Dr. Colleen Lewis / December 13, 2016
I have always thought opossums are pretty cool. They can eat as many as 700 ticks per day; this fact amazes me. They do not run very fast, cannot run up my pantleg or drop down on a thick web. Opossums are rabies free and I have never heard of anyone getting bit by one. But most people think of them as vermin: objectionable, nuisance animals, pests or parasites. So, what is equine verminous arteritis? It has nothing to do with opossums, but everything to do with parasites and inflamed arteries in the intestinal tract of horses, donkeys and zebras. Like opossums, one particular class of horse intestinal worms, the large strongyles, are also defined as vermin: objectionable, nuisance animals, pests or parasites! These verminous worms can do some pretty wicked damage to our horses that can sadly be fatal.
Large strongyles (Strongylus vulgaris) are a parasite that horses harbor in the small intestines as an adult worm. However, the younger stages of migrating larvae are the actual culprits of serious gastrointestinal damage. These young forms are causing all the ruckus as they weave in and out of blood vessels and intestines, often leaving a wake of damage with swelling and blood clots. The life cycle of the large strongyle starts with the adult worm laying eggs that are passed out of the horse, hidden in the feces. Our horses unknowingly drop fecal balls that may contain thousands of these eggs every day. Hatching in the feces can occur in as early as three days, depending on the temperature and humidity. The larvae mature through a series of molts that take place while still in the dung. The larval stages emerge from the fecal matter and hang in moisture droplets on blades of grass or films of water, waiting to be eaten by the same horse, or by a new horse. The horse ingests the 3rd stage larvae (L3) from grass, typically while grazing, however, larvae may also be picked up from hay or water that has been contaminated with feces. The larvae then make their way down to the large bowel, namely the ventral colon or the cecum. The next molt advances the larvae to stage 4 (L4) which then penetrate the lining of the intestine and enter the blood vessels. The larvae migrate along the intestinal (mesenteric) arteries for two to four months until entering the blood stream and landing in the arteries of the small intestines. The final molt into the L5 larvae occurs and as the young adults penetrate the small intestinal lumen. In six to seven months, their maturity into adults will be complete.
Imagine waves of thousands of larvae migrating through the tissues of the gastrointestinal tract. Most of the damage is caused by the L4 larval stages penetrating the arteries, causing bleeding (hemorrhage), swelling (inflammation) and even devitalized tissues (necrosis). Severe bleeding can lead to blood clots (thrombi) which can ultimately block the blood supply to the intestines resulting in colic and death. Elimination is possible in closed herds. Treatment and prevention is the key to keeping large strongyle numbers reduced if elimination isn’t successful. Unfortunately, we do not have a product on the market that will kill 100 percent of adult worms and their migrating larvae. The adult worms will be killed by the common classes of equine dewormers (anthelmintics), but eliminating the larval stages along with the adult is paramount when creating a deworming program. To provide a more complete reduction or possible elimination of large strongyles in your horses; look for benzimidazoles (products containing fenbendazole at the larvicidal rate), pyrantel tartrate (daily formulations) and macrocyclic lactones (ivermectin and moxidectin). A fecal egg count reduction test (FECRT) will help you and your veterinarian identify worm burden, product efficacy and resistance. It is a great idea to know what you are faced with and hit it with your best shot.
About the Author
Dr. Colleen Lewis is a 1996 graduate of Kansas State University, College of Veterinary Medicine. Her career has taken her to many places as a practice owner, consultant, embryologist, and mentor. She enjoys mixed animal practice, teaching, traveling, farming and high school sports with her husband, Andrew and their three boys.
