Digestive Health

Image of two horses eating hay.

A horse’s digestive system begins with the mouth, ends with the anus and incorporates all the organs in between that are involved in consuming and processing food. Its purpose is fourfold: to digest food, absorb nutrients, move food through the digestive tract and eliminate waste products in the form of feces. Digestive problems can occur anywhere along the way. Changes in bowel movements — including a change in the frequency of bowel movements, a change in the color or consistency of feces or the presence of blood — are common signs that something is wrong.

Examining Your Horse’s Digestive System

Your equine veterinarian will need to thoroughly examine the horse to make a diagnosis. This starts with a health history and a physical examination. To diagnose digestive troubles, your vet may do any of the following:

  • Visually inspect your horse’s mouth for moistness and changes in color
  • Check your horse’s heart rate
  • Manually feel the chest and abdomen
  • Tap your horse’s abdomen or listen for abdominal sounds through a stethoscope
  • Rectally examine your horse
  • Take a sample of your horse’s feces
  • Order additional tests, such as a bacterial culture
  • Use other equipment, such as a stomach tube or long needle, for tests
  • Take X-rays
  • Take a biopsy of intestinal or liver tissue
  • Conduct blood tests

Infectious and Noninfectious Diseases

Digestive issues can be divided into infectious and noninfectious diseases. Infectious diseases usually enter your horse’s digestive tract through its mouth, such as by accidentally ingesting contaminated water or food. Contaminants could be bacteria, viruses or parasites.

Noninfectious causes of digestive problems include overeating, consuming chemicals, obstructions, poor nutrition or a digestive system injury. Serious diseases or problems affecting non-digestive organs could cause gastrointestinal (GI) problems as side effects.

Whether your horse is suffering from an infectious or noninfectious digestive health problem, it might exhibit the following symptoms.


Diarrhea is one of the most common symptoms of digestive health issues, and can be very serious. Causes include bacteria, malabsorption of nutrients and viral infection. If your horse has a severe case of diarrhea, it could become so dehydrated and have such a poor electrolyte imbalance that it goes into shock.


Colic can be divided into three types; all are painful and potentially life-threatening. Gas colic is the most common. It stems from gas collecting in the horse’s bowels. If your horse has gas colic, it may swing its head and stomp. Watch for rolling eyes and pinned ears, and listen for loud stomach rumblings.

An accumulation of parasites, food masses or other foreign bodies in the bowels or sand in the intestines causes obstructive colic. Symptoms include restlessness, lethargy, sweating, pawing and an abnormally quiet stomach.

The third form, twisted gut, means your horse’s intestine is physically twisted. This extremely painful condition causes sweating and restlessness and doesn’t respond to ordinary pain medicines.


Colitis has both infectious and noninfectious causes, including bacteria, parasites, Potomac Horse Fever and equine coronavirus. In many cases, no cause is identified, but the symptoms are clear. Horses with colitis have such severe diarrhea that the loss of fluid causes low blood pressure. As blood flow decreases to vital organs, horses can go into shock and die.

Once a horse has colitis, its colon loses the ability to efficiently absorb nutrients and water. The bacterial population necessary for normal digestion also shifts. Depending on the case, colitis may affect only part of the GI tract or the whole thing.

Overo Lethal White Syndrome (OLWS)

Overos — that is, horses with white patterns of coloration over a dark coat — are most likely to develop OLWS. When two carriers of a defective gene mate, they can produce an all-white foal with blue eyes whose GI system fails to develop regularly. These foals aren’t able to pass feces and, thus, get colic. Unfortunately, there’s no way to successfully treat OLWS. Equine veterinarians typically advise euthanizing foals with OLWS and colic. However, some white foals don’t have OLWS.


Many horses have ulcers, including horses confined to stalls, show horses and, especially, racehorses. Horse biology is responsible for the high preponderance of ulcers. Both humans and horses produce hydrochloric acid to aid digestion. Humans only produce this acid when eating. Horses produce it all the time. If a horse goes too long without eating, acid accumulates and irritates the stomach.

To prevent ulcers, horses need to eat many small meals. Eating lots of roughage stimulates saliva production, which helps neutralize the stomach acid. Increasing your horse’s exercise without balancing the need for roughage and frequent meals can lead to ulcers. Regular use of NSAIDS, the common type of anti-inflammatory pain relievers, also promotes ulcers in horses.

Symptoms of ulcers in horses include decreased appetite, weight loss, poor coat, dullness and the urge to lie down frequently. In foals, look for intermittent colic, lack of appetite, diarrhea, excessive salivation, grinding teeth and lying on their backs.

If you suspect your horse is suffering from any digestive problems, call us today. We’ll help diagnose and solve your horse’s digestive health issues.


Find us on the map

Office Hours

Our Regular Schedule


8:00 am-5:00 pm


8:00 am-5:00 pm


8:00 am-5:00 pm


8:00 am-5:00 pm


8:00 am-5:00 pm


9:00 am-2:00 pm




Read What Our Clients Say

  • "Kath, thank you so much for being Greyson's vet, your gentle hand, compassion and great skill made all the difference for my buddy."
    --Deirdre Dekking, Portland
  • "Dr. Mertens is a very friendly, knowledgeable equine veterinarian (and a friend to the rest of the farmyard, including the chickens!). Always up to date on the latest in equine health, and even though Mertens Mammals is a small clinic giving you the personable experience, Dr. Mertens also has equipment for more advanced diagnostics and procedures!"
    —Ari Gordon, Boring
  • "I have been a client of Mertens Mammals for 10 years. Kath (Dr. Mertens) is the most kind, thoughtful and caring Vet I have ever had for my animals. I have 3 horses: my Appy "Breeze" who turns 30 this year, my Paint " Rocket " 14 and my other Paint "Fergie " 3 are all under Kath's care. I also had a dog "Cowboy" who I lost to cancer last July. Kath took care of him also. Dr.Mertens came out on Mother's Day about 7 years ago to treat Breeze for a cut above her right eye. After a few weeks of recovery we were happy to not have a huge scar. Kath also spent 2 hours with me 3 years ago when Rocket had a small case of colic. (Thank God). I couldn't ask for a better person to take care of my girls. Kath always calls to check in on the patients. When people ask who I use for my veterinary needs for my horses, I'm always proud to refer them to Mertens Mammals. I am so thankful for an honest and caring Vet. Thank you, Kath!"
    --Michelle Schatz, Portland
  • "I got looking at Sassy’s records and it will be 6 years in July when you first met Sassy—I feel so fortunate that you became her vet and have looked after her all these years. You are such a wonderful and caring doc and I know you always have Sassy’s best interests at heart. I also know I can always rely on you to help me or Sassy when needed. Thanks for all you do for both of us—will keep you posted on Sassy.”"
    —Judy Becker, Eagle Creek
  • "My horse, Pester, would have lost her eye had it not been for the great care and dedication by Mertens Mammals. They are truly there for you any time you need them. I could go on and on. Kath Mertens has been so good to us!"
    --Troy and Wendy Mohr, Estacada
  • "She has always been there for us. She is forever in my thoughts as the best vet. She saved our hours old filly from certain death. She knew exactly what to do in an instant after just seconds in seeing her. We are forever in your debt...thank you!"
    Barbara T.
  • "Thank you Dr Mertens for such a thoughtful and careful job with our sweet Daisy the Donkey this morning! She is well on her way to health again after that abscess!"
    Suzi C.