![]() ![]() Three species of large strongyles affect horses: Strongylus vulgaris, Strongylus edentates, and Strongylus equinus. This is of particular concern with ascarids and small strongyles. Some parasite species have become very resilient, adapting to survive dewormer treatments. A large concern with internal parasites is resistance to dewormers (known as anthelmintic resistance). 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. Each type of parasite impacts the horse differently and has unique infective states (time in the parasite's life cycle when it can infect the horse). In adult horses, small strongyles and tapeworms are the largest concerns. Large strongyles, small strongyles, ascarids, tapeworms, pinworms, bots, and threadworms are common internal parasites that infect horses. Parasite management should incorporate multiple methods to reduce populations, and not solely rely on dewormers. 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. It is expected that all horses will have some level of parasite burden. These clinical signs and symptoms are typically found in horses with large, overwhelming parasite loads. ![]() Internal parasites can cause gastrointestinal lesions, oral lesions, liver damage, stomach and intestinal lining issues, telescoping of the intestines, impactions, and colic. While all symptoms are undesirable, internal damage is more concerning for horses with parasite overload. In young horses infected with ascarids, diarrhea is also a common symptom. Various types of colic can also occur from internal parasite infestations. Externally, you may notice weight loss, poor growth, coarse and dull coats, potbellies and/or excessive tail scratching. Heavy internal parasite loads can cause undesirable effects on your horse's health. Why are we concerned about internal parasites? Some parasites, such as ascarids, will stay in the body for weeks, while others, such as small strongyles, can remain in the gut for over two years. How long a parasite resides inside the horse varies by the type of internal parasite. For example, ascarids (roundworms) migrate from the gut to the heart, liver, and lungs, and then back to the gut. A lot of parasites find this new home within the horse's digestive tract, but some migrate to other tissues and organs. They can cause damage while migrating through and feeding on these organs. Once consumed, the parasite will migrate to the desired area of the body where it matures into an adult and reproduces. While grazing, the horse accidentally ingests the parasitic larvae or eggs. Eggs and/or larvae are found in the horse’s environment (most commonly pastures). The stage of life when the parasite can infect the horse is referred to as the infective state. The life cycle of a parasite includes stages within the horse and stages outside the horse. To start with, a parasite is defined as an organism that lives on or in a host (in this situation, the horse is a host). How do horses get infected with internal parasites? This article will describe how parasites infect horses, why we are concerned about internal parasites, and the basic life cycles and symptoms of 6 common equine parasites. ![]() While there are many species of internal parasites that infect horses, some of the most common are strongyles, ascarids, tapeworms, pinworms, bots, and threadworms. They inhabit specific tissues, organs, and systems of the horse, deriving nutrients from the horse so they can continue to grow and develop. Internal parasites are small organisms that live out part of their life cycle within the horse's body, with the potential to cause illness. Deworming, along with other management practices, helps to keep a horse’s internal parasite load at an acceptable level. Many horse owners know that deworming is important, but they may not be aware of all the specific types of parasites that dewormers (also known as anthelmintics) treat. ![]()
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