Equine Parasite Control Part 1: Common Internal Parasites

Equine internal parasite management is a confusing and seemingly ever changing subject.  Daily dewormers? PowerPaks? Rotate dewormers? Don't rotate dewormers? Fecal flotations? High, Medium, Low shedding programs? 

Parasitologists are constantly updating their parasite control recommendations based on new research to try to control parasite resistance to dewormers while continuing the excellent control of internal parasites that we have become accustomed to. 

Let's start from the beginning. Part 1 of this discussion will be.....

What are the common internal parasites we are attempting to control in our horses?


Small Strongyles (Cyathostomes) - Have become an important group of intestinal parasites in horses.  Larvae burrow into the lining of the intestine and remain dormant or "encysted" for several months before completing their life cycle.  During this time the larvae are resistant to most dewormers.

Small strongyle larvae can cause severe damage to the lining of the intestine, especially when large numbers of larvae emerge from the encysted stage all at once.  Colic and diarrhea are common in heavily infected horses.  These parasites also cause weight loss, slowed growth in young horses, poor coat condition and lethargy, or lack of energy.  While lighter infections are not obvious, it is common for a horse's general health and performance to improve after treatment for these parasites.

The early and late larval stages (before and after they burrow into the lining of the intestine) and the adult parasites are susceptible to several dewormers.  But currently there are only two types of dewormers that are effective against the encysted larval stage.

Large Strongyles - As larvae they penetrate the lining of the bowel and migrate along the blood vessels that supply the intestines.  Even small numbers of these larvae can cause extensive damage.  Infection with large strongyles can cause unthriftiness, weight loss, poor growth in young horses, anemia (low number of red blood cells) and colic.  In most cases, colic caused by these parasites is relatively mild, but severe infections can result in loss of blood supply to a portion of the intestine, leading to severe and potentially fatal colic.  These intestinal parasites have been controlled for the most part by our routine deworming protocols.

Roundworms (Ascarids) - Greatest concern for horses under 6mo of age.  Healthy older horses have immunity but may still shed eggs (mostly eliminated by modern deworming programs).  Adults cluster in the small intestine causing impaction, often with colic, can result in rupture and death

Most often a problem in young horses (especially foals, weanlings, and yearlings).  Adult roundworms are several inches long and almost the width of a pencil; in large numbers they can cause blockage (or impaction) of the intestine.  In addition, roundworm larvae migrate through the internal organs until they reach the lungs.  They are then coughed up and swallowed back into the digestive tract to complete their lifecycle.  Large infections can lead to damage to the liver or lungs due to migration of these larval forms.  

Roundworm infection in young horses can cause coughing, poor body condition and growth, rough coat, pot belly, and colic. Colic is most likely in older foals (over 3months of age) that are heavily parasitized with roundworms when dewormed for the first time.

Tapeworms - Found in virtually all grazing animals.  Contribute to colic by causing inflammation, ulceration and bowel obstruction.  The tapeworm lifecycle involves a tiny mite as an intermediate host, and horses are at risk of developing tapeworm infection when they eat this mite in grass, hay or grain.   Praziquantel has been demonstrated to be highly effective against tapeworms.  

Pinworms - Lay eggs on the skin around the horse's anus.  The irritation they cause makes the horse repeatedly rub its tail. The eggs can sometimes be found by placing a piece of scotch tape on the skin around the horse's anus and then viewing under a microscope.

Bots - Not usually a cause of major health problems, although they can damage the lining of the stomach where they attach.  They may also cause small areas of ulceration in the mouth where the larvae burrow into the tissue for a time after the eggs are taken into the mouth.  

Stay tuned for Part 2 of Equine Parasite Control; Fecal Egg Counts and Control Programs