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To begin, run fecal egg counts (FEC) on all horses prior to their next scheduled deworming.
- If fecals are negative or counts very low, don't treat the horse.
- Horses that need deworming should have follow-up fecal egg counts done to determine the FECRT (see sidebar) to make sure the product used actually is effective.
- In addition to targeted treatment of only horses with high egg counts, consider timed treatments at the beginning, middle and end of grazing season rather than regularly year round.
- Periodically monitor post-treatment egg counts in high-risk horses to make sure parasite resistance isn't developing.
- Establish a sane cutoff for FEC results that lead to a deworming. Not treating horses with low egg counts (below 100 to 200 eggs per gram) helps to keep that horse's immune system well stimulated and decreases the percentage of eggs in the environment from parasites that have been exposed to drugs and may have become resistant.
- Instead of rotating between dewormers with each treatment (called fast rotation), consider a slow rotation approach where each drug is used for at least a year, or until post-treatment egg counts show resistance. This is the method commonly used to reduce resistance problems with other farm animals, but it's been quite slow to make its way over to horse management strategies.
- Isolate new horses and don't allow access to paddocks or fields until the horse has been treated with ivermectin or moxidectin. If using any other dewormer, you must do an after-treatment egg count to be sure it was effective and the horse isn't shedding resistant worms into your environment.
Follow this with a larvicidal dosing of five-day, double-dose fenbendazole, and keep the horse on ivermectin every six to eight weeks or moxidectin every 10 to 12 weeks for one or two treatments.
- Never allow your horse to eat off the ground or graze away from home when in areas with heavy horse traffic.
- Remove manure from paddocks and stalls at least once a day.
- Drag the fields, especially when there is crowding, to break up manure piles and expose larvae to the killing effects of drying and sunlight. If you can remove the horses after dragging the field for two to three weeks, all the better.
Fecal Egg Counts And Fecal Egg-Count Resistance Testing
With evidence of parasite, especially small strongyle, resistance to dewormers on the rise, it's important to know which drugs are actually going to work for your horse/farm. Fecal egg counts are relatively inexpensive, and it's wise to incorporate at least one a year into your health management program. The best time to do this is mid-to-late grazing season, when burdens are likely to be large if you have poor control. With large groups of horses, representative sampling of horses that are in good condition and samples from all horses with problems holding their weight, history of colic, etc. should be done. It's common to find wide variations in egg counts within a group, from zero counts to high infestations of eggs.
If you've been using a rotation deworming system and have problem horses, you'll need to know which drugs are effective and which aren't. To determine this, horses need to have fecal egg counts redone after a deworming. This is called FECRT - fecal egg count reduction testing.
In an extensive review article published by parasitologist Ray Kaplan from the University of Georgia (Veterinary Research, 2002), he explains that different cutoffs should be used for determining if the degree of egg-count reduction after a treatment is indicating resistance or not. This is because of the inherent differences in effectiveness of the drugs. The following cutoffs were suggested:
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Ivermectin and moxidectin: Minimum 95% reduction
- Benzimidazoles (oxibendazole, fenbendazole): Minimum 90% reduction
- Pyrantels: Unclear, probably minimum 85 to 90% reduction
If horses don't show these drops in their fecal egg count following a deworming, the drug used should be removed from your deworming program. Since parasites may be resistant to more than one drug at a time, all products in use will need to be tested.
Get An Accurate Fecal Egg Count Before You Deworm
Manure samples that aren't properly handled will give misleading results. Eggs can hatch or become dehydrated beyond recognition. Larvae and tapeworm segments also can be damaged, making your test a waste of money. For most accurate results, follow these steps:
- Collect only freshly passed manure.
- Use a disposable plastic glove or a sheet of plastic wrap to pick up the sample (sample size about one fecal ball).
- Turn the glove inside out, so sample is inside, or wrap the plastic around the sample.
- Place the wrapped sample into a small plastic or glass container with a lid, or seal it inside a self-sealing plastic bag after pushing out air.
- Keep samples cool (refrigerator temps about 45° F) until tested, including in transit to a testing lab, if that's required.
Drag the fields only during the hottest times of the year, when the drying-resistant infectious larvae of strongyles are the most metabolically active and will die quicker.
BOTTOM LINE
Deworming by the calendar, according to product recommendations, is easiest, but growing parasite resistance problems are making it less feasible. If you own several horses or manage a large farm, the measures above are particularly important. If you own only one or two horses and keep them on your own farm, you still need to realize some of the current approaches to deworming may have already induced resistance in your horses.
Doing fecal egg counts doesn't have to mean a lot of expense. Talk to your veterinarian. Costs can probably be reduced to about the same as a deworming if you collect the samples yourself and take them to the office.
Odds are you may not have to deworm after all, and not treating horses that don't need it will delay resistance problems developing. If your horse does need treatment, sampling two weeks after deworming will tell you if your product choice was effective, information you need to know before serious problems develop.
As far as choosing a brand of dewormer, determine first the type of drug(s) you need in the product, and then let cost be your guide. There are a large number of "generic" brand dewormer choices, especially in the category of ivermectin. Purchasing dewormer pastes in quantity can be a money-saving step, but it's not going to help if your horse doesn't need all those frequent doses.
