Measures taken to lessen the burden of equine Ascarids, also known as roundworms, are wrought with pitfalls due to the tenacious nature of the roundworm, its increasing resistance to chemical dewormers, and the naive population of horses it targets.
Roundworms are typically a problem in young horses, especially foals, weanlings and yearlings, where it can take advantage of inexperienced immune systems. Adult horses develop a good immunity and thus rarely have a problem. However, they do retain enough worms to continually shed eggs onto the pastures.
Due to the particularly hardy nature of the roundworm egg, pastures can accumulate an unusually high number of Parascaris eggs over the years. Eggs can remain infectious in the environment for several years even under the harshest of conditions. The egg also possesses a sticky coating which enables it to travel on a horse’s hair coat and remain on buildings, fences and feeders.
It is likely that foals become exposed very early in life as they nuzzle their dam during nursing or snuff around in their newfound environment.
The adult roundworm is large and easily visible to the naked eye. It can be 20 to 30 cm in length and resembles a large earthworm. Ascarids live in the small intestine where the female is a prolific egg layer, laying up to 200,000 microscopic round eggs a day. The eggs are passed out in the feces and remain in the environment until ingested.
When ingested, the eggs hatch into the larval form and continue with their life cycle migrating through the lining of the intestinal wall into the internal organs. Once the larvae reach the lungs, they are coughed up and swallowed back into the digestive tract where they mature into the adult roundworm. Considerable damage can occur in the liver or lungs as a result of larval migration. Heavy burdens of adult roundworms in the gut have the ability to block the intestinal tract and trigger life-threatening colic. This colic can also be triggered by a deworming treatment when the paralyzed worms fall away from the intestinal lining and create an impaction.
Young horses harbouring the migratory phase of the parasite often exhibit signs of respiratory distress, including fever, coughing and nasal discharge. In addition to respiratory illness and varying degrees of colic, common symptoms of roundworm infestation are poor body condition, depression, loss of appetite, anemia, failure to thrive and gain weight, a rough hair coat, pot-belly, and diarrhea.
Roundworm infestations become common when foals are raised on the same pastures year after year. Therefore the soundest course to reduce infestation in young stock is accomplished by raising foals on different pastures from year to year. It is also advisable to segregate yearling and two-year-olds from mares with foals. Deworming expectant mares 30 days before foaling further reduces the new foal’s exposure to parasites.
In addition to pasture hygiene, a strategic deworming program involving all horses out at pasture is often necessary. Parasite resistance to chemical dewormers is becoming an extremely important problem. Levels of resistance have been documented to ivermectin, moxidectin and more recently pyrantel.
Resistance has not yet been confirmed in benzimidazole drugs. The label on this class of drugs will show the active ingredient to be either fenbendazole, oxfenbendazole, or oxibendazole. Currently it appears to be the best choice for Parascaris treatment on many premises.
Deworming is generally started when foals are seven to eight weeks old, and treatment is performed at regular intervals until the foal is about one year old.
Extra care is taken when young stock becomes heavily infested. Decision-making can become increasingly complicated as levels of infestations rise, a foal’s health declines, and resistance is discovered. Deworming heavily infected foals with a potent dewormer may cause shock or obstruction to the bowels.
As a result veterinarians generally recommend a double course of fenbendazole. The first dose at half-strength has been shown to kill a proportion of the worms, partially reducing the worm burden. A followup higher dose is then given a week or two later to remove the remainder.
Veterinarians are usually asked to intervene when resistance to chemical dewormers becomes apparent and/or when young stock becomes heavily infested. Decision-making takes into account farm history, age and health of animals infested, deworming schedules, and fecal egg count analysis.