Whole-Farm Management Strategies for Equine Internal Parasites

By Brittani Kirkland     

This article is part 3 of a 3-part series on equine internal parasites. Here are the links to part 1, Prevalent Parasites: Common Types of Equine Internal Parasites  , and part 2, Decoding Dewormers: Types, Resistance Concerns, and Use for Horses.

It is impossible for a horse to be completely free of internal parasites, and most of the time, the horse can remain in good health with a low level of parasite burden. It is only when the horse has large, overwhelming parasite loads that we see clinical signs and health issues. From a management perspective, horse owners should focus then on trying to manage these loads to prevent illness. Dewormers are often the first thing we think about when we think of parasite management, but this is only one tool out of many that we have at our disposal. Internal parasites commonly found in horses   spend part of their life cycle outside in the horse's environment and part inside the horse's body. This allows us to implement strategies both in the horse's environment and within the horse’s body to aid in reducing the whole farm parasite population.

Parasite eggs and larvae in the horse's environment can be reduced through improved pasture, facility, and manure management. Parasites are unknowingly ingested by horses as eggs or larvae in the environment. By managing target areas on your farm more thoroughly, you can reduce the overall chance of parasite ingestion. Once inside the horse's body, our only option for parasite management is strategic treatment with dewormer. It is essential to stop the old practice of deworming every 2 months and move to a targeted deworming strategy to combat parasite resistance to dewormers  .

Pasture Management

Keeping pastures healthy is important for horse management in general, but especially for internal parasite management as many parasites infect the horses when they consume pasture grasses that have larvae or eggs on them.

1. Avoid having too many horses and/or other livestock on the pasture at once (overstocking) and overgrazing pastures (grass height becomes shorter than 3 inches). When this happens, horses will begin to graze in the "roughs" of the pasture – areas of taller grass where manure accumulates. They typically avoid grazing near manure, which is where parasite eggs are deposited onto the pasture. But when forage is limited, they may be forced to graze the roughs, and there will be a higher risk of horses ingesting parasite larvae. To prevent this, rotate pastures   and have a designated sacrifice lot to promote pasture growth and avoid overgrazing.
2. Mow pastures regularly (keeping the grass between 4-8 inches tall). This helps to keep pastures healthy during the grazing season. It is also recommended that pastures be mowed at the end of the grazing season before wintertime (no lower than 4" for plant health) to reduce winter survival of parasites. However, it should be noted that some internal parasites are extremely hardy and still can survive through the winter (even in the northeast U.S.), so you can’t expect pastures to be "clean" in the spring. Parasites survive particularly well if they are in fecal balls and under snow, which helps insulate them from the cold.
3. Co-graze pastures with other livestock, such as cattle or goats, that are not affected by the species of internal parasites affecting horses. This allows the other animals to consume some of the parasite eggs that your horse might eat without causing them any harm, since those species of internal parasites do not affect them. This essentially stops the parasite's life cycle from continuing. You could either graze the animals together or in a rotational system you could graze them one after the other.
4. Harrowing pastures to spread and break up manure piles can help reduce parasite loads if done during very hot, dry weather conditions. In the northeast US, we often don’t experience dry heat waves that last long enough to kill parasite larvae on pasture. Therefore, in Pennsylvania horses should be removed from that area prior to harrowing and stay off the pasture for the rest of the growing season. This gives time for the parasites to be exposed to the elements and die before the horses return the following spring.
5. Be strategic about which pastures you place your horse in. Due to their lower immunity, foals should be on pastures that are suspected to have a smaller parasite load, for example pastures that have had fewer horses on them or have had low shedders on them regularly. 

Read Basic Pasture Management for the Equine Owner  , Grazing Systems for Livestock and Horses  , and How to Make Rotational Grazing Work on Your Horse Farm   for more tips on pasture management. It is important to realize that even with the best management, horse pastures will always have some level of parasite contamination. Even if horses were effectively dewormed and put on a "clean" pasture that hasn't been grazed for years, encysted small strongyle larvae will migrate out of the horse’s gut and begin producing eggs to deposit on the pasture. Our goal is to reduce, not eliminate, the parasite burden on pastures.

Manure Management

Many equine parasite eggs are shed into the environment through the horse's manure. This means that manure management can greatly reduce the presence of eggs in the horse’s environment.

1. Remove manure from small areas where horses are typically confined (stalls, paddocks, etc.) within a short timeframe to prevent larvae development (within 24-72 hours is best).
2. Remove manure from pastures regularly. This can be very time-consuming, so it may only be an option for smaller farms. There are manure vacuums and sweepers for pasture management if you are interested in machinery to help with this.
3. Only spread manure on pastures if it has been properly composted beforehand. Don't spread manure that has not been composted. Never spread foal manure. Both ascarid and small strongyle eggs, two prevalent parasites in horses, have been found to be eliminated when manure is composted in windrows. This can occur in as little as eight days! However, you must turn and mix the compost piles appropriately and ensure the pile reaches a minimum internal temperature of 104° F. If you leave the pile alone it can actually do the opposite of what we want and promote parasite development rather than eliminate them. 

Read Horse Stable Manure Management   and watch Composting on Horse Farms: Why and How   for more information.

Facilities Management

Some exposure to parasite eggs and larvae can be reduced through a few management changes to your facility, particularly those that improve sanitation.

1. Feed your horses in troughs, buckets, and/or feeders that are raised off the ground. This can help reduce the chance of fecal contamination and therefore reduce the risk of parasite infection.
2. Frequently clean and disinfect water and feed troughs/buckets. This can reduce any fecal contamination that may have occurred, and again lower potential contact with parasites.
3. Quarantine any new horses coming onto the farm to help reduce the chance of introducing new or genetically unique parasites to your farm. Perform fecal egg count reduction tests to find out if the horse is infected with parasites that are resistant to the dewormers you use on your farm. (See section on Dewormers for Management). If the dewormer did not reduce the internal parasite load as needed, an additional deworming treatment with a different chemical class should be given immediately. Wait until reduction is greater than 95% before turning the horse out with other horses and on pasture grasses.

Dewormers for Management

All adult horses should receive one to two treatments with dewormer annually according to the American Association of Equine Practitioners. However, it is important that these treatments be given strategically, as anthelmintic (dewormer) resistance (a parasite’s ability to survive deworming treatments) has been documented in all three of the dewormer chemical classes currently on the market. It is important to take the following steps when using dewormers for management:

1. Talk to your veterinarian about a detailed plan for your farm. Each situation is unique, and therefore, partnering with your veterinarian to develop deworming strategies can help ensure you are making the best decisions for you and your horses. Some things to discuss with your veterinarian include:
    - Your farm type (single- or multi- horse farm, breeding operation, boarding facility, etc.) and internal parasite concerns with that type of operation
    - Current parasite load of horses on the farm
    - Current health status of the horses and any additional things that may need to be adjusted due to the horse’s state (immunocompromised, pregnant, young, etc.)
    - Environmental conditions/climate in the area
    - Timing of deworming based on the season
    - Other factors that might affect the parasite load on your farm (livestock presence, pasture rotation, etc.)
2. Conduct fecal egg counts (FEC) on all horses on the farm and identify their individual shedding rates (typically for small strongyles but ascarids can also be identified).
3. Strategically determine type and frequency of dewormer treatments based on the horse's individual need (based on the FEC results) and which drug classes are effective on the farm (see #5). Some horses may need to be treated more frequently than others.
4. Give the dewormer at the appropriate time. During certain seasons (particularly spring and fall in the northeastern US), the risk of parasite infection may be higher. Therefore, your veterinarian may suggest treatment during certain months. Also, make sure the dosage you are giving is the correct amount based on the horse's body weight. Do not just guess your horse's weight but measure it to get a more accurate estimate   before giving a treatment.
5. Monitor small strongyle anthelmintic resistance by conducting a fecal egg count reduction test. This is to make sure the drug class is still effective on your farm. Have a veterinarian take a fecal egg count and then treat all horses with the most common dewormer treatment used on the farm. After 14 days, you can have your veterinarian conduct a fecal egg count reduction test to see how much of the parasite population was eliminated (ideal is a reduction of 95% or greater). Adjust treatments as needed if resistance is suspected.

Conclusion

Managing internal parasites is important to help promote health in your horses, and there are many ways you can help reduce your horse’s parasite load. Not all farms will be able to use each technique outlined in the article, but the more you can adopt, the better. Avoid only using one management technique. Instead, use a combination of the strategies discussed above to help reduce the farm load and reduce all horses’ chance of exposure.