Deworming

The concern over the increasing resistance of parasites to our available dewormers remains high for expert parasitologists. When we discuss deworming, the “one-size fits all” analogy has become extremely inappropriate and we must look at management factors to determine the most appropriate and effective approach for each horse. In doing so, it is important to differentiate between the “closed herd” situation from a barn with the constant influx of horses. The “closed herd” implies a limited number of horses that graze the same pasture(s) in which new horses are introduced infrequently. In this situation, pasture management will help to control the parasite burden. Our current recommendations include:

  • Avoiding overgrazing
  • Removing manure from pasture
  • Adequate composting of manure to kill eggs
  • Rotation of other grazing livestock

Many horses will develop some natural immunity to infection, while others continue to shed high amounts of parasite eggs into the environment, acting as reservoirs of contamination. Fecal Egg Counts (FEC) can be used to monitor the parasite burden and type for each horse and should be performed on all horses at least 10-12 weeks following the most recent deworming. Fecal egg counts consist of processing a small amount of manure (one fecal ball is sufficient) and using a microscope to obtain an accurate count of the number of eggs present. Samples of manure should be fresh (

In a “closed herd” with appropriate pasture management and FECs to identify the parasite burden of each horse, it often possible to deworm significantly less frequently and modify the plan based on the results of each horse’s FEC. Deworming should be concentrated in the Spring and Fall when parasite shedding is most active. FEC should be done at least twice per year in order to identify the type of shedder each horse is (low, moderate, or high) and modify the deworming program accordingly.

In a large barn with an “open herd,” or frequently changing population, it is very difficult to control the potential for exposure to parasite eggs. When horses graze on communal areas, whether they are on pastures of hand-grazed at competitions, there is the constant threat of infection. In these situations it is more appropriate to consider a more aggressive approach to deworming and more frequent FECs should be performed. Your veterinarian can then establish a deworming plan for each horse.

We recommend a bi-annual FEC for horses on daily dose dewormer, such as Strongid-C. Note that horses on daily dewormers do still require paste deworming as well.

Young horses (birth through yearling) require a more intensive deworming schedule as their natural immunity increases. The difference in schedule is primarily due to ascarids (round worms), which can cause significant problems in young horses, but not commonly in adults. Foals require monthly deworming in the first year of life, but it is important that the dose is adjusted to the weight of the foal. Horses younger than 6 months of age should not be dewormed with Quest or Quest Plus. Talk to your veterinarian for details.

As your horse’s primary care veterinarians, we are constantly evaluating the newest literature, research, and products to make sure we are offering the most up-to-date information and treatment for your horse’s continued health. If you have questions or concerns, please contact us so we can discuss your specific inquiry.

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