You’ve finally recognized the folly of deworming every horse several times throughout the year. You’ve accepted your veterinarian’s advice and abandoned traditional deworming practices for an evidence-based program that has regular fecal examination (fecal egg count, or FEC) results as its basis. But change is always a little scary, and you want to understand your new program as thoroughly as possible. So, what do the results of an FEC tell you about an individual horse? The answer is “it depends.”

An FEC is a quantitative technique whereby an individual mixes weighed or measured quantities of feces with a known volume of flotation solution. The veterinarian or lab technician examines a measured subsample of that mixture microscopically to identify the eggs of various parasites occupying the gastrointestinal tract. This technique allows him or her to calculate the type of eggs being passed per unit weight of feces (e.g., eggs per gram, or EPG).

All horses and ruminants always harbor some worms because of the self-perpetuating cycle of fecal contamination and pasture grazing. However, high egg numbers in the feces based on FEC translate into large numbers of infective stages on pasture. This can potentially result in higher parasite numbers in the host (the horse), with a greater likelihood of disease. That said, FECs are not very useful for predicting the absolute numbers of parasites that produced those eggs. A horse passing 800 strongyle eggs per gram of feces won’t have twice as many worms as a pasturemate with 400 EPG. The FEC can be influenced by the horse’s age or immune status, time of year, age of the parasite population, and the residual effects of recent deworming.  

So, what is an unacceptably high egg count? It depends. If your horse was dewormed two weeks ago and is still passing lots of eggs, that treatment obviously didn’t work. One of the FEC’s most powerful applications is measuring fecal egg count reduction (FECR), which is a comparison of paired fecal samples collected before and after treatment. We calculate FECR on a percentage basis to determine anthelmintic efficacy (see chart). If you don’t know your herd’s resistance status, then you should evaluate every class of anthelmintics used at the stable after the next use. Re-evaluate effective drugs by FECR every year or two