Fecal egg per gram (EPG) counts are valuable to actually determine the number of worm eggs in a horse’s manure. Routine fecal floats determine if parasite eggs are present but cannot differentiate a heavy-shedding horse from one shedding fewer worm eggs. Manure for EPG determination is weighed, floated in a standard volume-of-egg flotation solution, and worm eggs are counted under a microscope using a special slide.
The most commonly observed worm egg is called a typical strongyle-type ova (eggs) because the eggs of small and large strongyles look alike. Small strongyles are the most common internal parasite of horses. Roundworm eggs are also observed, especially in younger horses. Tapeworm eggs are seldom seen, even when horses are infected with large numbers of tapeworms.
An excellent study from Denmark found that horses tend to be consistent in the amount of worm eggs shed in their manure. Less than 200 EPG is considered a low level of shedding. Horses that had two previous EPG counts performed six months apart that were both less than 200 EPG had an 84% chance that their third EPG count would also be low. Similarly, if a horse had two previous EPG counts greater than 200, the chance of being greater than 200 EPG on the third count was 59%. This demonstrated that horses with low EPG counts tend to keep low EPG counts and those that are high also tend to stay high.
These findings help to target deworming to reduce resistance and allow for the classification of horses into low or high EPG shedders. Horses that consistently have low EPG counts do not require frequent deworming, and deworming twice a year is probably sufficient. Horses that have a consistently high EPG count can be segregated to reduce overall pasture parasite egg contamination, and these horses may benefit from more frequent deworming.
Your veterinarian can perform EPG counts and is the best source of information to tailor a deworming program that is best for your farm. EPG counts also help slow the development of resistance to dewormers, and less frequent deworming reduces costs.
Reference: Nielsen, M.K., N. Haaning, and S.N. Olsen. 2006. Strongyle egg shedding consistency in horses on farms using selective therapy in Denmark. Veterinary Parasitology 135:333-335.