Worms are a natural occurrence in horses. Left unchecked they can affect a horse’s overall health and in extreme cases lead to death. Thankfully, there are steps you can take to prevent internal parasites from becoming a concern for your horse. The first step is knowing the common types of internal parasites and understanding their life cycle.

For the common internal parasites, the mode of transmission relies on the ingestion of actively infective larvae from the environment, which can come from overgrazed paddocks or pastures with too many horses. Once in the horse’s intestinal system, they mature through several life stages to become adult worms; the females then lay eggs and the eggs are passed in the feces into the environment where, in favorable climatic conditions, they develop into infective larvae ready to be consumed by a horse. 

Here we highlight four of the most common internal parasites you need to know about.

Small Strongyles (Cyasthostomins)

Known for their tendency to burrow into the lining of the intestine and remain dormant in this encysted form for a period of time, small strongyles create inflammatory conditions in the intestinal lining. If large numbers of these larvae emerge simultaneously, an infected horse can experience intestinal upset including colic and diarrhea.

While in an encysted state, small strongyles are “protected” against many available deworming medications. To date, only moxidectin (Quest) or a 5-day course of fenbendazole (Panacur PowerPak) has been shown to have larvicidal action (the ability to attack the encysted stage of the parasite). There are indications that worm resistance is developing to these previously effective dewormers. Fecal egg counts help determine how often it is necessary to deworm based on numbers of eggs seen through microscopic examination of a manure sample and which horses are the primary shedders in your herd.

Large Strongyles (Bloodworms)

Historically, large strongyles posed one of the greatest threats to horse health and caused death in many. The advent of new deworming products in the 1980s led to aggressive deworming strategies where horses were wormed every two months has significantly reduced this threat.

Bloodworms live inside blood vessels that feed the intestinal tract; there they can cause damage and emboli (blood clots) that compromise blood circulation to the bowel. Affected horses appear unthrifty, suffer weight loss, and can develop colic or anemia. Most commercial dewormers are able to kill this parasite, so it no longer poses the health threat that made it infamous in the past.

Ascarids (Roundworms, Parascaris equorum)

Roundworms are primarily a disease of young horses, yearlings or younger, that have not yet developed effective immunity against them. The adult worms can grow as thick as a pencil and up to five inches long. When they tangle in the small intestines a complete blockage and/or intestinal rupture can occur. Their life cycle also involves migration through the lungs and liver, where they can set up inflammatory conditions to elicit cough and respiratory problems. A young horse with a large ascarid burden may display a rough hair coat, an unthrifty, pot-bellied appearance, weight loss, coughing, and have colic episodes.

Caution must be taken when deworming an infected horse as rapid die-off from the dewormer can cause serious impaction colic. To minimize infection risk, a pregnant mare should be dewormed against ascarids within 30 days prior to her foaling due date. Foals should not be dewormed against ascarids before 60-70 days of age in order to minimize the development of dewormer resistance, which is currently occurring with the avermectins (moxidectin and ivermectin).

It takes 10-12 weeks from the time of ingestion of the parasite larvae for the worm to develop into an adult, egg-laying form. Ascarid eggs are incredibly tenacious in the environment, lasting for as long as 5-10 years even in extreme weather conditions.

Tapeworms (Anoplocephala perfoliata)

Tapeworms in horses are transmitted through an intermediary host, an orbatid “forage” mite that is consumed in the pasture, hay, or grain. The tapeworm develops into an infective larva within the mite, which when ingested by the horse, develops into an adult tapeworm, a female sheds eggs within 6-10 weeks. The tapeworms collect at the ileocecal junction located between the small and large intestines. There they may elicit sufficient inflammation and bowel disturbance to cause impaction or spasmodic colic, and in some cases may cause a segment of the small intestine to slide into the opening of the cecum (intussusception), which is a seriously painful surgical condition.

Young horses and those over age 15 seem to be most at risk of tapeworm infection probably due to reduced immunity with youth or age. Control is managed using praziquantel given once or twice a year.

By Nancy S. Loving, DVM