Deworming

Commonly used parasite control measures involving rotational deworming are based on concepts that are more than 40 years old.  Recommendations have changed as more research has become available and more resistance has developed to many of the common deworming preparations.  There are no new dewormers coming to market in the near future.  The original deworming strategies were based on the control of Strongylus vulgaris, the large strongyle, which was considered the most important parasite of adult horses.  Deworming every 2 months prevented eggs from being shed on pasture.  This method was successful and now disease from S. vulgaris is very rare in managed horse populations.  Small stongyles (cyathostomins) were not considered important pathogens at that time.  However, that situation has changed and currently the small strongyles are recognized as the primary equine parasite pathogen.  Similarly, Parascaris spp. (roundworms) are the major parasitic pathogen of foals and weanlings, and Anoplocephala perfoliate (tapeworm) has been recognized as a cause of colic in the horse.

We now have a situation where there is increased resistance of parasites to the currently available dewormers.  The new recommendations for parasite control is to use a strategic deworming program based on fecal egg counts and fewer yearly dewormings.  Horses can be classified based on egg shedding numbers and this can be used to determine when and how often and with what product to deworm horses.  Low shedders have 0-200 eggs per gram, moderate shedders have 200-500 eggs per gram, and high shedders have greater than 500 eggs per gram.

Current recommendations are to deworm all horses with two yearly treatments with further treatments targeting those horses with high strongyle egg counts.  Spring and fall are the best times of year for treatment.  We are happy to help you set up a strategic deworming program for your horse or stable.

During the first year of life foals should receive a minimum of four anthelmintic treatments.  First deworming should be at two months of age with a benzimidazole drug.  Second deworming just before weaning (4-6 months of age).  Do not exceed 3 months without a deworming.  Fecal egg counts are needed at this time to determine if worm burdens are primarily strongyle or ascarids so the right choice for a dewormer can be made.  Third and fourth treatments should be at 9 and 12 months of age, primarily targeting strongyles.  Tapeworm treatment should be included in the 9 month treatment.

Fecal egg counts should be performed to evaluate deworming efficacy.  Yearlings and two year olds should be treated as high shedders and receive at least 3 yearly treatments with efficacious drugs.