Egg counts key to parasite control

Egg counts key to parasite control

Jul 01, 2006

Eggs of the bot fly attach to the hairs on the leg of a horse.
Overcoming clients' preconceived notions can be difficult, especially as newer research suggests that treating horses as individuals might be more effective than a shotgun deworming regimen.

"Talking to a client, I say: 'Your horse doesn't have any eggs right now, you don't need to deworm. And they say, 'Oh, but it's been two months, I have to deworm,'" says Natalie Carrillo, MV, Dipl. ACVIM, professor at the University of Florida College of Veterinary Medicine. "Even though you've got hard proof that their animal doesn't necessarily have to be dewormed, it's very difficult to convince any owner otherwise."

A heavy ascarid infection.
Practitioners have two options; the first is prevention, Carrillo says. The alternate is to get fecal egg counts (FEC) on a monthly basis, and only deworm those that have high parasite load as you rotate pastures.

"It's very difficult to get a client to believe that," she says. "That's the biggest problem that we're facing. Nobody wants to go ahead and say 'I'm not going to deworm my animals because we just don't see any fecal eggs'. "

While current 8-week protocols that rotate drugs works by and large, some horses on some farms still suffer infestations, requiring the individual treatment of some animals differently from the herd as a whole.

"Today most horse owners continue to follow recommendations based on knowledge that is 30-40 years old," states Ray Kaplan, DVM, PhD, professor at the University of Georgia College of Veterinary Medicine.

A small strongyle tail shown at 160x.
For more than 20 years, some species of small strongyles have been fighting back. A recent study suggests that some animals from some farms can pose serious treatment problems. So much so, some veterinarians advocate a more individualized approach as a measure to protect the long-term efficacy of anthelmintics.

"I will tell you that a majority of veterinarians in practice in the horse world are not particularly interested in parasitology, do not stay current with the literature, and don't read the articles in the journals," says Rose Nolen-Walston, DVM, Dipl. ACVIM, professor at the Cummings School of Veterinary Medicine at Tufts University. "I would have to say that most of the advice that horse owners get from their veterinarians is very out of date and not particularly evidence based. I think we have a lot of excellent evidence on what the most appropriate way to deworm horses is, but I don't think DVMs are necessarily giving that advice to their clients. They are recommending to deworm every eight weeks, and rotate dewormer each eight weeks, which is not supported by the literature at this point."


A small strongyle egg.
Parasitologists speculate that refugia, the proportion of the overall parasite population that is not being selected by anthelmintic drugs, can be the key to unlocking proper protocols in any given horse. It is a balancing act because too much refugia means that you are not controlling parasites. The way to achieve the balance is by not treating all the animals all the time. Most of the horses don't need to be treated most of the time. The few horses that need to be treated are usually not treated often enough.