Experts say the traditional bimonthly deworming program is no longer the best option

TO BE MOST effective, a deworming program to control internal parasites in a horse should be tailored to that horse's specific needs.

The deworming drugs you use and how often you administer them should depend, in part, on the total number of horses you have, their ages and uses, whether they are on pasture or kept in a barn, and the region and climate. Also, some horses might need to be dewormed more frequently than others.

Ray Kaplan, D.V.M., Ph.D., a veterinary parasitologist and associate professor at the University of Georgia, studies dewormers and works with horse farms in parasite-control programs. He said three chemical classes of drugs are used today as dewormers: benzimidazole, pyrantel, and macrocyclic lactone.

Familiar white pastes

The benzimidazoles ("white pastes"), which have been used the longest, include fenbendazole (Panacur, Safeguard), oxibendazole (Anthelcide), and oxfendazole (Benzelmin). These are effective against several types of worms, including roundworms, but will not kill bots, tapeworms, or encysted small strongyles.

"These drugs are basically the same, and when worms become resistant to one, they are resistant to others in this class, as well," said Kaplan.

Bill Clymer, Ph.D., parasitologist for Fort Dodge Animal Health in Amarillo, Texas, said these products are referred to as purge dewormers. They knock out the majority of worms in the digestive tract, and it takes about three weeks before worms are found in the horse's feces again.

According to Kaplan, oxibendazole is the most effective of the benzimidazoles.

"Fenbendazole doesn't work well at all anymore," Kaplan said. "I have yet to test it on a farm where it worked--as a single dose. It's also sold in a [Panacur] PowerPac, administered at double dosage for five days in a row, to kill encysted cyathostomes [small strongyles]. At double dose, it kills some of the worms that are resistant to a single dose, especially when given for five days. It's not so much the increased dosage but the duration that kills them, overcoming their resistance. But it also will produce super-resistant worms if used frequently, since only the most resistant worms survive."

Daily feed additives

Another class of dewormers is pyrantel salts (pyrantel pamoate, marketed as Strongid paste, and pyrantel tartrate, marketed as the daily feed additive Strongid C). This drug kills mature large and small strongyles, pinworms, and ascarids, but it will not kill bots, tapeworms, or encysted small strongyles.

When using Strongid C, recommendations suggest giving a purge dewormer first to clean out all adult worms in the digestive tract, then mixing the daily dewormer with the horse's grain.

"This regime does a good job of killing whatever worms are in the gut that day," Clymer said. "But if you miss a day, it doesn't work as well. If a horse has encysted cyathostomes, he could be on daily dewormer for a year and encysted worms could still emerge."

The most recent development in this class is the double-dose pyrantel pamoate for tapeworms. Craig Reinemeyer, D.V.M., Ph.D., president of East Tennessee Clinical Research, is studying equine internal parasites and drugs to control them. He said Phoenix Scientific Inc. markets generic pyrantel dewormer called Pyrantel Pamoate Paste.

"Their new dewormer involves an increased dosage of their currently approved product, but the syringe has different dosage levels on it; the plunger can be adjusted for a single dose or double dose," Reinemeyer said. "You can treat two adult horses for regular nematode infections, or one adult horse for tapeworms."

Throughout the body

The third class of dewormers, endectocides that kill internal and external parasites, includes avermectins (ivermectin) and milbemycins (moxidectin, or Quest). They both kill immature larvae at various stages and locations in the body, not just the worms in the gut, but they have a slightly different effectiveness for various parasites.

"Avermectins give 42 days of egg-shed protection," Clymer said. "Moxidectin gives about 84 days of egg-shed protection and provides more effective control of encysted small strongyles. Thus, the recommended frequency for deworming with Quest is four times a year, whereas avermectin products are recommended six times a year. For full protection with the white paste dewormers, technically you'd have to deworm about once a month."

Frequency intervals

Most people do not deworm monthly, and deworming might not be needed in hot, dry weather when worm larvae do not survive well in a pasture. If you monitor egg counts, you will have an indication when you need to deworm.

"But I remind people that fecal egg counts are just an indicator that you have at least one egg-laying female," Clymer said. "You may have 10,000 encysted small strongyles and not a single egg. But it may be good to do fecal examinations before and after treatment to see if resistance is a problem and to know whether the products actually are working."

Most horsemen deworm for bots at the end of fly season and after a hard frost has eliminated the egg-laying bot flies. Bots can be controlled with a good boticide 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. Several products claim efficacy against bots, including ivermectin and moxidectin.

Kaplan advised against using moxidectin too often because it could cause parasite resistance.

"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," Kaplan said. "Every yearling, toward the end of worm transmission season that year, should be dewormed with moxidectin."

Kaplan said the end of strongyle transmission season varies. In the North, it will be the fall. In the South, the end of transmission season will be the spring. In the South, the summer is so hot that transmission is not significant. Worm transmission starts about September when conditions are wetter and peaks in early spring when the weather is wet and the temperatures are rising.

"Moxidectin should be used toward the end of grazing season, after horses have built up larval burdens," Kaplan said. "Deworming at that time gives a good clean-out. In the South, where we don't have much summer transmission, the first worming at the start of grazing season would be in early September. Moderate and high egg-shedders may need to be dewormed in November.

"The next treatment for all horses would be in December or January with ivermectin or moxidectin. Using a [combination] product with praziquantel [Equimax, Zimecterin Gold] enables this treatment to clean out a broad spectrum of nematodes, plus get rid of bots and tapeworms at the same time."

Kaplan, however, does not 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," Kaplan said. "If horses aren't grazing or the parasites aren't surviving on pasture because it's too hot and dry [or when pastures are covered with snow], there's very little transmission occurring and no reason to treat."

At the beginning of grazing season, Kaplan recommends using ivermectin to take care of small strongyles, other nematode parasites, and residual bots. This cleans out egg-laying adults of small strongyles and adult and larval stages of most other nematodes.

"Thus, you can start the grazing season with no eggs being shed by the horses," Kaplan said.

Worm resistance

According to Kaplan, the traditional bimonthly deworming program that was recommended in 1966 is out of date.

"At that time, the only broad-spectrum dewormer was thiabendazole [a benzimidazole no longer on the market]," Kaplan said. "It didn't kill any larval stages of any parasites, just egg-laying adults. The biggest problem in the 1960s was Strongylus vulgaris [large strongyle or 'bloodworm']; it was estimated that 90% of all colics were caused by bloodworm damage."

Kaplan said frequent deworming treatments were designed to kill adult worms before they could shed eggs to prevent future infection, not to help the horse at the time by ridding it of larvae.

"That program became widely adapted and, as a result, S. vulgaris became rare by the 1980s," Kaplan said. "Ivermectin became available and killed all stages of the bloodworm. One dose of ivermectin every six months would theoretically prevent any egg-shedding. Most people began using ivermectin at least twice a year, if not more. By the 1990s, the only place we could find bloodworms were in feral horses or on farms where worm control was neglected.

"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 1960s when that deworming program was introduced, small strongyles were not considered to be an important cause of disease, just a nuisance parasite. But now that we've gotten rid of 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."

Kaplan said drug-resistant strains of worms are everywhere.

"The every-two-month program no longer prevents egg-shedding," Kaplan said. "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 and the worms were not already resistant. Then the eggs reappear in large numbers. So, if you're waiting eight weeks until you treat again, there's a lot of egg-shedding occurring in between, which defeats your control program."