New Concepts In Parasite Control In Adult Horses

 By Rebecca Bott SDSU Extension Equine Specialist

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.