Preventing Parasite Infestation

Pasture and Herd Management Reduces Need for Chemical Dewormers

By Michael Hipp


Photo credit Kim Roe

Parasites in our horses are a constant battle. Most large animal veterinarians I know readily say that all horses have (or will have) parasites at some point. Most of them prescribe a regular deworming schedule. While this has been accepted as a good standard practice, there are some things to consider.

Chemical dewormers, while safe and effective, do compromise horses’ immune systems to some extent. This is a concern especially in working or performance horses.

Another problem is that the parasites we commonly find in horses are adapting and becoming resistant to these chemicals. Over the years, roundworms and strongyles have developed a resistance to some drug classes, including benzimadazoles and the tetrahydropyrimidines or pyrantel salts, which include pyrantel pamoates and pyrantel tartrate.1

With no new drugs on the horizon, we need to consider how we use the ones we currently have and also look at other ways to manage for parasites.

Pasture Management

Parasites enter through your horse’s mouth, carried on grazed grass in the pasture. To prevent these parasites from hitching a ride on that next grass blade, follow these proven practices for preventing infection:

Monitor grass height. Never allow your horse to graze grass below three inches. Most parasites will only travel up the first three inches of the grass blade. When the horse is removed from the pasture be sure to mow the grass so that all the grasses are no more than 3 inches tall. This evenly exposes the parasites to sunlight and drying out, increasing the chance of reducing their population. You can reintroduce the horses when the grass grows to between six and eight inches.

Rotational grazing. This will help lower the worm count on any one pasture and allow the grass to recover to a safe grazing height.

Graze multiple species in sequential order. The same parasites that cause a problem for horses do not cause problems for other livestock, such as cows or goats. This is because those animals are ruminants (multi-stomached) instead of mono-gastric (single stomach). So, instead of mowing you can graze other livestock, such as cows or goats, behind the horses. These animals will graze the taller grass left behind by the horses and ingest the infective parasites. When this happens, the parasite will go into their rumen and not be able to complete their life cycle, thus reducing the overall population on the pasture.

Never feed directly on the ground. If you must feed a horse in pasture do so from a feeder off the ground or a bucket of some kind. Throwing hay onto the bare ground allows the parasite to climb into the hay and be ingested by your horse.

Be careful when you harrow! Horses naturally will not graze near manure, which is a natural defense against parasite infection. When you harrow a pasture, you spread the manure over more area and thus take away that natural defense. If you do need to harrow, follow these two guidelines: only harrow during the hottest and driest periods to give the best chance of exposing parasites to sun and heat to dry them up, and only harrow when you can keep horses off the pasture for at least two weeks to allow the parasites to die off.

Make sure the manure you spread on your pasture is properly composted. If you have properly composted your manure before spreading, then all the parasites in the manure will have been killed off. If the manure has not been properly composted before spreading, then follow the guidelines for harrowing.

Horse Specific Parasite Management

For many years it’s been standard practice for horse owners to assume their horses have parasites and to use a regular deworming schedule. But in order to slow down the adaptation of these parasites to our current chemical treatments and to reduce the impact to your horse’s immune system and energy, I suggest taking a horse-specific approach. This means:

Never assume your horse is infected. Look for some basic signs of infection: dull, rough coat; lethargy or depression; decreased stamina; loss of condition; pot belly (especially in younger horses); colic; diarrhea. If your horse shows no signs of infection then there may not be a concern, especially if you only have one horse. Some horses are immune to some parasites while others are not.

If you suspect infection, get a fecal egg count. If your horse is infected with a parasite there will be eggs from that parasite in their manure. Your veterinarian can do an examination of your horse’s manure to determine 1) if eggs are present, 2) how many eggs are present (which will determine the amount of dewormer to use), and 3) what kind of parasite is present (which will determine the type of dewormer to use). Testing for these three things will ensure that you don’t treat an uninfected horse, don’t use too much chemical medication on your horse, and you use the right type to make sure you kill the parasite you’re after. Call your veterinarian to find out their specific procedure for obtaining samples of manure for testing.

Always deworm based on the weight of the horse. If you are administering dewormer yourself, follow label instructions and use only the amount required for your horse’s weight. Most horse owners underestimate their horse’s weight, so it’s easy to under dose. To estimate the weight of your horse, use a tape measure to measure their heart girth (from the base of the withers down to a couple of inches behind the front legs, then back around and up the other side to the base of the withers) and the length (from the point of the shoulder to the point of the rump), then plug those numbers into this formula: heart girth x heart girth x length divided by 300 + 50. For example, if you have a heart girth of 70 inches and a length of 62 inches, the estimated weight of your horse would be: [(70 x 70 x 62) / 300] + 50 = 1,062 pounds. If you are working with your veterinarian, they will guide you on the amount and type to use.

Keep separate records for each horse. Each horse should be treated individually and records kept individually because every horse will vary in their susceptibility to infection and the amount of dewormer required.

By taking steps to manage your pasture well and treating each of your horses specifically and individually you will have the greatest success in battling those common parasites that plague every horse owner and your horse will be better for it.

1 Giedt, D.V.M, Elizabeth et al, “Controlling Common Internal Parasites of the Horse”, Oklahoma State Extension Service, Fact Sheet VTMD-3976, downloaded August 28, 2017.

2 Ensminger, M.E. et al, “Feeds and Nutrition”, 2nd Edition, The Ensminger Publishing Company, 1990 – p. 1522.


Published October 2018 Issue


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