Managing to Prevent Parasite Infection 

Parasites in our horses are a constant battle. Hang around two horse owners talking long enough, and you will eventually hear the word – “worms”.

Photo credit: Alayne Blickle

Photo credit: Alayne Blickle

Most large animal veterinarians I know readily say that all horses have or will have parasites at some point. Most of them also prescribe a regular de-worming schedule. While this has been accepted as a good standard practice, there are some things to consider.

Chemical de-wormers, while safe and effective, do compromise the immune system to some extent when in use, which is a concern especially in a working or performance horse. Another problem is that the parasites we commonly find in horses are adapting and beginning to become 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 or near the horizon, we need to not only consider how we use the drugs we currently have but also to look at other tools to manage for parasites. One of the best tools to prevent parasite infection is good pasture management. The other is a horse-specific approach to parasite management.

Pasture Management

Hiding on grass being grazed in a typical pasture, parasites enter your horse through their mouth. To prevent parasites from hitching a ride on that next grass blade going into your horse’s mouth, follow these proven prevention practices:

  • Monitor grass height. Never allow your horse to graze grass below 3 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 so that all the grasses are no more than 3 inches tall. This evenly exposes the parasites to sunlight and drying out, increasing chances to reduce their population. You can reintroduce the horses when the grass gets to between 6 – 8 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 these animals are ruminants (multi-stomached) instead of mono-gastric (single stomach). So, instead of mowing, graze other livestock such as cows or goats after horses have been removed. These animals will graze the taller grass left behind by the horses and ingest the parasites. When this happens, the parasite will go into their rumen and be unable 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 a pasture, do so from an off-the-ground feeder or bucket of some kind. Throwing hay onto the bare ground opens the hay up for the parasite to climb into and be ingested by your horse.

  • Be careful when you harrow! Horses naturally will not graze near its own or another horse’s 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. So, 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

    • 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, follow the previous guidelines for harrowing.

Horse Specific Parasite Management

Photo credit: Julie Allen

Photo credit: Julie Allen

For many years it has been a standard practice among horse owners to just assume that their horses have parasites and to use a regular worming 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 level, I always suggest taking a horse-specific approach.

A horse specific approach involves:

  • 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; or 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 or the chances of, 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 there are eggs present (2) how many eggs are present, which will determine the amount of de-wormer to use, and (3) what kind of parasite is present, which will determine the type of de-wormer to use. These three things are very important so that you don’t treat an uninfected horse, don’t use too much chemical medication on your horse, and so 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 de-worm using the weight of the horse. If you are administering de-wormer yourself, always follow label instructions and use only the amount required by your horse’s weight. Most horse owners underestimate their horse’s weight so it is easy to under dose.

  • To estimate the weight of your horse you simply 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) / 300] + 50.2

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 lbs

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 de-wormer 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.

Photo credit: Alayne Blickle

Photo credit: Alayne Blickle


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.