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.
Put It To Use
- Deworm by using fecal egg counts.
Bots And Tapes
Because bots and tapeworms aren't readily detected on fecal exams and aren't sensitive to most dewormers, they need special attention. The new combination dewormers that contain the drug praziquantel are the ideal choice for both. Use these at least once a year, in late fall or early winter.
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.
If He's Really Wormy
It's a Catch-22: Horses that are in poor condition because of parasites are also the ones at highest risk of adverse reactions to deworming. Even dewormer packaging warns about use in debilitated horses. What do you do? Don't let fear of reactions paralyze you. This horse will only get better when you get rid of his parasites. First, involve your veterinarian for ways to reduce the risks. A commonly used approach is to treat the horse with flunixin meglumine (Banamine) and sometimes an anthistamine as well on the deworming days and maybe for one day after. This will help reduce the risk of adverse reactions to large parasite die-offs.
As for the actual dewormers, an initial treatment with a drug that will only kill off the mature worms (e.g. pyrantel, oxibendazole or fenbendazole) may be tried, but resistance to these drugs is common so a post-treatment egg count is advisable. If adult parasites persist, the next step is ivermectin (first step with some veterinarians), followed a week or two later by a larvicidal deworming.
Larvicidal Deworming
Effective deworming of heavily parasitized horses should include treatments that remove as many of the immature forms within the walls of the intestine as possible. Ivermectin does a good job of this for many parasites but isn't effective against two life stages of small strongyles (cyathostomes), which are now the most common parasite of horses. Treatment with double doses of fenbendazole for five consecutive days remains the most effective at removing immature stages of small strongyles and has a low rate of adverse effects. However, resistance to this drug is growing, too, and horses may continue to shed small strongyle eggs even after a five-day, double-dose treatment. If positive egg counts persist, the horse will need to also be dewormed with a single dose of ivermectin or moxidectin, and fenbendazole will need to be avoided in the future.
Don't Trust "Natural" Dewormers
While there is no shortage of alternative, herbal and/or diatomaceous earth-based dewormers out there, nor any shortage of glowing testimonials to their use, there are still no published well-conducted studies to show these products actually work (see Horse Journal, May 2004). The gold-standard test for effectiveness is the FECRT (see page 10). Without taking horses known to be parasitized and retesting them after treatment, there's simply no way to know if the product works. It's not at all unusual to find negative fecals or low egg counts in healthy adult horses regardless of whether they're being dewormed, and too easy for problems with sample handling (e.g., sample sent through the mail) to result in false-negative egg counts. Furthermore, many of the herbal ingredients that do have the potential to kill worms are also potentially toxic to the horse. Most modern paste dewormers have a wide margin of safety. If equine studies become available that show alternative treatments work, we'll be delighted to hear about them. In the meantime, the intelligent, safe choice is to use dewormer drugs.
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.
