Problems in our horses with recurrent colic, gut and arterial damage related to blood worms (those are the large strongyles) have all but been eliminated thanks to today’s effective, affordable deworming products. Paste dewormers are simple to use, easy to obtain and relatively inexpensive to purchase.
Most deworming packages have manufacturer-recommended schedules to follow, making it seem a no-brainer project. However, unless you’re keeping up with the most-recent veterinary literature regarding current deworming problems or working closely with your veterinarians on your own barn’s deworming program, you may be headed for trouble using these old methods.
It’s Not A One-For-All Deal
Healthy adult horses often do quite well with minimal dewormings. This is because their digestive tract develops a strong immunity to parasites over time. Horses with low levels of exposure and in stables with good barn-management practices are further protected by not having to deal with large parasite challenges on a routine basis.
However, the strength of adult-parasite immunity varies tremendously among individual horses. In any given group of horses on an identical deworming schedule, you’re likely to find some that test with low/no parasites and others with high burdens. The stress of an injury or illness can also reduce immunity in any horse, and both very young and very old horses have special needs.
Daily Dewormers
The use of daily, low-dose pyrantel tartrate dewormers (Strongid C was the flagship product and now there are a number of brand choices for the same drug) has been popular for many years. The goal is to kill recently ingested larvae of commonly problematic worms before they get a chance to mature or do damage.
Horses on daily dewormers also need periodic treatments for bots. Although not currently labeled for this use, studies have shown that daily low-dose pyrantel tartrate is effective in tapeworm infestations. The problem comes in with effectiveness against small strongyles (Cyathostomes), now considered to be the major parasite problem of horses.
Even when first introduced, this drug showed variable ability to reduce small strongyle egg counts; sometimes high, sometimes only 80% reductions or less. This could actually be a bit of a plus for a healthy adult horse, since the few left behind means there will still be low-level stimulation of natural immunity. However, for high-risk horses, including the very young and old, daily dewormers often don’t offer enough protection. Resistance to the pyrantel family of dewormers is also becoming increasingly common. There’s some epidemiological evidence to suggest that widespread use of daily dewormers might be hastening the process of resistance. This means that if you think a daily dewormer is the ideal choice for a high-risk horse, it may be exactly the opposite.
If starting a daily dewormer for a horse with a known history of parasite problems, be sure to do a FECRT (see page 10) four weeks after starting the product, or eight weeks after starting if you dewormed with ivermectin before starting the daily.
All horses on daily dewormers should have FEC (fecal egg counts) checked at least once a year to make sure the product is getting the job done. This can be done mid-grazing season, or right before your regular, annual ivermectin bot deworming. High-risk horses should be checked more frequently, especially if there are any signs of problems (poor growth, pot belly, poor coat, diarrhea or other digestive upset).
Resistance Worries
Before considering deworming schedules, we need to understand what’s going on in terms of parasite resistance to current drugs. Widespread problems have been documented for resistance to the benzimadazoles (fenbendazole and oxibendazole currently on the market), the pyrantels and phenothiazine. This had led to growing concerns that the important parasites will eventually become resistant to ivermectin and moxidectin, too, although it hasn’t happened yet.
You may have seen claims that rotating dewormers helps to avoid appearance of resistant parasites. Unfortunately, there’s no strong proof that is true. With the benzimidazoles, for example, resistance develops quickly after only a small number of treatments. This has led to a sharp drop in the number of benzimidazole products available on the market and a tendency to substitute pyrantels in a rotation with ivermectin instead.
However, as we stated in the daily-dewormer section, pyrantel resistance is becoming widespread too. Higher doses and combinations of drugs (e.g. fenbendazole with pyrantel) generally don’t solve the problem. This leaves moxidectin and ivermectin as resistance-free options (except for possibly ivermectin and roundworms).
Changing Strategy
The growing resistance problems has led to increasing emphasis on changing deworming programs from the usual “deworm every X number of days” approach to a more selective approach that also involves proper management to reduce risks and identifies horses that need special attention:
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 ex pose 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.
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.
Also With This Article
“Put It To Use”
“Bots And Tapes”
“Dewormer Brands And Drugs”
“If He’s Really Wormy”
“Special Considerations For Foals”
“Fecal Egg Counts”
