Impaction colic in horses is a painful condition caused by an obstruction in the gastrointestinal (GI) tract. It is a serious and sometimes fatal condition. 
The term colic describes generalized abdominal pain or discomfort in horses. In many cases, this abdominal pain originates from the GI tract; but it’s important to remember that other organs in the abdomen can be a source of pain.
Cases of gastrointestinal colic are generally broken down into three types: spasmodic (gas), non-strangulating obstruction, and strangulating obstruction.
Impactions fall under the category of non-strangulating obstructions and refer to any type of blockage within the lumen of the GI tract. Often, it forms from dry feed (or digesta), but can also be caused by other foreign material, such as parasites, sand, and enteroliths. 
These blockages can prevent the normal passage of feed, gas and fluid through the digestive system, leading to mild or severe abdominal pain depending on the location of the impaction and its severity. 
Treatment for impaction colic includes giving analgesics for pain relief, rehydration and administering laxatives or psyllium to support intestinal motility. More complex cases may require surgery to remove the obstruction. 
The best way to prevent impaction colic is to offer ad libitum (free choice) access to good quality forage and fresh, clean water as well as keeping your horse’s diet and exercise schedule consistent. You should also avoid feeding your horse in sandy areas and follow a veterinarian-recommended deworming protocol.
If your horse is showing signs of colic, this is a medical emergency, and you should contact your veterinarian immediately.
Clinical Signs of Impaction Colic
Impaction colic symptoms are often easy to identify, but not all horses show the same signs. Symptoms vary greatly between individuals, based on the placement and severity of the blockage.
- Reduced intestinal sounds
- Increased heart rate (over 64 bpm) or other changes in vital signs
- Reduced fecal output
- Pale gums
- Gastric reflux
- Abdominal distention
- Aversion to feed or loss of appetite
- Pacing, stretching and rolling
- Pawing at the ground
- Kicking or biting at the flanks or barrel
Affected horses often look depressed, uncomfortable or uneasy to their owners. They may seem uninterested in their surroundings and unable to focus.
Colic is always an emergency and should be considered whenever there is a deviation from normal behaviour in the horse.  If you suspect impaction colic, consult your veterinarian to obtain a diagnosis.
Is My Horse at Risk of Impaction Colic?
Any horse can colic, even with the best care, management and feeding. But some modern management practices, such as long-term stall confinement and intermittent feeding, are linked to a higher risk of colic.
Research shows that keeping a horse stalled most of the day without access to ad libitum forage can increase the risk of colic by over 50%.  Lack of movement and fasting between meals can also reduce gut motility and mucosal water transport and affect the microbiome, increasing colic risk. 
Sudden alterations to the horse’s routine and diet can upset the sensitive digestive system. If your horse is placed on stall rest due to illness or injury, the sudden reduction in mobility can affect digestion and the transit of feed through the gut.
Factors such as poor dentition, dehydration and rapid ingestion of feed may also put horses at higher risk of impaction colic.  Other suspected risk factors for colic include:
- High concentrate, low forage diets
- History of previous colic
- Administration of medication or dewormers in the previous month
Equine Digestive Tract Anatomy
Horses are hindgut fermenters, breaking down and extracting energy from forages using bacterial digestion. 
When the horse consumes feed, it first passes into the foregut – the stomach and small intestine – where the feed is broken down by enzymes.
The horse has a relatively small stomach compared to its body size.  Feed passes through the foregut quickly, allowing the horse to graze continuously throughout the day.
Feed then passes into the hindgut, which is the site of fibre fermentation. The hindgut includes the cecum, large colon, and small colon. Bacteria in the hindgut allow the horse to break down cellulose from forage and convert it into energy.
Interestingly, the horse is unable to vomit because the esophagus only allows feed and gas to move in one direction. This makes digestive upsets more severe for horses and can contribute to colic. 
Common Sites of Impaction
The equine GI tract is full of twists and turns, and blockages can occur in any part of the system. However, some sites are more commonly affected than others.
Most impactions occur in the large colon, specifically at the pelvic flexure and transverse colon. At both of these locations, the diameter of the colon dramatically decreases in size, which increases the likelihood of digesta becoming stuck. 
The severity of colic symptoms and the horse’s physiological reaction depends on the location and size of the blockage. 
Partial obstructions occur in wider areas of the GI tract, such as the stomach, cecum and parts of the large colon. In this type of impaction, some fluid and gas can still pass around blockages or trapped digesta.
Total obstructions occur at sphincters or narrow parts of the intestine, such as the small intestine. In these cases, a part of the gastrointestinal tract is entirely blocked. Total obstructions can be fatal without intervention.
Pelvic Flexure Impaction
The most common area of feed impaction in the large colon is the pelvic flexure, which is the juncture between the left ventral colon and left dorsal colon. The pelvic flexure can be found in the lower left side of the abdomen. 
This part of the colon has a significant and sudden decrease in diameter and surface area, as well as a 180-degree hairpin turn. These features make it a common site for poor-quality hay or other dehydrated digesta to get stuck. 
Signs of pelvic flexure colic in the horse include mild or moderate intermittent pain and reduced fecal output. Risk factors associated with pelvic flexure impaction in horses include: 
- Poor quality forage
- Poor dentition
- Lack of water intake or dehydration
- Frequent and sudden management changes (stall rest, inconsistent exercise routine)
Pelvic flexure impactions can be common in areas with low-quality forage, and they usually respond well to prompt medical treatment.
Ileal impaction occurs when the last 18 inches of the small intestine, or the ileum, becomes obstructed by digesta or a foreign body. The regular passage of feed, gas and fluids may be blocked, causing significant gut distension and colic symptoms. 
Cecal Impaction (Cecum)
The cecum is a large, blind-ended sac on the right side of the horse’s abdomen where the fermentation of feedstuffs occurs. It is 4-5 feet in length and contains bacteria capable of breaking down the cellulose in plants. 
Less Common Sites of Impaction
Gastric Impaction (Stomach)
Feed spends little time in the stomach (around 2-6 hours) compared to the entire transit time of digesta through the horse’s GI tract. However, the stomach can become overfilled if there is a foreign body or obstruction to the pylorus – the outflow tract from the stomach into the small intestine – that prevents normal clearance of feed material from the stomach.
Impaction in the stomach is rare and often goes undiagnosed until the horse happens to undergo gastroscopy, usually to diagnose gastric ulcers. Gastric impactions are generally resolved without surgery.