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Horse health: The search for natural dewormers
by Denise Steffanus
Diligence of horse owners in keeping their horses free of parasites has resulted in overuse of dewormers. As a result, resistance to current classes of dewormers is a mounting problem worldwide, especially in roundworms and strongyles.
Roundworms, or ascarids, hatch in the stomach and then migrate through the bloodstream to the lungs and liver, where they can cause serious damage and even death, especially in foals and young horses.
Large strongyles during their larval stage migrate to organs and arteries, causing damage and blood clots. These blood clots may break away, blocking arteries to the hind legs and intestines, and cause colic, lameness, and sometimes death. The larvae of small strongyles lodge themselves in the intestinal wall. Both types of strongyles can cause loss of appetite, weight loss, depression, progressive weakness, anemia, diarrhea, and episodes of recurring colic.
Four major classes of deworming agents used today are benzimidazoles (fenbendazole and oxibendazole products); tetrahydropyrimidine pyrantel (pyrantel products); macrocyclic lactones (ivermectin and moxidectin products); and praziquantel, one of the active ingredients in duo-dewormers that combine the drug with ivermectin (Equimax, Zimecterin Gold) or with moxidectin (Quest Plus).
In a 2014 joint review, researchers in the United States, Canada, and Brazil found that benzimidazoles (Panacur, Safe-Guard, Anthelcide) have produced resistance worldwide, and parasites in the U.S. have developed a degree of resistance to pyrantel (Strongid, Exodus). They also saw early signs of resistance to ivermectin because of its widespread overuse.
One of those researchers is Martin Nielsen, D.V.M., Ph.D., a board-certified parasitologist at the University of Kentucky’s Maxwell H. Gluck Equine Research Center. After the joint paper was published, Nielsen surveyed managers of Central Kentucky farms in 2014 to document their parasite-control programs. Those results showed that 70 percent still were following a rotational deworming program while performing little or no fecal testing, despite recommendations from experts to adopt an individualized deworming program.
An individualized deworming program identifies the parasite load in individual horses and how most effectively to address it. Some horses need to be dewormed less often than others, and the most effective deworming product may vary depending on the parasites found in a fecal test. Seasonal and regional factors also play an important role.
Nielsen advocates fecal testing before deworming and follow-up fecal testing 14 days after deworming to determine how effective it was.
“Pre-treatment [fecal] samples can help identify the high shedders and tell us if a group of animals is also dealing with ascarid infection,” he said. “But for many people the most important information to have is whether the dewormer works or not. Lots of people are using drugs that don’t work anymore, but they don’t know this because they are not doing the testing. That’s where the post-treatment samples are important.”
New strategies versus old ways
Researchers and veterinarians have an uphill battle to convince horsemen to use fecal testing to guide their deworming programs. A fecal test costs from $13 to $20 and always involves a veterinarian, but dewormers can be purchased over the counter for $10 or less. The prevailing thought among horsemen is that it’s cheaper to just deworm without a fecal test, especially when they are managing a lot of horses in this tough economic time for the horse industry.
“Even a cheap dewormer is expensive if it doesn’t work as intended,” Nielsen argued.
Nielsen has undertaken an intriguing endeavor to determine if the same natural substance, Bacillus thuringiensis, used to kill Eastern tent caterpillars that caused mare reproductive loss syndrome can be used as a new, oral dewormer. This bacterium produces crystal proteins that kill insects without harming animals, which makes it ideal to eradicate tent caterpillars from pastures containing pregnant mares and their young foals.
Since 2001, when MRLS caused the death of 516 foals and the loss of 3,000 pregnancies with an economic blow of $336 million, many farms use B. thuringiensis annually in their pasture-care programs. Genes from B. thuringiensis are incorporated into genetically modified (GMO) corn as a pesticide. So horses already have been exposed to B. thuringiensis in their feed and forage. A question Nielsen and his colleagues may have to address in the future is whether this repeated exposure could accelerate development of parasite resistance to B. thuringiensis in Central Kentucky.
“We have yet to treat a horse with the crystal proteins that B. thuringiensis produces,” Nielsen said. “What we’ve done so far has been to test it in vitro. We culture parasite eggs in a petri dish, and we expose them to different concentrations of the crystal proteins to see if we get a good dose response. The results are very encouraging with the small strongyles, so it looks like it could work very well.”
If his research produces the results he hopes to achieve, B. thuringiensis will provide a new, effective class of natural dewormers.
Nielsen added that just because a substance exists naturally doesn’t mean that parasites can’t develop resistance to it. He pointed out that ivermectin is derived from a naturally occurring fungus.
Over the years, horsemen have tried a wide range of natural approaches to battling intestinal parasites. One of those longtime beliefs is that occasionally feeding a small handful of tobacco or a few filter-free cigarettes to a horse can kill worms.
Richard J. Martin, B.V.Sc., Ph.D., who specializes in veterinary pharmacology and toxicology at Iowa State University, has studied nicotine. He said very old studies into tobacco’s use as an antiparasitic found that its dangers outweigh its benefits. Tobacco, which is toxic, causes a dramatic increase in the horse’s blood pressure and heart rate, and it adversely affects the gastrointestinal tract, he said. Martin advised horsemen to use the safe, proven dewormers on the market, not tobacco.
Other researchers at Iowa State investigated hedge apples, the fruit of the osage-orange tree, for their purported ability to repel insects and kill parasites, if ingested by an animal. They extracted compounds from these softball-size fruits that look like a green brain to test the theory. One of the compounds, when concentrated, did repel insects, but the amount of the naturally occurring compound in the fruit was too little to have the same effect.
Hedge apples contain resveratrol, the same potent antioxidant and anti-inflammatory found in red wine, but no evidence supports that hedge apples can act as a dewormer if consumed. The fruit’s taste is so offensive that nothing eats it.
Diatomaceous earth as a dewormer has many proponents. Product packaging, however, does not state that it is more than a dietary supplement. This flour-fine powder is made from fossilized remains of phytoplankton that contain silica (sand). The sharp edges of each grain purportedly cut into parasites, causing them to die.
“What amazes me about diatomaceous earth is that there are so many strong believers out there who get really upset when I tell them there is no scientific evidence that this works,” Nielsen said. “I’m sorry, but it has been examined in a number of different ways, and it doesn’t seem to be doing anything.”
Moreover, even the arguments supporting diatomaceous earth defy logic, he said. Larvae and worms, which are hardy enough to survive grinding by the horse’s teeth and stewing in stomach acid, would not be damaged by four ounces of the powder mixed into grain and forage during digestion. If diatomaceous earth remained abrasive in the gut, it would cause irritation and erosion of the gastrointestinal lining, similar to the damage that causes sand colic. Following this line of thought, horses in regions where there is silica sand, such as Florida and the Gulf Coast, would never have intestinal parasites – but they do.
“Don’t waste your time on diatomaceous earth. It’s not cutting it,” Nielsen quipped.
Garlic is another purported natural dewormer that has been touted by those who discourage horse owners from using commercial dewormers.
“There is some potential antiparasitic effect of garlic, but again can you feed the animals enough?” Nielsen said. “It looks like with horses, probably not. You could do an extract of the garlic, or you might even be able to identify the chemical in the garlic that kills the worm. I haven’t seen anybody do that, but again, there’s so much work going on with all different types of roots, plants, fruits.”
Predacious fungi, organisms that attract and trap nematodes (roundworms), have been investigated over the years as a potential dewormer, but no company has commercialized them. They do not kill worms inside the horse, but they trap and kill them in the manure before they can contaminate the pasture. Nielsen believes the fungi could be useful as a daily supplement to deworming.
New smartphone device
Nielsen and his colleagues at Gluck Center also are working on new diagnostic tools for parasites. They have developed a smartphone device, Parasight, that counts the number of eggs in a fecal sample. Using Parasight, the veterinarian collects fecal samples on site, passes them through a simple filter system, and stains the eggs with a proprietary reagent that makes them glow. Then the veterinarian takes a photo of the glowing eggs and lets the smartphone do the counting. Results, along with treatment recommendations, are delivered via e-mail within minutes.
The device links up with a database that provides information about resistant parasites, outbreaks, and trends in the area where the sample was taken. The Parasight System App is free, but the veterinarian must purchase the device.
Parasight eliminates the need for shipping the fecal sample to a laboratory, centrifuging the sample, and manually counting the eggs via a microscope. For more information on Parasight, visit www.theparasightsystem.com.
ID worming program
The individual deworming program promoted by Zoetis enlists the stable veterinarian to perform a four-step program for each horse:
◗ Step one establishes a baseline egg count in the horse or herd by performing a Fecal Egg Reduction Test (FERT);
◗ Step two measures the shedding levels of a particular horse to determine whether parasite eggs are being passed to other horses;
◗ Step three assesses the risk of parasite infection; and
◗ Step four determines the horse’s individual needs and establishes a safe and effective deworming program.
“The problem is that we’re grossly over-deworming without knowing whether we’re doing any good or not,” said Tom Lenz, D.V.M., M.S., Zoetis’s senior director of equine veterinary services. “So not only are we often not being effective, but we’re stimulating resistance in the parasites.”
Learn more about this program at www.IDMyHorse.com.