The Modern Equine Vet
June 2024
Vol 14 Issue 6 2024
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Infectious Diseases

The Evolution of Deworming Strategies

By Joy Curzio

When deworming products were first brought to market, all horses were dosed every 2 months. Rather than this strategy annihilating parasites found in horses and in the pasture, it led to widespread resistance in small strongyles (Cyathostomins), the most prevalent equine internal parasites, tapeworms, and Parascaris, a type of ascarid found in foals. Now, a more strategic targeted approach to deworming is taken, using a combination of testing to determine the heavy shedders in a herd and the types of parasites that are being shed at different times of the year.

During a Cornell Equine Seminar presented by Elisha Frye, DVM, DACVPM, of the Animal Health Diagnostic Center (AHDC) at the Cornell University College of Veterinary Medicine, recent evidence was reviewed that showed a reduced egg reappearance window for commercially available deworming products used in the United States. Dr. Frye explained that the aims of resistance control are to first prevent the development of resistance and then to delay the accumulation of resistance genes.

“Resistance has a very specific kind of framework so we have to apply it to the same animal, the same type of parasite and the same dose of the same drug,” she said. “The goal of deworming is not zero parasites. It is to lighten the parasitic load of heavy shedders.”

Modus Operandi of Different Parasites
Small strongyles
Small strongyles are sometimes referred to as small red worms. They are a type of roundworm and extremely common in horses. Small strongyles produce eggs in the horse’s colon, which are passed into the pasture where they larvate and are consumed by the horses when grazing. The maturation of an egg to L3-stage larvae can be as quick as 3 to 4 days if the temperature is between 77° F and 91° F, but when temperatures reach extremes (hotter than 104° F or colder than 32° F), eggs do not develop. Geographic areas that experience freeze-thaw cycles are less hospitable to egg survival.

Once the L3-stage larvae enter the gastrointestinal tract, they can encyst in the mucosa of the intestine. After a period of 4 months to 2 years, the L4-stage small strongyles will be passed into the pasture in the spring in large numbers, which may result in diarrhea and severe illness. Any L4-stage larvae that do not encyst in the intestine travel to the colon where they lay eggs, which are passed in feces, and the life cycle completes.

Large strongyles
Large strongyles are deadly to horses. They have a similar life cycle to small strongyles in that the horse ingests larvae while grazing, but large strongyles have a longer prepatent period of 6 to 7 months for the full life cycle, which is helpful in avoiding dewormer resistance.

Large strongyles travel up the mesenteric artery against the blood flow, which forms a thrombus. The parasite then swims back down the artery into a blood vessel of the gut, where it can cause necrosis. If the necrosis, or infarction, isn’t surgically removed, the entire intestine will die, requiring the horse to be euthanized.

Ascarids also are commonly called roundworms. There are several types of ascarids, but Parascaris equorum affects only foals until approximately 1 year of age, at which time they develop natural immunity. Again, the horse eats the parasite’s eggs, but the larvae leave the small intestine and go to the liver, where they travel via blood vessels into the lungs and then the trachea. Foals cough up the Parascaris and then swallow them again, where they go back into the intestine.

“It’s a unique but not so pleasant life cycle that Parascaris takes inside the foal. We worry about impaction colics in foals because these roundworms can fill up the inside of the intestine,” Dr. Frye said.

Tapeworm eggs are passed in horse manure, but it is the oribatid mite that eats the eggs. The eggs develop into larvae in the mite, surviving for approximately 2 to 4 months in the mite. Again, the infestation occurs when the horse grazes. The larvae attach to the cecum and cause colic when found in high numbers. Tapeworms have a prepatent period of 6 to 10 weeks, but they can be shed intermittently in “packets” so the actual egg/parasite count is irrelevant. In addition, these parasites are not recognizable by a fecal float, which is a common test for parasites. As adult tapeworms can grow up to 3 inches wide, potentially congesting the colon and blocking absorption of nutrients, Dr. Frye emphasized that if tapeworms are present, they need to be treated.

Types and Timing of Testing
A fecal float is a technique that floats the parasitic eggs, allowing a count to be taken. One feces ball (10 g) is filtered with a sugar solution to detect strongyles and a zinc sulfate solution for other parasites. The mix is then spun twice in a centrifuge. This is known as the Modified Wisconsin Double Centrifugation Technique and is highly sensitive, detecting down to 1 egg per gram of manure. Another technique, the Modified McMaster, does not use a centrifuge; rather it uses grids in which the eggs are manually counted and a mathematical formula is applied. This technique is less sensitive, detecting down to 25 eggs per gram of manure.

Regardless of the technique used for the fecal float, it should be applied consistently across testing for the same herd, according to Dr. Frye. “Don’t bounce around with the type of technique because then it is going to be hard to interpret the numbers,” she said.

After a fecal float is done, each horse in the herd can be identified as a low, moderate or heavy shedder, according to the AAEP’s classification definitions Heavy shedders eliminate more than 500 eggs per gram of manure, with moderate shedders eliminating between 200 and 500 eggs per gram. Low and moderate shedders do not always have to be dewormed when taking a more targeted approach.

“All of the horses in a herd do need to be dewormed once or twice a year, regardless of shedding, however. If targeted deworming alone is used as a sole practice, large strongyles could thrive,” Dr. Frye noted. “Deworming foals is different because fecal egg reduction tests are not performed on foals. AAEP guidelines recommend deworming foals 4 times in their first year of life.”

According to Dr. Frye, in an ideal world, a fecal float would be done 4 times per year for 1 year, including the spring and fall when large parasite burdens would be expected (depending on climate). Each horse would be categorized as a low, medium or heavy shedder, and then a dewormer would be administered. A fecal egg reduction test would be performed on just the heavy shedders. This test, available at AHDC, which is also the New York State Veterinary Diagnostic Lab, comprises a fecal float, administering a dewormer and recording the product name, and then approximately 14 days later, another fecal float is performed. Results show the reduction in eggs per gram (EPG), or lack thereof in the case of dewormer resistance, after the dewormer was administered using this mathematical formula:

 EPG (pre-treatment) – EPG (14 day post-treatment) X 100 = FECRT
                             EPG (pre-treatment)

EPG, eggs per gram; FECRT, fecal egg count reduction test

Any targeted deworming would only be done during the heavy shedding time. Fecal egg reduction tests would be performed annually on heavy shedders and, if resistance is seen, then a veterinary parasitologist should be consulted, who may recommend that the egg reappearance window be shortened by giving a dewormer that works more frequently.

Treatments and Efficacy
There are 4 classes of anthelminthic agents approved for use in the United States. Fenbendazole which is a benzimidazole-class agent, and pyrantel, which is a tetrahydropyrimidine, are both used for strongyles and Parascaris. Ivermectin and moxidectin both fall into the macrocyclic lactone class, and are both used for strongyles and Parascaris, although moxidectin is also effective for encysted strongyles. Praziquantel is an isoquinoline and is used with other agents solely for the elimination of tapeworms.

No resistance to fenbendazole has been found in ascarids; however there is resistance to ivermectin and moxidectin. Interestingly, small strongyles are resistant to fenbendazole but not to ivermectin and moxidectin. It is also important to note that fenbendazole kills the worms without paralyzing them, reducing the
risk for impaction colic.

Based on the AAEP Internal Parasite Control Guidelines for foals, fenbendazole should be given when the foal is 2 to 3 months of age to kill Parascaris. A fecal egg count can be performed at 6 months of age to determine if strongyles or Parascaris are present, and this fecal egg count will dictate choice of dewormer. At 9 months of age, foals should be dewormed and praziquantel should be added to eradicate any tapeworms; deworming should be repeated at 12 months of age.

Macdonald et al. have shown that the egg reappearance periods (ERPs), which is the time between deworming and egg shedding, have shortened for all these available agents. The 2 most used dewormers, fenbendazole and pyrantel, had ERPs of approximately 6 weeks when first introduced to market; their current ERPs are 4 to 5 weeks. The ERP for ivermectin has been reduced from 9 to 13 weeks to 6 to 8 weeks, and the ERP for moxidectin has been more significantly reduced—from 16 to 22 weeks to 10 to 12 weeks.

“People want to think that more is better—if we give more fenbendazole, it will kill the parasites. That’s not true though necessarily. If there’s dewormer resistance with fenbendazole, it doesn’t matter how much you give—it’s still not going to work,” Dr. Frye noted. Fecal flotation to determine the heavy shedders, coupled with fecal egg reduction tests to monitor dewormer resistance, will keep available dewormers working longer. Horse owners and their veterinarians should move toward a targeted approach to reduce dewormer resistance. MeV

For more information:
Macdonald SL, Abbas G, Ghafar A, et al. Egg reappearance period of anthelmintics against equine cyathostomins: The state of play revisited. Int J Parasitol Drugs Drug Resist. 2023;21:28-39

Kaplan RM, Nielsen MK. An evidence-based approach to equine parasite control: It ain’t the 60s anymore. BEVA. 2010;22(6):306-316.