Large strongyles, Strongylus vulgaris, live in the horse’s large intestine. Their eggs pass out of the digestive tract in feces and then hatch. Horses eventually consume the larvae, which pass through the intestinal wall into the various arteries that supply the digestive tract. Eventually, after several months, they return to the intestines as adults, and the cycle starts again.

Small stronglyes also reside in the large intestine, and their larvae migrate to and hibernate in the gut wall. When these larvae come out of hibernation, they can cause substantial damage to the gut wall. Some parasites, such as roundworms, are a greater issue for young horses, and the ingested larvae of these worms pass through the liver and lungs before maturing in the small intestine where their activity can cause substantial irritation and inflammation.

From a nutritional perspective, the problem with these parasites isn’t just that they’re stealing nutrients from the horse to survive, but that they are causing potentially permanent damage to digestive tract tissue and altering gut function.

The gut wall is vital to nutrient absorption. The first section of the small intestine, the duodenum, is responsible for secreting the many digestive enzymes needed to break molecules—such as sugars, starch, amino acid, and fatty acids—into their component parts. These are then absorbed into the gut epithelia in the subsequent jejunum and ileum of the small intestine. Damage to the tissues and scarring in these regions of the intestine can negatively impact cells that secrete enzymes, as well as those responsible for nutrient absorption. Parasites in the arteries of the intestinal tract could limit blood flow, reducing the amount of oxygen reaching intestinal tissue. Damage to vessels in this area might limit the nutrient movement away from the gut. This damage might be irreversible and could last a lifetime, resulting in a horse that’s a hard keeper.

In other species parasites are known to negatively impact the enteric nervous system (ENS), the integrity of which is necessary for gut homeostasis. The ENS is part of the autonomic nervous system. Autonomic nervous functions act largely subconsciously and regulate such things as heart rate, respiratory rate, and in the case of the ENS, digestion. When certain parasites impact the gut ENS in other species, gastrointestinal motility can be affected. In humans, conditions such as irritable bowel syndrome can result due to disruptions in the gut-brain axis.

If internal parasites impact horses’ ENS and, thus, gut motility, the horse might be at an increased risk of impaction colic if motility slows and poor nutrient digestion and absorption—and potentially diarrhea—if it speeds up.

The digestive tract’s main functions include digesting and absorbing nutrients, as well as serving as a barrier to potentially harmful agents, preventing them from entering the body. Internal parasites impact all these functions, both in the short term and potentially for the horse’s life. They’re not just robbing nutrients from the horse in that moment. They’re also creating a situation that might rob the horse of nutrients long after the parasites are gone.