The Insidious Ascarid
- Hagyard Equine Medical Institute
In our January 6 Facebook post about ivermectin toxicity, several people questioned why someone would deworm foals. Foals are susceptible to threadworms, strongyles, pinworms and tapeworms, but roundworms, or ascarids, can pose a serious health threat to very young horses. Ascarids are a large family of parasites that can attack dogs, cats, cattle, swine, birds, raccoons, skunks, people and horses. The good news is that most ascarids are host-specific. The equine ascarid, Parascaris equorum, usually infects only horses, zebras and donkeys. These worms are approximately the size of a pencil and can sometimes be seen in the manure of foals. Ascarid infection occurs when foals ingest eggs (with larva inside) that have been passed in manure, usually when grazing with the mare or rummaging around a stall. These parasites can infect any age of horse but prefer foals less than 15 months of age. Once the eggs travel to the small intestine, the larvae are released. This insidious creature then moves through the gut wall and travels to the liver via blood vessels or the lymphatic system. Once in the liver, the worms mature to the next larval stage and then migrate to the lungs through the bloodstream. In the lungs, they burrow into the terminal air sacs (alveoli). From there, they journey to the trachea through airways (bronchioles). Once they reach the top of the trachea (pharynx), they are swallowed and moved back to the small intestine, where the larvae mature into egg-laying adults, usually 2 ½ - 3 months after original infection.
Ascarid infections can cause unthriftiness in foals, failure to gain adequate weight, rough hair coat, lethargy, a “pot-bellied” appearance and even colic that can be fatal. Additionally, if the worms are migrating through the lungs, foals can develop respiratory disease, fever, coughing, nasal discharge and pneumonia. Once a large population of adults has migrated back to the small intestine, the worms can bundle together and create an intestinal impaction that can lead to intestinal rupture. Fenbendazole (Panacur), pyrantel pamoate (Strongid) and pyrantel tartrate (Strongid-C) are approved dewormers in foals. These medications are very good for roundworms, among other things. However, if foals have a heavy infestation of any type of worm, a mass die-off can create an impaction in the intestine. Because of this, the best plan to deworm foals begins with a consultation with your veterinarian to determine the best way to combat parasites in your area. Along with the aforementioned dewormers, macrocyclic lactones (ivermectin and moxidectin) are good choices but should be used according to the proper doses and not before the foal is 2 months of age (ivermectin) and 6 months (moxidectin). We recommend starting to deworm foals around 2 months of age in order to target ascarids, which have a prepatent period (time of infection to time of clinical signs) of about 60-80 days. Please consult your veterinarian before giving any medication, so that you can develop a treatment plan that is right for your horse.
