Parasites
“Spring shots” season is upon us, and it’s time to think about worms too!
This is a good time to have a fecal egg count (FEC) done by your vet when they’re out for spring preventative care visits. Plan ahead by grabbing a half a poop ball from each horse/donkey/mule in a ziplock bag and label the horse’s name on the bag with sharpie. Store in the fridge until the vet arrives, or if it’s still chilly in your part of the world it’s ok to keep outside in temps 40 and below. The cold fridge/outside will keep eggs from hatching and help you get an accurate count. No need to send a huge sample- I can’t tell you how many times people would drop off a grocery bag or larger with most of a poop pile in it when I worked at the lab! Too funny.
Your vet will either do the FEC in-house or end it off to a lab for testing. Often, a Modified McMasters Egg Count is performed. Part of the fecal sample is mixed with a special solution that makes the parasite eggs float to the top. Then a small amount of the liquid suspension is placed on a special microscope slide and the tech counts how many parasite eggs are in the grid. You’ll get a report from the vet telling you how many “eggs per gram” though some vets will just tell you if the horse is high, medium or low.
What is FEC good for? It lets you know whether your horse is a high or low shedder, which helps guide you as to how many times per year they need to be dewormed. It doesn’t diagnose disease and doesn’t tell you the total worm burden in the horse. It doesn’t indicate whether the horse may have a reaction to a large number of worms passing after giving a chemical dewormer. A FEC does not tell you about tapeworm/bot larvae/pinworm/encysted strongyle status.
AAEP has a lot of resources on their updated parasite guidance if you want to learn more, https://aaep.org/wp-content/uploads/2024/05/Internal-Parasite-Guidelines_Updated.pdf
If you prefer video format, I hosted extension agent Lara Worden for an in depth discussion about best practices for equine parasite management, you can watch it here: https://youtu.be/VrdsK0shZ3s?si=gYKoFWXXKgDh-8bG
The short version is- if your adult horse is a high shedder, plan on de-worming 4 times per year. Three times with ivermectin, which has pretty good effectiveness in the US for large strongyles, most small strongyles and stomach bots; and one time with Quest Plus in late winter/early spring to cover encysted strongyles and tapeworms. This will cover your bases- large strongyles, pinworms, encysted small strongyles, tapeworms, bots. Horses that are low shedders typically just need two rounds of dewormer- fall ivermectin and spring Quest Plus.
In an ideal world, you’d check for parasite resistance with a fecal egg count reduction test (FECRT) which lets you know how well the chemical dewormers you’re giving are working on your horses specifically. Some places have worms with increased resistance to dewormers- meaning that even if you give the “correct” dewormer, it isn’t killing back the worms like expected. There’s increasing resistance to certain types of dewormer- for example, you should assume that Safeguard and Strongid dewormer won’t work for anything except pinworms and large strongyles (bloodworms). Ivermectin and Quest won’t work for Ascarids or pinworms.