There are several different types of worms that can be found in horses, ponies, donkeys and mules, so having a good idea of the types of worms you may be looking for as well as the signs of worm infestations is important. Since commercial worm treatment pastes or medications kill most types of worms commonly found in horses when used at the correct time they are effective in developing a management and worm control program that will ensure your horses are worm free year round.
Young foals are particularly susceptible to a type of worm known as an ascarid. The foal swallows the ascarid when he or she is grazing, usually by contact with old fecal material that contains ascarid eggs. Once inside the digestive tract the ascarid hatches and grows to the larva stage, moving to the lungs to develop and then returning to the intestine, often reaching up to 20 inches in length. While in the lungs the ascarid can contribute to respiratory problems such as pneumonia. The adult worm will rob nutrients from the digested food as well causing possible blockages of the intestine. The adult ascarid continues to lay eggs that are passed out with the fecal material, further contaminating the pasture. The young foal will usually, on his or her own, develop immunity to the worm and they are very rare in adult horses. Often owners will notice that the foal seems to be thin and somewhat lethargic, although they are eating normally and appear otherwise healthy. Threadworms may also be present at the same time; these worms will typically cause a problem with reoccurring cases of diarrhea that may be mild to severe.
Adult horses are more likely to develop infestations of strongyles, often known as bloodworms. Strongyles may be of two varieties, small or large, and both are highly problematic. Bloodworms spend part of the lifecycle in the heart and bloodstream, leading to anemia and serious complications if other diseases or circulatory or blood disorders are present. Colic, aneurisms, and chronic inflammations in the joints may be noted with severe strongyle attacks.
Botflies are responsible for the small yellow to cream colored eggs seen on the hair of the horses legs, especially in the late summer and early fall seasons. The flies lay the eggs and then the horse licks them off, allowing the larva access to the mouth and the body. The larva lives in the intestines during the winter and may cause ruptures and ulcers, then are eliminated from the body in the fecal material in the spring. Owners can easily remove the eggs with combs or grooming stones.
Pinworms are most problematic in the lower intestines and colon and cause an itching sensation around the anus where the eggs are deposited. Often these infected horses will constantly rub their rumps against fences, poles and even trees.
Tapeworms may be found in horses although not as commonly as the other worms. Lungworms will cause symptoms of coughing and respiratory problems and are common in some areas.
Each worm species and type will have a different life cycle so treatment must be given at the right time. Horses on pasture with other horses are more likely to have worm problems than those fed dry feeds and kept relatively isolated. Removal of all wastes and proper cleaning of manure and old feed is very important.
Use worming pastes and medications according to the recommended dosage and never use more than the correct dosage or combine products, as they can be fatally toxic. Avoid worming a pregnant mare, young foal or a horse that is ill unless you have consulted with your vet.