Parasite Control: The Old, The New, and What To Do

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In the 1960s, deworming was done by a veterinarian, who treated a horse by passing a stomach tube through his nose to deliver potent chemicals that killed everything that wiggled. Horses hated it, and so did owners.

It was considered a breakthrough when owners were able to avoid the stomach-tube experience by simply squirting gooey, over-the-counter deworming paste into their horses' mouths.

By the 1980s, more was known about parasite life cycles, and owners began faithfully following recommendations to administer medications on a six- to eight-week schedule.

If you're still following that or certain other protocols from that time, guess what? This is 2010. After the passage of decades, much more has been learned about equine parasites and how to control them.

As a result, owners are now being urged to make another change in how they think about and approach parasite control. The change is considered so important by the veterinary community that it's been addressed at the last several American Association of Equine Practitioners (AAEP) conventions.

The new protocol relies heavily on fecal-egg-count testing. You'll read more about that here. I've been helping my own practice's clients understand and adjust to the recommended new protocol, and now I'll pass the info on to you.

The goal: to keep you as up to date as possible on ways to protect your horses' health from the damages of internal parasites.

CHANGE OF ENEMY

Common (older) plan: You've been deworming your horses every six or eight weeks. To help yourself stay on schedule, you may deworm each time your farrier comes to trim or shoe.

What's new? Over the years, shifts in the parasite populations have changed the way these pests respond to medications. In the past, the large strongyle was an owner's most serious concern. But in recent years, the small strongyle and the tapeworm have emerged as more significant threats.

Trouble is, the six- to eight-week-interval deworming programs of years past were designed around the life cycle of the large strongyle, no longer the most important enemy in our war against worms.

Even more important, widespread and frequent use of deworming drugs has led to resistance within some parasite populations. While the drugs ivermectin and moxidectin remain effective against most parasites, and although praziquantel is still effective against tapeworms, overuse of these or other deworming agents risks further resistance. If we're not careful, we could be left without an effective parasite-control weapon.

What to do:It's now recommended that you deworm only when it's necessary, and with a medication that's effective against the right worms. This minimizes pressure on the remaining effective deworming medications, and ensures they continue working. With this protocol, your basic plan would be to deworm only twice a year, with fecal egg counts used to determine whether more frequent dewormings are required. (For particulars, see "Making It Work," on page 81.)

Sample Deworming Schedule

Spring (March/April)

  • Check fecal egg counts.
  • Deworm all horses with moxidectin/praziquantel.
  • Recheck fecal egg counts on all horses with EPG >200 prior to deworming.

Summer (June/July)

  • Check fecal egg counts.
  • Deworm all horses with EPG >200, and recheck after deworming.

Fall (September/October)

  • Check fecal egg counts.
  • Deworm all horses with ivermectin.
  • Recheck fecal egg counts on all horses with EPG >200 prior to deworming.

Winter (December/January)

  • Check fecal egg counts.
  • Deworm all horses with EPG >200 and recheck after deworming.

REDUCED EFFECTIVENESS

Common (older) plan: You've been rotating dewormers frequently to avoid resistance. This plan also allows you to take advantage of feed store or catalog sales.

What's new? Resistance to various deworming medications has become widespread. If you conduct random rotation of deworming medications based on what's on sale, you could be treating your horse ineffectively?thus putting him at higher risk for parasite-related health problems. If you're treating a group of horses in a barn or pasture, ineffective deworming means more eggs shed on the pasture, and an escalation of parasite burdens, even in less susceptible horses.

What to do: Use fecal egg counts to monitor the efficacy of your deworming program. There's no real need to rotate, as long as you're testing to determine whether resistance is an issue. But if you discover you have a resistant population of parasites to a specific dewormer, that dewormer should no longer be a part of your parasite-control strategy?even if it's on sale!

NO 'ONE WAY FITS ALL'

Common (older) plan: To ensure a consistent program, you've been deworming all horses in your care the same way, they all get the same medication and are on the same schedule.

What's new? Thanks to research, we now know that certain horses in a group are much more susceptible to parasites than others. They may harbor larger numbers of worms, and may be more resistant to deworming medications. These resistant horses may not only suffer health problems of their own because of their heavier parasite loads, but they'll also shed more eggs, putting other horses on the same farm at higher risk.

What to do:Identify high-risk horses with a strategic deworming plan that involves careful scheduling of fecal egg counts performed at regular intervals. When you know which horses are high-risk, you can target them for more frequent treatment, while maintaining other horses in your herd on a less-aggressive schedule.

NO GUARANTEE

Common (older) plan: You've been using a daily pellet dewormer and administer ivermectin every six months.

What's new? Resistance to pyrantel tartrate (the drug that's in daily dewormers) has been reported. So, while daily deworming is still a great strategy when it's effective, you can't just take for granted that a daily dewormer is effective for your particular horses and horsekeeping set-up.

In addition, this common older plan of daily dewormer, boosted by ivermectin, doesn't include effective treatment for tapeworms, nor does it provide a way to monitor effectiveness of your ivermectin treatments.

What to do: Use the same monitoring schedule for horses on a daily dewormer as for those that aren't. Also evaluate the effectiveness of ivermectin with properly timed fecal egg counts. Consider including praziquantel in the program to kill tapeworms.

OWNER RESISTANCE

Common (older) plan:You've sidestepped fecal egg testing for your horses, because you've heard (accurately) that such tests can be negative even when parasites are present. You'd rather spend money to deworm more often than worry you're wasting it on testing that's not 100-percent reliable.

What's new? While it's true that fecal samples can yield false-negative results (the test is negative, yet parasites are present), such testing is still the best measure available for evaluating overall parasite burdens. And without some kind of monitoring, there's no way to identify resistance to deworming medications.

What to do: Once again, fecal egg counts are now considered a key part of developing a strategic deworming plan, one that targets the most important parasites, minimizes pressure on effective deworming medications, identifies high-risk horses, and monitors resistance development on your farm.

Biannual deworming with an appropriate medication for your farm is still recommended, even if egg counts are zero, because this helps overcome concerns about possible false-negative tests. In fact, re-emergence of large strongyle populations has been reported on farms that deworm less frequently than twice a year?strong evidence that regular deworming is still important, just at a lowered frequency.

MAKING IT WORK

At this point, you may be thinking that an individually tailored deworming program sounds great in theory, but in reality, is complicated to manage.

We thought the same thing at first in our veterinary practice. "How on earth can we make this kind of program work for individual horse owners, boarding stables, and training barns? Can it really be done?" Absolutely. All you really need to do is make sure fecal egg counts are performed at proper intervals.

Most equine veterinarians are equipped to perform accurate fecal egg counts in their own clinics, it's a fairly simple test to perform. If your vet isn't equipped to run the test in-house, s/he can easily send your samples to the lab.

You can collect a fresh fecal sample by simply donning a rubber exam glove, picking up a single fecal ball, and turning the glove inside out around the sample. Use a permanent marker to write the horse's name on the glove. Fecal samples will still yield accurate results for several days following collection. If your vet typically deworms your horses for you (especially for a boarding stable on a regular health-maintenance program), s/he can easily collect samples when on the farm.

The new schedule calls for regular dewormings twice yearly, in the spring and fall months. Prior to every deworming, fecal egg counts are performed in order to identify potential resistant horses.

Even if the egg count is zero at this time, however, a regular deworming is still recommended. This helps protect against possible false negative tests, accounts for parasite larvae that aren't producing eggs, and prevents re-emergence of large strongyles seen on farms where a deworming frequency of less than twice each year is used.

If fecal egg counts are high (>200 eggs per gram, or EPG) at the time of this regular deworming, a follow-up fecal check should be performed seven to 14 days later to ensure there's no resistance to the deworming medication. If resistance is detected, a different medication should be used for future treatment.

During the summer and winter months, no deworming treatment is required except for horses identified as high egg shedders. Fecal egg counts are recommended during these off-months to identify resistant horses. A deworming medication would only be administered to horses with higher counts.

Is it more expensive to run all of those tests? You'll actually spend less over time, because the cost of running fecal egg counts will be less than the cost of more frequent deworming medications. Just as important: By making the change to a strategic parasite control program, you'll not only save money, you'll also have more confidence that your deworming program is effective. You'll minimize contamination of the environment with chemicals, and will help preserve the effectiveness of the deworming medications you depend on to protect your horses.