermectin is an example of a natural de-wormer that has undergone quality control for the concentration and activity of its active ingredient, and has been meticulously safety-tested in horses. Ivermectin is produced by a naturally-occurring fungus, so it qualifies as natural even though many would regard it as a chemical. At the end of the day, reach for a de-wormer that’s undergone strict quality control, standardization and safety testing.

Are there any benefits or consequences to not de-worming your horse?

At the University of Kentucky, we keep a herd of horses that has not been de-wormed since 1979. These horses have very high parasite loads, including bloodworms, ascarids and tapeworms, but they are in good health and do not show any signs of parasitic disease. Being a research herd, all horses are monitored carefully every day, as we do not allow any kind of distress or suffering. Their stress levels are probably lower than what most horses experience since they are never moved, ridden, or trained in any manner, and they are on pasture every day year-round. These factors may all contribute to their health status.

Research in human medicine has suggested that intestinal parasite burdens can be beneficial in preventing allergies and autoimmune disease. We don’t know if this is the case with horses as well, but it cannot be ruled out.

Why not skip de-worming?

  1. In foals, we want to avoid the very high ascarid burdens that typically occur at around four to six months of age. This coincides with weaning, which is very stressful for foals, and large parasite burdens put them at risk for small intestinal impactions – which are life-threatening.
  2. We want to make sure yearlings and youngsters do not accumulate large burdens of encysted small strongyles, which can be harmful and cause severe diarrhea.
  3. In all age groups, we want to prevent bloodworm from occurring, as it can still cause serious and life-threatening disease.

So there are still reasons to de-worm your horse with an effective product. But the blindfolded chemical warfare strategies where all horses are treated with an arsenal of products at fixed calendar dates year-round need to be abandoned and replaced by approaches based on diagnostic information, and by taking parasite biology and treatment susceptibility into account.


Dr. Martin Nielsen graduated with his DVM degree from the Royal Veterinary and Agricultural University in Denmark in 2001. He spent three years in equine veterinary practice before joining graduate school in 2004. He received his PhD in equine parasitology at the University of Copenhagen in 2007, and served as assistant professor there until 2011. He then joined the M.H. Gluck Equine Research Center at the University of Kentucky where he currently serves as associate professor in equine parasitology and holds the title of Schlaikjer
Professor in Equine Infectious Disease. Dr. Nielson is board certified in veterinary parasitology with the European Veterinary Parasitology College (EVPC) and with the American College of Veterinary Microbiologists (ACVM). He is chair of the AAEP Equine Parasite Control Subcommittee , which published its guidelines in 2013.