The War on Parasites

When caring for horses, it is best to stay ahead of problems by preventing them rather than reacting to a problem that has already presented. The same is true for deworming them to control internal parasites. In fact, successful controlling of intestinal worms is a cornerstone to managing healthy horses and is a constant pursuit of caretakers. Each horse’s deworming needs vary based on their housing situation, region, and season. Unfortunately, science has revealed that deworming is no longer as simple as administering a rotation of products every 8-12 weeks. As such, it is imperative to work with your veterinarian to tailor the proper schedule for your unique horse’s needs and risks.

First, let’s identify the four main types of parasites that negatively affect horses so we can better understand what we are up against.

• Small strongyles (cyathostomins) consist of over 40 different species and are generally not harmful to a horse’s gut. However, when they present in large numbers, they can disrupt the healthy digestion and absorption of vital nutrients. This can lead to weight loss and colic. This type of worm is affected by ivermectin (only for a certain portion of their lifespan after they are no longer encysted in the intestine walls) and moxidectin.
• Ascarids (parascaris equorum) present mainly in foals where infections can lead to slowed growth, a dull coat, low energy, and in serious cases blocked instestines which can lead to colic and ruptures. Luckily, as horses mature, they develop an immunity to ascarids. This type of worm is affected by ivermectin, moxidectin, fenbendazole, oxibendazole, and pyrantel pamoate.
• Tapeworms can cause anemia and lead to colic, specifically with a heightened risk of perforations of the intestines and peritonitis. This type of worm is affected by pyrantel pamoate and praziquantel.
• Large strongyles (bloodworms) are rare but can lead to significant issues. The most dangerous species is the strongylus vulgaris variety which migrate through the walls of the abdominal arteries. This can leave them prone to rupturing and blood clots which halt proper circulation to the intestines. In some cases, they can even cause severe damage to the liver and other organs. This type of worm is affected by ivermectin, moxidectin, fenbendazole, oxibendazole, oxfendazole, and pyrantel pamoate.

Now that we understand the enemy of the battle, let’s talk about the weapon we must use against them. An anthelmintic chemical is one used kill worms within the gut of a horse, commonly referred to as a dewormer. While these chemicals are effective, caretakers must understand that worms can develop a resistance to them. Each time a specific chemical is administered, most of the specifically targeted parasites within the horse’s gut will die. However, a few (who are less susceptible to the chemical) will survive and reproduce. Unfortunately, that means they then pass on their resistance to the following generation of parasites - yikes.

Okay, we have an understanding of the enemy and the weapon. Let’s chat through some guiding pillars to successfully deworming your herd. First off, it’s important to understand that it is unrealistic to think you will ever fully eliminate parasites. The goal is the keep them at a healthy level, as few worms will do serious harm to your horse’s gut. Second, it will go a long way in your fight against parasites to identify the high shedders within your herd. High shedders are horses who are responsible for most of the strongyle eggs released within a given pasture. To do this, your vet will need to conduct regular fecal egg counts which can also help determine if a certain species is developing a resistance to dewormers. Your vet will test a fresh fecal sample, deworm the horse, then conduct another test 10-14 days later. If a specific dewormer is working, you can expect the egg count to depreciate by at least 90%. By identifying your high shedders, you can keep them on a more strict schedule and a lower parasite total count and healthier herd will result.

While you can expect parasites to always be part of your horse’s life, there are some things you can do to lower that exposure. As a general rule, the more pasture space your horse has access to, the better. If given enough area for turnout, horses will generally designate certain parts of pasture as grazing areas and others as “bathroom” areas. This means they will be less likely to graze where parasite eggs have been dropped in their manure. A good rule of thumb for this is to offer approximately 2 acres per horse within the herd. But don’t fret if your farm is not big enough! You can help control parasites by picking up manure regularly from paddocks. Helpful hint: if you plan to spread this manure across a certain area that horses will have access to, compost it first so the heat will kill the eggs present! You can also rotate grazing areas or consider safe, temporary fencing to divide up a given area.

That’s it, soldier. You now have some solid arsenal to use on the war on parasites at your farm. Remember, they’re always going to be there – the goal is to control them, not eliminate them.