Prevalent Parasites: Common Types of Equine Internal Parasites

common symptom. While all symptoms are undesirable internal damage is more concerning for horses with parasite overload.
Internal parasites can cause gastrointestinal lesions oral lesions liver damage stomach and intestinal lining issues telescoping of the intestines impactions and colic. These clinical signs and symptoms are typically found in horses with large overwhelming parasite loads. It is expected that all horses will have some level of parasite burden. The goal as horse owners then is not to completely eliminate internal parasites but to keep parasite loads at a level that does not cause illness. Parasite management should incorporate multiple methods to reduce populations and not solely rely on dewormers.
Types of Internal Parasites
Large strongyles small strongyles ascarids tapeworms pinworms bots and threadworms are common internal parasites that infect horses. In adult horses small strongyles and tapeworms are the largest concerns. Each type of parasite impacts the horse differently and has unique infective states time in the parasites life cycle when it can infect the horse. The age of a horse will also impact which species of parasites they are more susceptible to for example ascarids are more likely to be found in foals and weanlings than mature horses. A large concern with internal parasites is resistance to dewormers known as anthelmintic resistance. Some parasite species have become very resilient adapting to survive dewormer treatments. This is of particular concern with ascarids and small strongyles.
Large Strongyles             
Three species of large strongyles affect horses Strongylus vulgaris Strongylus edentates and Strongylus equinus. Large strongyle larvae are found on pastures and are ingested while the horse grazes the larvae swim up blades of grass in water droplets to increase their chances of being eaten. Once ingested S. vulgaris larvae migrate to the large intestine and can then migrate to the blood vessels and burrow through the walls

of arteries and disrupt blood flow. Because of this S. vulgaris are commonly known as bloodworms and are considered the most pathogenic and damaging of the large strongyles. The other two species of large strongyles will migrate from the digestive tract into the liver and then back to the large intestines but these are not as damaging since they do not affect the blood stream to the same extent. Currently large strongyles are not a large concern for domestic horses as treatment strategies from decades ago have substantially reduced their presence.
Small Strongyles
Similar to large strongyles there are a number of