What is Your Horse's Fecal Egg Count Telling You?
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.  

Fecal Egg Count Reduction Calculation for Strongyles

But what’s an unacceptably high FEC in horses that haven’t been dewormed recently? Again, it depends. Mature horses’ typical FECs vary widely within a population. Approximately 40 to 50% of mature horses in a herd consistently have low FECs (0 to 100 EPG), regardless of their deworming status. Another 30 to 40% will be “moderate shedders” (200 to 500 EPG) and will require more anthelmintic treatments per year. About 20% are “high shedders” (600 to 3,000 EPG), and they often produce more worm eggs than the rest of the herd combined. Clearly, this subset requires the most intensive deworming program. A horse’s shedding status is about 80% repeatable—so about eight of 10 horses in your herd should fall under the same category as the previous year.  

You can estimate each mature horse’s shedding status from a single fecal sample collected about three or more months after the most recent deworming. One currently recommended evidence-based parasite control program is known as “selective treatment,” whereby low shedders might receive treatment only twice annually or not at all. Moderate shedders might benefit from one additional treatment during the main seasons of pasture transmission (spring through autumn in the North; autumn through spring in the South). Treat high shedders intensively all through the main seasons of transmission.  

Overall, the FEC is still a fairly crude diagnostic technique, but when performed at the appropriate times it can help you identify which horses should be easy keepers at deworming time and which ones present the greatest risk for everyone else. Paired samples can help identify the dewormers that still work in your herd, as well as those to which the worms have developed resistance. Both bits of knowledge will help you improve your parasite control program’s efficacy, reduce annual medication costs, and prolong the utility of the few effective anthelmintics we have left.