When temperatures drop in the winter, impaction of the large colon becomes a common cause for emergency farm calls. Impaction colic occurs when digested material within the large colon becomes stuck. Because the material stays in one place, the large colon absorbs moisture from it, generating a doughy to hard, dry mass. Impactions commonly occur where the large colon narrows and turns at the pelvic flexure.

Causes often associated with impactions include decreased water consumption and changes in routine. In the winter when the weather changes, especially abruptly, horses might have reduced turnout and exercise and might not be interested in drinking, particularly if the water is very cold. Other causes of impaction can be related to the inability to chew and break down feed well (e.g., due to pain or poor dentition), an abnormality in the gastrointestinal (GI) tract (e.g., a tumor), or a problem with the GI tract’s ability to move digesta efficiently and effectively—called motility.

With an impaction blocking newly ingested material, the horse stops passing manure, could exhibit mild to severe signs of abdominal discomfort, and might or might not show a decrease in appetite. It’s possible on physical exam that your veterinarian will find signs of dehydration (tacky gums and skin that returns to normal slowly when pinched during a skin tent test) and decreased gut sounds. Rectal palpation often allows the veterinarian to confirm and characterize an impaction at the pelvic flexure. On some occasions an impaction of the large colon can be out of reach during a rectal exam. In these cases a veterinarian might decide to treat for an impaction, while also ruling in or out other causes of colic (sometimes impaction can precede serious colics with intestinal displacement).

The first treatment step is to stop adding to the blockage. Hold the horse off all feed until the impaction ­resolves; more food will not push it out. The ­second step is hydrating the horse, which moistens the impacted material and allows for the large colon to move it. An effective way to do this is administering water directly into the GI tract via a nasogastric tube. Your veterinarian might also administer Epsom salts to draw water into the colon, as well as mineral oil to both moisten and act as a marker of GI transit. Mineral oil can sneak around impacted material, however, so seeing oil alone might not confirm impaction resolution. Sometimes impactions require more aggressive fluid therapy with a combination of oral and intravenous fluids, and on rare occasions surgery is necessary. Once impactions resolve, evidenced by manure passage and confirmed on rectal palpation, owners can reintroduce feed slowly over a few days.

The key to avoiding impactions is prevention—your horse and wallet will thank you for it. While there’s likely little you can do to keep winter weather from altering your horse’s turnout and physical activity, you can make sure he is well-­hydrated. You can’t make a horse drink, but you can lead him to enticing water.

Horses should have unlimited access to clean water all the time, particularly during winter. Clean and fill buckets and troughs regularly, and add heaters if needed to prevent water from freezing. Even if your buckets and troughs don’t freeze, some horses don’t like to drink cold water, and a heater might help.

When plain water isn’t palatable enough, additives are available to pique your horse’s interest. These might ­include:

• A few handfuls of grain (senior feeds work well);
• Electrolyte powders made specifically for horses; or
• Diluted fruit or vegetable juices.

However, always keep a bucket of plain water available in case your horse prefers it over the flavored water you offer. Keep in mind that various flavoring products’ sugar content and electrolyte levels might not be appropriate for all horses.

In addition to doctoring your horse’s water, you can add moisture to his diet by wetting feed and forage. Soaking only takes a few extra minutes, and most horses don’t seem to mind the added water. For finicky horses, it might take some trial and error to get the water proportions and temperature right to make an appetizing mash.