It is not possible—or desirable—to eliminate worm burdens in horses. Instead, the goal is to eliminate diseases related to parasites. Fecal egg counts can help you develop a deworming regimen that leaves enough parasites to improve a horse’s immune system while preventing an infestation.
In general, 20% of horses excrete 80% of the total egg output in a herd. Fecal egg counts are useful for identifying high strongyle egg shedders that need more aggressive deworming protocols and for identifying those individuals that need fewer deworming treatments.
Fecal Egg Counts
Certain times of year are most conducive to finding parasite eggs in the feces due to seasonal variations when adults lay the most eggs. Also, the emergence of small strongyle larvae from the intestinal wall coincides with lengthening days of spring. The best opportunity to examine fecal egg counts is in springtime and early summer months.
Your veterinarian needs only a couple of fresh fecal balls obtained from the top of a manure pile from an identified horse. Quantitative fecal egg count testing gives an exact count of parasite eggs per gram (EPG) of feces. In adult horses, 95% of all worm eggs shed in the feces are small strongyles. Ideally, a horse has a negative fecal egg count or at least no more than 200 eggs per gram (EPG). An individual with 500 or more EPG is considered a high shedder and needs more frequent deworming treatments.
A digital counting technique that is being researched at the University of Kentucky uses an app on a smartphone to photograph a fecal sample on a filter with fluorescent staining. Then, strongyle and ascarid egg counts are quantified. It is hoped this stall-side fecal egg count device will be available to the industry in the next couple of years.
The absence of parasite eggs in a fecal sample doesn’t necessarily mean the absence of adult egg-laying worms in a horse; it just means that eggs aren’t being laid at that time. Small strongyle larvae encysted in the bowel lining can cause damage, yet have not molted to the stage of egg-laying adults.
Adult ascarids can shed large numbers infrequently, and fecal sampling isn’t able to reveal the extent of infection with migrating ascarid larvae.
Tapeworms are difficult to find on fecal flotation samples and require ELISA testing of saliva or blood, or special centrifugation techniques available only in certain labs.
Fecal Egg Count Reduction Testing (FECRT)
Testing for the efficacy of deworming drugs compares a pre-treatment fecal egg count on an individual horse with a fecal egg count 14 days following deworming treatment. The pre-treatment fecal count should be done at least 10-12 weeks after a previous deworming. Fecal egg count reduction testing (FECRT) helps to identify which anthelmintic drugs are effective on your farm.
The expectation for egg reduction with ivermectin or moxidectin is at least 95-98%. For all anthelmintics other than ivermectin or moxidectin, a drug is considered effective if there is a greater than 90% reduction in numbers of eggs present in the post-deworming sample. Achieving only an 80% reduction suggests some drug resistance.
Once a drug is found to lack efficacy, it shouldn’t be used again on that premises, or at least not unless combined with another effective dewormer. For example, combining pyrantel with oxibendazole has been shown to decrease egg counts by 95%.
Practical use of the FECRT method enables each equine farm to select from a variety of possible anthelmintics that still retain effectiveness. Horse owners need to limit the temptation to use only ivermectin or moxidectin as deworming treatments. Using only these drugs stimulates equine parasites to develop resistant genes to these medications. As yet, there is no development of any new deworming medications on the horizon.
By Nancy S. Loving, DVM