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New Parasite Control Recommendations

Recommendations for intestinal parasite control in adult horses are changing. These changes are based on new evidence for the types of parasites commonly affecting horses as well as the development of parasite populations that are becoming resistant to treatment with an anthelmintic (de-wormer). Evidence now exists to suggest that adult horses tend to vary greatly in both their susceptibility to parasites as well as in their tendency to shed, or release parasite eggs into the environment. Geographic location, weather, and housing systems also play a role in the prevalence or exposure to parasites. For example, horses housed in pastures where the manure is not routinely removed are exposed to a cycle of shedding eggs and ingesting larvae. Collectively, these discoveries have led the American Association of Equine Practitioners to move away from the 40-year old recommendation of deworming every two months. Instead, they recommend a parasite prevention program that involves deworming adult horses once or twice each year with a comprehensive anthelmintic and then further treating individuals as needed.

The new recommendation is based upon the understanding that while most adult horses are infected with small strongyles, only heavy loads pose a significant health threat. Additionally, the prevalence of parasites such as large strongyles, which warranted more frequent control has subsided. Anthelmintic resistance, or the development of parasites that have evolved to survive when hosts are treated with an anthelmintic is also a great concern. Parasites that are able to survive treatment pass their capability of resistance on to subsequent generations. With every subsequent treatment, non-resistant parasites die, and resistant parasites survive preferentially. Therefore, it seems more appropriate to treat once or twice a year towards the end of the grazing season, timed to also coincide with the greatest susceptibility of the parasites (spring and fall).

Parasite Control Program Goals

According to the American Association of Equine Practitioners, the goal of any parasite control program can be summarized as follows:

  • To minimize the risk of parasitic disease.
  • To control parasite egg shedding.
  • To maintain efficacious drugs and avoid further development of anthelmintic resistance as much as possible.

Even when adult horses are treated once or twice each year with a comprehensive anthelmintic designed to target large and small strongyles, bots, and tapeworms, horse owners are encouraged to perform fecal egg counts (FEC) to identify horses that may need additional parasite management. FEC testing involves collecting a small sample of manure and submitting it to a veterinarian or veterinary diagnostic lab for analysis of the number and types of parasite eggs present. It is important to realize that parasites go through life cycles which will result in periods of time when little to no eggs are being shed in manure. Thus, a FEC test that comes up with few to no eggs is not necessarily indicative of a horse without intestinal parasites. Sampling manure 2-3times throughout the year and performing a FEC will provide a more accurate reflection of whether each horse is a high or a low shedder of parasite eggs. Those that are high shedders will require additional parasite management as compared to low shedders.

More Information

For more information on parasite control in horses of all ages please view AAEP’s Parasite Control Guidelines. The recommendations for use of anthelmintic in horses under one year of age differ from what has been described in this article. Additional methods for minimizing exposure to, and therefore ingestion of parasite eggs include frequent removal of manure from pastures and dry lot areas. Manure should be composted prior to application to pastures as fertilizer to ensure that parasite eggs have been heated to a high enough temperature to destroy them. View Manure Management for Horse Owners for more information on manure management on horse farms. Consult your local equine practitioner to develop a customized parasite management plan suitable for your horses.

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