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THE 3 BEST-KNOWN LARGE STRONGYLES ARE:


◗ Strongylus vulgaris—both larvae and adult forms can be very harmful to the horse. While embedded, larvae can cause blood clots that block the arteries, causing thrombo-embolic colic. When the larvae return to the large intestine as adults, they at- tach to the wall of the intestine and begin to suck blood, which can cause severe anemia.


◗ Strongylus edentatus larvae migrate through liver and the lining of abdomen and can cause severe damage to the liver.


◗ Strongylus equinus larvae migrate through liver and pancreas.


SMALL AND LARGE STRONGYLE DEVELOPMENT


One drop of dew on a single blade


of grass can contain 100 larvae, and some experts have stated there may be as many as 1,000. So at any given time, great numbers of larvae are developing in a pasture. Additionally, both develop through very similar stages once the eggs are passed in the horse’s stool. Under ideal conditions of moist tem- peratures between 65 and 75 degrees F, the eggs will hatch.


LARVAL SURVIVAL TACTICS


T e fi rst two larval stages feed on


T is lit le guy burrows into its host and hangs on with those lit le black hooks.


bacteria and other microscopic organic material in the stool. When the third stage is reached, the larvae develop a protective coating that covers their mouths and prevents them from feed-


ing. During the third stage, larvae must live on stored energy until they are consumed by a horse. T e same coating that prevents them from eating also protects them from the environment and digestion by the horse’s stomach. T ird-stage strongyle larvae have an amazing ability to survive.


◗ As the temperature decreases, the larva’s metabolic rate de- creases in order to save energy and allow them to survive freez- ing temperatures until spring, when they will be ingested. Hot weather may be more harmful than cold as it speeds up this metabolic rate and the larva dies due to lack of energy.


◗ Upon arriving in the large intestine and cecum, larvae shed their coating, penetrate the mucosal lining and encyst in the wall of


1. Pinworm eggs (Oxyuris equi) 2: Roundworms (Parascaris equorum) 3: Strongyle 4: Tapeworm


the intestine. T ey remain encysted until it’s time for them to excyst, become adults and start laying eggs. T e amount of time they remain encysted can vary from weeks to two years and may be determined by the number of adults in the intestine.


◗ Small strongyle-encysted larvae appear to be unaff ected by all deworming medications except two—moxidectin and Panacur Powerpack (fenbendazole). T e decreasing amount of time it takes for strongyle eggs to reappear in a horse’s stool dewormed with moxidectin seems to indicate that small-strongyle en- cysted larvae are becoming resistant to moxidectin, the newest dewormer on the market.


TAPEWORMS—PARTS CREATE THE WHOLE All tapeworms share a very similar life cycle that takes approximately


six months to complete. Only three species in United States aff ect horses. T e tapeworm has no mouthparts or digestive tract and can only absorbs nutrients through its skin. T e tapeworm at aches to the lining of the intestine with hooks that are part of the head (scolex). T e tapeworm’s entire body is composed of egg-producing segments which are shed in stool as they reach maturity. Oribatid mites eat the segments.


continued page 40


“Adult female roundworms lay as many as 170,000 eggs per day or 60,000,000 eggs per year. Eggs stick to many surfaces, including dust particles, making them highly transportable and able to stick to a dam’s nipples where a foal can ingest eggs while it nurses.”


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