Strategic deworming for horses

By Kimberly Cole, Extension Equine Specialist, Ohio State University

Internal parasites can cause a variety of problems in the horse ranging from a dull hair coat and unthriftiness to colic and even death. There are several species of internal parasites that can infect horses — most notably small and large strongyles, ascarids, tapeworms, pinworms and bots. Infected horses shed the parasite eggs in their manure, contaminating pastures, paddocks and pens. The eggs or larvae are ingested while the horse is grazing and mature within the horse’s digestive tract. Some parasites are able to migrate to other areas of the horse’s body, causing significant damage along the way.
In the past, traditional deworming recommendations were to treat horses with a different deworming product every 60-90 days. Research has shown that only about 20% of the horses in a herd shed the majority (> 80%) of parasites on a pasture. Treating horses with low numbers of parasites not only wastes money, but can promote resistance to dewormers. In several states throughout the United States, including Ohio, small strongyles are becoming increasingly resistant to dewormers that are currently available on the market. Therefore, the current recommendation is to strategically treat only those horses with high parasite numbers with the appropriate dewormer at the appropriate time.
A fecal egg count is a simple test that can determine the type and number of parasite eggs that are present in your horse. Results of a fecal egg count are expressed as eggs per gram (epg) of manure. A fecal egg count of less than 200 epg indicates a low parasite load. A fecal egg count of 200-500 epg indicates a moderate parasite load. A fecal egg count of greater than 500 epg indicates a high parasite load. Horses with low fecal egg counts may not need to be dewormed more than twice a year, while horses with high fecal egg counts may need to be dewormed 6 times a year. It is important to note that a fecal egg count of zero does not mean the horse is free of internal parasites. Some types of parasites produce eggs only intermittently, while larvae do not produce eggs at all.
Fecal egg counts can also help evaluate the effectiveness of a particular dewormer on your farm. Perform a fecal egg count on your horses and then treat them with a dewormer. Perform another fecal egg count 14 days later. If the fecal egg count is not reduced by at least 80%, the parasites your horse has are considered resistant to that particular dewormer. That dewormer should not be used as a treatment by itself but may still be effective when used in combination with another deworming product. Regardless of the type of dewormer you use, always remember to use a weight tape to estimate the weight of your horse and administer the correct dosage of dewormer.
Students in the Department of Animal Sciences at The Ohio State University recently conducted a preliminary research study investigating the prevalence of internal parasites and effectiveness of dewormers containing pyrantel pamoate in horses in relation to age (young vs. mature), season (summer vs. winter) and housing system (stall vs. pasture). Fecal samples were collected from 65 horses, ranging in age from 1 to 22 years, on a farm in central Ohio. Fecal egg count data were collected prior to treatment with pyrantel pamoate at the recommended dosage and 14 days post-treatment. The dewormer was considered effective if fecal egg counts were reduced by at least 90% by days 14 post-treatment. Strongyle, ascarid, hookworm, pinworm, whipworm and tapeworm eggs were identified in this study; however, strongyle, ascarid and tapeworm eggs were the most prevalent.
Young horses, less than 3 years old, had greater fecal egg counts compared to mature horses. The fecal egg counts were similar during summer months compared to winter months. Horses maintained on pastures had greater fecal egg counts than horses kept in stalls. Resistance to pyrantel pamoate was evident regardless of the age of the horses, season or housing system.
In this study, treatment with pyrantel pamoate was more effective at reducing total fecal egg counts in mature horses (27.8%) compared to young horses (14.3%). Pyrantel pamoate was also found to be more effective at reducing total fecal egg counts in the summer months (15%) as compared to the winter months (0%). Horses maintained on pastures were less resistant (21.4%) to treatment with pyrantel pamoate compared to stalled horses (16.7%).
Although pyrantel pamoate was not considered effective in reducing the total fecal egg counts of horses on this farm, it may still be a useful product if used in combination with other deworming products. Additional studies investigating the effectiveness of other dewormers are currently in the planning stages using additional farms throughout Ohio.

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