Published April, 2010

After a snowy winter, temperatures in Central Kentucky are seasonably warm once again. Grass is green, birds are singing, and equine parasites are stirring back to life. This is the time of year when horse owners and farm managers become aware of worms and make plans to combat them.

Many farms follow a calendar schedule dictating what type of deworming product to use and when, while others use a seasonal method and do not deworm at all during certain times of the year. According to the University of Kentucky's Mary Rossano, MS, PhD, assistant professor in animal and food sciences, many people who practice seasonal deworming assume parasites die during winter and there is no need to administer dewormer in the colder months.

Actually, Rossano said, the reverse is true south of the Ohio River, where summer is often hot and dry enough to kill most parasite larvae before emergence or on grass blades before ingestion. In colder climates some types of worms can survive under several inches of snow, although they are less likely to be ingested.

Deworming practices depend on the age of the horse under consideration. Since there are several types of parasites, each with different means of action, transmission, and temperature tolerance, it is important for horse farm managers to know which types are targeting their herd. Ascarids, commonly called "roundworms," hatch inside the horse and migrate through the liver and lungs to feed in the small intestine and primarily affect young horses until they are one to two years old. Small strongyles, on the other hand, are the major parasite of concern in adult horses and attack the large intestine. Most foals develop small strongyle infections in their year of life and gradually build immunity as they reach adulthood. Bots, pinworms, and tapeworms are rarer and seasonally affect adult horses. Each type of worm responds differently to the various types of dewormers and some (particularly small strongyles) are becoming highly resistant.

That is why, Rossano said, horse owners may want to reconsider the calendar method of deworming.

"The more frequently we deworm, the faster we select for drug resistance in parasites," Rossano said. "When it comes to small strongyles, there might be times of the year when it's a good idea to hold back, and that's appropriate if there are horses whose deworming frequency is reduced to twice a year. This will reduce selection pressure on the parasites and hopefully slow the development of drug resistance."

Rossano said there is a program designed to deworm only the most infested horses in a pasture group. Some horses seem to attract more worms than others and consequently become "high shedders." Fecal egg counts should be done to assess which horses are high shedders. Those are the horses that receive the most benefit from deworming products, while low shedders do not necessarily need them. The herd should be retested periodically, but high and low shedders usually remain in their respective categories for life. Controlling contamination from high shedders during the seasons when parasite survival is most favorable helps reduce the opportunity for reinfection of the high shedders. Deworming a limited number of animals limits the parasites' chances to become drug resistant.

Rossano cautioned that research on the topic of scheduled deworming is limited since drug resistance has increased so quickly and has yet to produce consistent guidelines. She also pointed out that scientists are still learning about the seriousness of certain types of parasites, which could affect the need for treatment.

"As we try to get more evidence to back up the protocols we recommend, there needs to be a better understanding of the true disease outcomes related to parasites. Few controlled population studies exist to help us understand those questions," Rossano said.

For more information on equine parasites, contact your veterinarian, your local Cooperative Extension agent, or see The Horse's "Parasite Primer."

Natalie Voss is a UK equine communications intern and undergraduate student in equine science and management.