A good deworming program can greatly benefit horses, but it should be tailored to their specific needs. The deworming drugs you choose, and how often you administer them, should depend in part upon how many horses you have, age and classes of horses, whether they are at pasture, your geographic region and climate, etc. Across the board deworming schedules may be inefficient; some horses won’t need deworming that often, while others may need more frequent deworming.
Chemical dewormers kill/remove internal parasites (to minimize the damage they do, and the nutrients they rob) and interrupt their life cycle–halting passage of eggs or immature forms that might reinfest horses. Your veterinarian can help you devise a good deworming schedule that will be most effective for your horses. In some instances, where horses are not continually exposed to egg and larvae, you may be able to deworm two or three times a year, while in other situations horses must be treated more often.
CLASSES OF DEWORMING DRUGS – Ray Kaplan, DVM, PhD (a veterinary parasitologist and Associate Professor at University of Georgia) has done a number of studies involving deworming and dewormers, and works with horse farms in parasite control programs. Regarding the deworming drugs available today, he says there are three chemical classes. Within each class, the drugs are very similar, but between classes they are very different.
Benzimidazoles – The benzimidazole class (“white pastes”) have been used the longest and includes fenbendazole (Panacur, Safeguard), oxibendazole (Anthelcide), and ox-fendazole (Benzelmin). These drugs are effective against several types of worms, including roundworms, but won’t kill bots, tapeworms or encysted small strongyles. “These dewormers are basically the same, and when worms become resistant to one they are resistant to other drugs in this class as well,” says Kaplan.
Bill Clymer, parasitologist (Fort Dodge Professional Services) in Amarillo, Texas, says these products are referred to as purge dewormers. They knock out the majority of worms in the digestive tract and it will be about three weeks before you find worm eggs in the horse’s feces again. “It takes that length of time before new larvae become egg-laying adults, so it gives you about three weeks of protection from egg shed,” says Clymer.
Kaplan says oxibendazole is the most effective of these 3 types of benzimidazoles. “Fenbendazole doesn’t work well at all anymore. I have yet to test it on a farm where it worked–as a single dose. It’s also sold in a PowerPac, however, administered at double dosage for 5 days in a row, to kill encysted cyathostomes (small strongyles). At double dose, it does kill some of the worms that are resistant to a single dose, especially when given for 5 days. It’s not so much the increased dosage but the duration that kills them, overcoming the resistance of a lot of those worms. But it also will produce super-resistant worms if used frequently, since only the most resistant worms survive. You’re already starting out with resistant worms, then treating with this massive dose, so the only ones that survive are the most resistent. If the PowerPac is used, it should be used in a very targeted manner rather than broad spectrum across the whole herd, or it will soon have very little usefulness,” he says.
Pyrantel – Another class of dewormers is the pyrantel salts (pyrantel pamoate, marketed as Strongid paste and pyrantel tartrate marketed as Strongid C, in pellets to be administered daily in grain). This drug kills mature large and small strongyles, pinworms and ascarids, but won’t kill bots, tapeworms or encysted small strongyles. When using Strongid C as a feed additive, recommendations suggest giving a purge dewormer first (to clean out all the adult worms in the digestive tract), then administer the daily dewormer with grain.
“This regime does a good job of killing whatever worms are in the gut that day. But if you miss a day, it doesn’t work as well. And if a horse has encysted cyathostomes, he could be on the daily dewormer for a year and those encysted worms could still emerge,” says Clymer.
The most recent new development in this class of drugs is approval of double dose pyrantel pamoate for tapeworm control. Craig Reinemeyer, DVM, PhD (formerly with University of Tennessee and now president of a company called East Tennessee Clinical Research, doing studies on equine internal parasites and the drugs to control them) says Phoenix Scientific, Inc. markets a generic pyrantel dewormer called Pyrantel Pamoate Paste. “Their new dewormer involves an increased dosage of their currently approved product, but the syringe will have different dosage levels on it; the plunger can be adjusted for a single dose or double dose. You could treat two adult horses for regular nematode infections, or one adult horse for tapeworms,” explains Reinemeyer.
Macrocylic lactones – The third class of dewormers is referred to as endectocides (killing both internal and external parasites), and includes the avermectins (ivermectin) and milbemycins (moxidectin, or Quest), says Clymer. The two types have some similarities and some differences. They both kill immature larvae at various stages and locations in the body (and not just the worms in the gut) but have slightly different effectiveness for various parasites.
“The avermectins have at least 8 or 9 different product names and give about 42 day egg shed protection. Moxidectin (Quest) gives about 84 days of egg shed protection and provides more effective control of encysted small stron-gyles. Thus the recommended frequency for deworming with Quest is 4 times a year (spring, summer, fall and winter) whereas avermectin products are recommended six times a year (every 2 months). For full protection with the white paste dewormers, technically you’d have to deworm about once a month,” says Clymer.
DEWORMING PROGRAMS AND FREQUENCY INTERVALS
Most people don’t deworm that often, and it may not be necessary under certain conditions, such as during hot, dry weather when worm larvae don’t survive as well in a pasture; horses won’t have as many problems with reinfection as in spring and fall. If you monitor egg counts, you have a clue about the need for deworming. “But I remind people that fecal egg counts are just an indicator that you have at least one egg-laying female. You may also have 10,000 encysted small strongyles and not a single egg,” says Clymer. It may be important, however, to do a fecal exam before and after treatment to make sure resistance is not a problem, and to know whether the products are actually deworming the horses.
Most horsemen deworm for bots after the end of fly season (after some hard frosts eliminate the egg-laying bot flies). Bots can be controlled with a good boticide (there are several products with label claims for efficacy against bots, including ivermectin and moxidectin), given once or twice a year. The second dose can be given in early spring to kill any bots still in the stomach, that might have been missed by the fall deworming.
Kaplan advises against using moxidectin very often, to prevent resistance problems. “But given the fact there’s no resistance to it yet, and it’s the best dewormer, it has a place in every deworming program on every farm. Horsemen should NOT be using it on every horse, on every treatment interval, but it should be used for horses that have chronically high fecal egg counts, and for yearlings. Every yearling, toward the end of worm transmission season that year, should be dewormed with moxidectin,” he says.
“The end of worm transmission will vary, depending on where you live. In northern climates, it will be in the fall. In the South, the end of transmission season will be in the spring, for strongyles. Here in the South, in summer it gets so hot that transmission for strongyles is not significant. Worm transmission starts in early fall (about September) when conditions are wetter, and peaks in early spring when it’s wet and warming up. Moxidectin should be used toward the end of grazing season, after the horses have built up their larval burdens. Deworming at that time will give a good clean-out. So in the South, where we don’t have much summer transmission (unless we have a really wet summer), the first worming at start of grazing season would be early September. Moderate and high egg shedders may need to be dewormed in November, and the next treatment for all horses would be in December or January (the end of grazing season) with ivermectin or moxidectin. Using a product with praziquantel will enable this tratment to clean out a broad spectrum of nematodes, plus get rid of bots and tapeworms at the same time,” says Kaplan.
The spring treatment, at the beginning of grazing season, should be given to all horses. “You should do an egg count for strongyles at the same time. I don’t recommend treating all horses throughout the year. We need to think of parasite control as a cycle that starts at the beginning of worm transmission season and ends at the end of that transmission season. If horses aren’t grazing, or the parasites aren’t surviving on pasture because it’s too hot and dry (or not accessible to horses when pastures are snow covered), there’s very little transmission occurring and no reason to treat,” he says.
At the beginning of grazing season, Kaplan usually recommends ivermectin to take care of the small strongyles as well as other nematode parasites and residual bots. This cleans out egg-laying adults of small strongyles and both the adult and larval stages of most other nematodes. Therefore you can start the grazing season with no eggs being shed by the horses. “The horses can then graze for a couple months without producing significant egg contamination and will pick up only small numbers of new larvae that survived winter,” he says.
Using ivermectin at the start of grazing and moxidectin later (for a total of two annual treatments) may be all the treatment needed for horses that are not high egg shedders or exposed to a lot of worm eggs and larvae. The every-two-month program that’s still so popular is out of date, says Kaplan.
“The reasons it was recommended, in 1966, no longer exist. At that time the only broad spectrum dewormer available was thiabendazole (one of the benzimidazoles that is no longer on the market). It didn’t kill any of the larval stages of any parasites, just the egg-laying adults. The biggest problem in the 1960′s was Strongylus vulgaris (the large strongyle bloodworm); it was by far the most damaging parasite. It was estimated that 90 percent of all colics were caused by bloodworm damage,” says Kaplan.
The large strongyle has a very long life cycle (6 to 8 months) and in early years we could only kill the adults, yet the larval stages were most harmful to the horse. The only way to improve the health of horses was to prevent them from getting reinfected. The dewormings could not help the horse, but you were killing the egg-laying females to minimize chances for infection later on.
“This is a concept horsemen often don’t understand. The frequent deworming treatments were designed to kill adult worms before they could shed eggs–to prevent future infection–not to help the horse at that point in time. This is still true today, but especially then, because we couldn’t kill the larval stages that were causing disease. If we treated every 2 months, we could almost totally prevent large strongyle egg shedding. That program became widely adapted, and as a result, S. vulgaris became rare by the 1980′s. Then ivermectin became available and killed all stages of the bloodworm. One dose of ivermectin every 6 months would theoretically prevent any egg shedding from S. vulgaris. Most people began using ivermectin at least twice a year, if not more. By the 1990′s, the only place we could find bloodworms were in feral horses or on farms where worm control was neglected,” says Kaplan.
“In the interim, this every-two-month deworming approach has caused drug resistance in small strongyles that encyst in the gut lining. Back in the 1960′s, when that deworming program was introduced, the small strongyles were not considered to be an important cause of disease–just a nuisance parasite. But now that we’ve gotten rid of the large strongyles, we realize that even though they are not as dangerous, the small strongyles can cause significant damage when they accumulate in large numbers,” he says.
RESISTANCE ISSUES – “What’s happened is that we’ve selected for drug resistant strains of worms, and they are everywhere. We did a study, looking at horse farms in Kentucky, Louisiana, Florida and Georgia, and despite major differences between farms (some were breeding farms or racing stables with expensive horses and higher levels of management, and some were pleasure horses), the patterns of resistance we found were exactly the same. There’s so much movement of animals today, it’s almost as if we have one big pasture in this country that our horses are sharing. The resistant worms get passed around very easily,” says Kaplan.
So now we have a lot of resistant worms, and the every-two-month program no longer prevents egg shedding. “The worms have learned to start producing eggs quicker. With all benzimidazole drugs and pyrantel, you only have about four to six weeks of egg control, assuming the drug worked in the first place (worms not already resistant). Then the eggs reappear in large numbers. So if you are waiting eight weeks until you treat again, there’s a lot of egg shedding occurring in between, which defeats your control program,” he says.
A study done by Dr. Reinemeyer compared the efficacy of two dewormers, treating groups of horses that had never been dewormed in their lives (Tennessee Walking Horses, all two years-old and younger, from the same farm). “There were two phases of the study. Horses in one pasture received Quest and similar horses in an identical pasture were treated with a double dose of fenbendazole (Panacur) for five consecutive days. Fifteen days after the first treatment, egg counts were reduced 99 percent in the Quest group, and 93.4 percent reduced in the horses receiving Panacur. At 30 and 45 days after treatment, egg count reductions were more than 99.7 percent for Quest, and 89.8 percent and 72.8 percent for fenbendazole. After 90 days the horses were dewormed again with the same treatments they received initially. The second treatment of Quest reduced egg counts by 100 percent, but five days of fenbendazole was less effective than originally noted, reducing egg counts by only 85.6 percent,” says Reinemeyer.
“The horses were kept in their respective pastures and left for eight weeks, then dewormed with a standard dose of fenbendazole, like the average horseman would do on a quarterly deworming schedule. After this dose (one tube of fenbendazole) the group of previously Quest-treated horses had a 45 percent reduction in egg count, meaning 55 percent of that worm population was already resistant to fenbendazole. When the fenbendazole-treated horses were treated again with fenbendazole, there was a reduction of negative 60.7 percent (in other words, egg counts increased by 60 percent after fenbendazole treatment). By using the same dewormer twice, that much in-crease in resistance occurred,” says Reinemeyer.