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03/18/2025
🔬Breaking Down New Recommendations for Equine Deworming 🪱
Deworming horses is a practice that seems to constantly evolve – the more we learn, the more recommendations change to ensure we are making the best decisions for our horses and the industry as a whole, and sometimes it can feel hard to keep up! We used to recommend rotating dewormers every 2 to 3 months to combat parasites, however, I hope by now, the only thing you rotate are your pastures (sorry – a little equine science humor). Rather, you should be utilizing targeted deworming for your mature horses. This means you are performing f***l egg counts to (1) identify your high shedders and (2) performing f***l egg count reduction tests on your herd to determine which dewormers are still effective. So let’s dive into how targeted deworming would work with your horses.
I want to make sure everyone is aware that through deworming, you will NEVER get rid of all the parasites – and that is not one of the goals of deworming. Rather, we want to minimize the risk of parasitic disease and delay further development of anthelmintic (dewormer) resistance. To accomplish these goals, we want to target parasites that are often associated with parasitic disease (primarily strongyles and tapeworms), provide more frequent dewormings to high shedders which contribute to environmental contamination, and only use dewormers that are effective for our herd.
However, not all horses are created equal and some variables can impact deworming recommendations such as age (see my previous post about deworming young horses), geographical region/climate, shedding level, and access to green grass. This is because many of our concerning parasites are transmitted when horses are on pasture, and we want to target them during periods of high transmission when they are more readily spread.
All horses should receive one to two baseline treatments of a macrocyclic lactone (ivermectin or moxidectin). Current recommendations are to deworm in the spring and once horses are removed from pasture going into winter. If you do not remove your horse from pasture, you can just time these for the spring and fall.
For high strongyle shedders (>500 eggs per gram), additional dewormings can be done during periods of higher transmission. This varies by geographical region and a study predicted the successful development of strongyles eggs for six different locations (Leathwick et al., 2015). This model suggests colder northern climates have a short window of transmission during the summer months. This window becomes larger as we move south to warmer climates until we reach southern states where transmission can be observed throughout the entire year. An exception here is arid states in the southwest (which I will discuss in a moment). If you want to see the graphs detailing these transmission windows, take a look at the 2024 AAEP Internal Parasite Guidelines.
Based on these findings, I would choose to deworm high shedders in colder climates during the summer months when transmission is observed. In comparison, high shedders in southern states may also benefit from an additional deworming in the winter when it is more mild. These recommendations could change based on the weather conditions observed each year.
I do want to specifically address arid regions of the United States (primarily in the southwest (including Nevada, Arizona, Utah, and New Mexico). When horses are located here and are not kept on green pastures, small strongyles and tapeworms are unlikely to require the primary attention in parasite control programs. As a result, the current recommendation for horses without pasture residing in these regions is to perform regular f***l egg counts to determine the necessary level of anthelmintic intervention required (Ramey and Nielsen, 2019).
So hopefully these recommendations help clear things up as to how to deworm! I still continuously see inaccurate and outdated information on this topic so it is important to stay educated for the sake of our horses!
Cheers!
Dr. DeBoer
Leathwick DM, Donecker JM, Nielsen MK. A model for the dynamics of the free-living stages of equine cyathostomins. Veterinary Parasitology. 2015 Apr 30;209(3-4):210-20.
Ramey DW, Nielsen MK. Limited strongyle parasite occurrence in horses kept in an arid environment. Equine Veterinary Education. 2020 Aug;32:37-40.
