A Changing Paradigm on Deworming Part 2

Work smarter, not harder, at preventing anthelmintic resistance.

This is the second in a two-part series. Need to review Part 1?

The highly permissive type of horse is what Dr. Cyprianna Swiderski, a veterinarian at Mississippi State University, calls a “Typhoid Mary,” and he will have to be dewormed more frequently than other types. Other horses will have a minimal egg count, reflective of a stronger immune system and a better ability to naturally fight off a parasite infection. Still others will fall somewhere in the middle.

It’s not a bad idea, Dr. Swiderski says, to deworm all horses at the start of parasite season, to kill off any larvae that might be encysted (and therefore wouldn’t show up in a fecal egg count test). Here’s where a veterinarian’s knowledge and judgment can really come into play. Some horses will require follow-up fecal egg-counts. For instance, the horse who went into parasite season with a low egg count: Was he low because he had little exposure? And will that change once the small strongyle season starts in earnest? Or was his worm egg count low because he has good immunity?

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Follow-up tests can ascertain this and will help direct your deworming plan for that horse for the year. He’ll need to be dewormed often enough to keep him healthy, but not so often that he is unnecessarily pushing the parasites toward drug resistance.

“It’s a fine balancing act between keeping the horse healthy and not treating him so often that you create a bigger problem than you had initially,” says Tom Kennedy, senior vice president of research and development at Farnam, an AQHA corporate partner that provides the Association’s official dewormers.

A veterinarian can help determine when a horse should be dewormed, and it won’t be based solely on the fecal egg counts. Dr. Swiderski says determining factors include:

    • How does the animal look clinically?
    • What is the history on the farm?
    • What is the stocking density?

Tom adds that veterinarians already are advising horse owners on things like nutrition and breeding practices, and now, “they should also pick up the anthelmintic piece.”

Pasture Management

Every horse will be different, as will be every situation. Are there just a few horses kept on a 100-acre pasture? They’ll have much less exposure to parasites than will horses kept in crowded conditions on small, grazed-down pastures that Tom calls “worm lots.”

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But no matter what your acreage is, pasture management can help limit the parasite load. Dr. Swiderski offers the following tips:

    • Dragging and harrowing pastures disperse parasite larvae, but this is only helpful if done in the summer months, and it necessitates that the pasture be left unoccupied for two to four weeks afterward. In the absence of climate conditions that favor parasite death, dragging the pasture disperses the larvae, which increases infection rates.
    • Any pasture that has been vacant for two months during the summer will have a reduced parasite load. The same goes for fields that have been swathed for hay or that have been grazed by other species of livestock.
    • Do not spread manure on pastures. While proper composting kills small strongyles, it may be impractical on farms to assure that the entire manure pile has been adequately composted.

Another idea, based on research in England: “We don’t have to use anthelmintics at all. We can just pick up the manure and prevent infection,” Dr. Swiderski says.

Horses are infected with small strongyles when they graze too close to manure piles that have infectious forms of the larvae. So if conscientious owners pick up the manure from their pasture every few days, before the larvae become infectious, there is no risk of the horses getting small strongyles.

That, of course, is labor intensive, but Dr. Swiderski jokes that her “army of children” helps her keep pastures clean. Pasture vacuums are another option.

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Cleaning pastures also opens up more grazing space – as much as twice as much – since horses will naturally avoid manure piles. A horse owner with a larger pasture might justify the cost of a pasture vacuum if he realizes he’ll go from 10 acres of grazeable grass to 20 acres, just by keeping it clean.

Management issues are just one piece of the puzzle, which Dr. Swiderski allows is a complicated one to figure out. It’s best done, she says, with the veterinarian and the horse owner working as a team.

“I doubt there are veterinarians out there that feel like they have to give every single dose of dewormer,” she says, “but they certainly need to be involved in developing a deworming plan. … and then the owner has a framework. Even if they’re going to the co-op to buy their dewormer, they’re buying it responsibly.”

And therein lies the new paradigm: to use dewormer less frequently, but more importantly, to use it optimally.