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Deworming-When, Why, and with What?
Types of Parasites:
The four most important types of intestinal parasites (worms) are large strongyles, small strongyles, ascarids (roundworms), and tape worms. All parasites have life cycles with multiple stages of development- eggs, larvae of several types, and adults. The larvae are generally responsible for the majority of the damage done to the horse, because they migrate through the tissues and organs of the body, leaving scar tissue behind.
Large strongyle larvae are picked up from grass in the early spring, and do their damage mostly to the arteries supplying the intestinal tract. Larval migration through the walls of these arteries causes scar tissue to form, which can lead to impaired circulation to the intestine, changing motility and function. When severe or chronic, the scarring can lead to slowed blood flow, clot formation and a showering of emboli
that cuts off the blood supply to a portion of the gut. The result is a life threatening colic. This catastrophic loss of circulation causes death of the intestine, with pain, shock, and ultimately, death of the horse from overwhelming toxicity and peritonitis (infection of the abdominal cavity).
While there is little that an owner can do to change damage done in earlier years, careful attention to deworming can prevent additional injury.
Small strongyle larvae, also ingested orally, “hibernate” in cysts in the walls of the large intestine, and are not killed by most of the dewormers while hiding in these cysts. Encysted strongyle larvae can cause exten- sive edema (swelling) and hemorrhage when the cysts burst to release the larvae for development into adults. This results in severe inflammation and damage to the absorptive surface of the intestine. The larval encysted forms can remain in hibernation for extended periods. When release of a large number of small strongyle larvae occurs, the horse can develop severe diarrhea, weight loss, chronic colic, and in severe cases, may die.
Ascarids, a danger primarily to foals, produce millions of sticky eggs that adhere to practically any surface and survive repeated cycles of cold and drying. These eggs are encountered as the foals explore their environments by licking and chewing. The eggs develop into larvae that migrate through the liver and lungs. From the lungs they are coughed up, then swallowed, and develop into adults in the intestine. The adults then produce eggs that are passed in manure and contaminate the environment. The resulting larval damage to the lung tissue predisposes the foal to
pneumonia. In addition, heavy infestation with ascarid adults can cause a physical obstruction and even rupture of the intestine.
Tapeworms are ingested through a secondary host, the orbatid mite, a tiny pasture mite that is inadvertently eaten while grazing. Adult tapeworms are commonly found at the junction of the small intestine and cecum, and cause ulceration, inflammation, and chronic colic. Occasionally intussusception, a condition where the bowel telescopes in on itself, results and requires surgical correction.
Types of Dewormers:
Chemicals used for deworming should ideally have a wide margin of safety, be useful against multiple types of parasites, and be easy to administer. In addition, parasites can become resistant to certain chemicals, so continued effectiveness must be shown.
There are four dewormers that are effective against largestrongyles, small strongyles and ascarids without significant resistance having developed. These dewormers are ivermectin (e.g. Zimectrin, Eqvalan, etc.), pyrantel pamoate (e.g. Strongid), oxybendazole (e.g. Anthelcide),
and moxidectin (e.g. Quest). Of these, ivermectin has efficacy against the largest number of larval forms as well as adults, but it will not eliminate encysted forms of small strongyles. Moxidectin is labeled for use against the encysted forms of small strongyles, so it is a wonderful addition to our arsenal. However, it must be used with care because the margin of safety is relatively small. The moxidectin chemical is distributed to body fat, so if the horse lacks sufficient fat tissue, too high a blood level may
result, leading to neurologic damage. Because of this, moxidectin should never be used in foals less than 1 year old, or thin, debilitated or elderly horses.
Other compounds on the market such as fenbendazole (Panacur, Safe- guard) generally have poor action against small strongyles due to the development of resistance, despite label claims of effectiveness. You should avoid their routine use, despite their low cost. However, these dewormers are very effective against ascarids, so they are useful for foals. In addition, five consecutive days of double doses of fenbendazole (Panacur Powerpak) can be up to 90% effective in eliminating encysted small strongyles, so their use is often prescribed for this purpose by a veterinarian. Horses with clinical signs of chronic parasitism may also benefit from administration of daily dewormer, such as Strongid C, which will remove all the newly emerging small strongyle adults as they appear, as well as treat new exposures.
None of the above-described dewormers are effective against tapeworms, but there are now several ways to eliminate this dangerous parasite. New products on the market combine ivermectin with praziquantel (Zimectrin Gold, Equimax), or moxidectin with praziquantel (e.g. Quest
Plus). Praziquantel has been proven extremely effective in eliminating
equine tapeworms. Previously the only effective dewormer for tapeworms was pyrantel pamoate (e.g. Strongid) given at a double dose. This meant 2 tubes of paste per 1200lb horse. Because of the difficulty delivering this volume of paste, many of our clients preferred to have us deliver the dose with liquid pyrantel pamoate through a nasogastric tube.
Deworming Schedules:
Many of you ask about appropriate deworming schedules for your horses. Individual circumstances at farms affect the frequency with which your horses need to be dewormed. Horses kept in small areas of pasture or in dirt lots need more frequent treatment, as higher levels of contamination of the available space occur. In contrast, a small number of horses on a large
pasture may pick up very few parasites.
The most effective way to determine the number of worms your horse is carrying is to have a fecal examination performed by our office. You can drop off a fresh fecal ball any time our office is open, and results will be available within 48 hours. Because of the impact of deworming chemicals on the environment (some pass through unchanged in the manure), minimizing their use through the use of fecal exams is wise. A fecal exam performed right before the next scheduled deworming will indicate if a longer or shorter interval would be more appropriate. Fecal egg counts of
less than 50-100 eggs per gram are considered acceptable. The sample schedules may be modified according to the results of a quantitative fecal exam done just before the next scheduled deworming in June or July. A fecal exam should also be performed at any time of the year when weight loss, diarrhea, or other signs of debilitation occur. Fecal exams will NOT show the presence of encysted small strongyle larvae or of tapeworms.
Because ivermectin has good effectiveness against larval forms, its use in the spring, when larvae are lying in wait on the grass waiting to be ingested, is excellent strategy. Ivermectin is also the safest dewormer effective against stomach bots (a larval form of the bot fly, which lays small yellow eggs on the horses’ hair). A dose of praziquantel is recommended on an annual basis and should be given in the winter or late fall, after a summer of exposure to the tapeworm-carrying orbatid mites results in a possible infestation. An annual dose of moxidectin or five day course of double dose fenbendazole to eliminate any encysted small strongyles is often recommended, and may be most appropriate at the end of the grazing season to kill any larvae which may have become encysted over the summer.
Alternatively, use in the spring may be indicated if there are clinical signs, as this is when the encysted forms generally emerge and cause inflammation. Rotation with oxybendazole and single dose pyrantel for other periods of the year is appropriate.
Horses that are stabled together should be dewormed on the same
schedule so that they do not become reinfected immediately. In situations where this is impractical, the use of daily low dose pyrantel pamoate (e.g. Strongid C) is an excellent choice. It is also very useful in situations where overcrowding is unavoidable and contamination inevitable. Some of our clients choose to use daily dewormer with their young stock to ensure optimum growth. Horses with heavy small strongyle infestations manifesting clinical signs of weight loss, diarrhea and poor hair coats canshow
dramatic improvement after treatment with daily dewormer for 90 days especially following larvicidal deworming with moxidectin or double doses of fenbendazole, as prescribed by a veterinarian.
Foals should be dewormed every 30 days beginning when they are 30 days of age, until they are 1 year old. This is the single most important thing you can do to ensure their good health and long life. Alternating oxybendazole or fenbendazole, pyrantel pamoate, and ivermectin on a monthly basis is appropriate. Either ivermectin with praziquantel or a double dose of pyrantel pamoate should be administered as the youngster nears his or her first birthday.
Internal parasites are a leading cause of colic and unthriftiness inhorses. Fortunately, much is understood about parasite control and strategies to minimize their deleterious effects. Please do not hesitate to contact our office with your questions or ask us directly during your horses’ annual spring visit. If you are interested in Strongid C, please call the office for more information or to borrow an explanatory video.
February Pyrantel pamoate ( e.g. Strongid)
March Oxybendazole (e.g. Anthelcide)
April Pyrantel pamoate (e.g. Strongid)
May Ivermectin ( e.g. Eqvalan, Zimectrin, Equell)
June Ivermectin (e.g. Eqvalan, Zimectrin, Equell)
July Ivermectin (e.g. Eqvalan, Zimectrin, Equell)
August Pyrantel pamoate ( e.g. Strongid)
September Moxidectin (e.g. Quest) OR Fenbendazole double dose daily for 5 days (e.g. Panacur Powerpak)
October Ivermectin + praziquantel (e.g. Zimectin Gold, Equimax ) or double dose Pyrantel pamoate (e.g. Strongid)
November Oxybendazole (e.g. Anthelcide)
December Pyrantel pamoate (e.g. Strongid)
Sample Deworming Schedule for 2 horses on a 20-acre pasture:
January Moxidectin (e.g. Quest) OR fenbendazole double dose daily for 5 days ( e.g. Panacur Powerpak)
April Ivermectin (e.g. Eqvalan, Zimectrin, Equell)
July Ivermectin (e.g. Eqvalan, Zimectrin, Equell)
October Ivermectin + praziquantel (e.g. Zimectin Gold, Equimax ) or double dose Pyrantel pamoate (e.g. Strongid)