Horse Illustrated Magazine

Antibiotic-induced clostridial colitis must be recognized and treated promptly to prevent a horse owner’s worst nightmare.

It’s safe to say we love horses—all horses. But every once in a while, we meet a heart horse. Like a soul mate or best friend, a heart horse is one that’s a perfect match for you. Together, you’re two peas in a pod, and you can’t imagine life without this four-legged friend.

For Lisa Sherrodd, her heart horse is a Norwegian Fjord mare named Fiona. When Fiona got into her teens, Sherrodd decided to breed her and pass on some of her amazing genes, which include great looks, a steadfast temperament, a love of people, a hardy nature, and kind eyes complete with a sunshade of long lashes.

In the summer of 2018, Fiona gave birth to a perfect filly named Rosie. Rosie was everything Sherrodd had hoped. In the afternoons, she’d lay in the pasture, and Rosie would lay beside her, with her head on her lap. Who is so lucky to get a second heart horse?

Rosie grew and developed perfectly. At age 3, Sherrodd was just beginning to ride her, and Rosie was a dream, just like her mom. Then one day she noticed Rosie was off her feed. She had some swelling in her jaw that the vet thought was an abscess. She prescribed antibiotics.

Rosie developed diarrhea, a fever and rapid heart rate, and Sherrodd took her to the local vet clinic. There, they administered a different antibiotic via IV, but Rosie’s condition deteriorated. Sherrodd rushed her to Colorado State University (CSU) Veterinary Teaching Hospital. By the time they arrived, Rosie was down and wouldn’t get up. Within minutes, she died in Sherrodd’s arms inside her trailer.

It was horrifying and devastating. A healthy filly suddenly dying while under veterinary care—what on earth had happened?

A Diagnosis of Colitis

Sherrodd requested that the CSU Veterinary Teaching Hospital conduct an autopsy on her horse. The results were sepsis brought on by antibiotic-induced clostridial colitis.

As lifelong, experienced horse people, neither of us were familiar with this condition, and we didn’t realize antibiotics put a horse at risk for developing it. To learn more and help prevent future fatalities, we spoke with two of the veterinarians who worked on Rosie’s case at CSU.

Yvette Nout-Lomas, DVM, Ph.D., explains that colitis in general refers to the inflammation of the lining of the large colon or hindgut in the horse. This leads to thickening of the wall with subsequent dysfunction, such as reduced absorption of water and nutrients. In addition, undesired substances can enter the bloodstream through the damaged sections.

“When the horse starts absorbing substances from the bowel into their blood stream—for example, bacteria or bacterial products—a severe disease process called sepsis can occur that is sometimes fatal,” she explains.

Like the name implies, antibiotic-induced colitis is brought about from antibiotic use. While any antibiotic can cause it, some carry a higher risk. Of course, the horse’s general health also plays a role.

Nowadays, we’re learning and hearing more and more about the importance of the microbiome in our guts. This is even more complicated in a horse’s gut. The disturbance antibiotics cause to this microbiome are paramount.

“Lots of different microbes live in the gut of mammals, and in the horse, the microbiome is particularly expansive and diverse in the hind gut,” Nout-Lomas says. “These microbes and the balance of different ones is critically important for normal function of the gut. Antibiotics can kill off the microbes that live in the hindgut and help keep everything functioning normally.”

In this altered state, other microbes may rise, throwing off what was a balanced system. This is referred to as dysbacteriosis, and can occur suddenly.

“For example, lactic acid is a product that is often present in the hind gut in small amounts, and during dysbacteriosis it can significantly increase,” Nout-Lomas explains. “These sudden changes usually lead to intestinal inflammation and dysfunction, the severity of which is variable.”

Although antibiotic-induced colitis isn’t very common, it does occur more regularly in hind gut fermenters like rabbits and horses, and the doctors do see these cases regularly in the hospital.

Because of the disruption to the gut microbiome and issues around antibiotic resistance, everyone should exercise caution when using antibiotics.

“We should be very careful in using antibiotics, and they really should only be used when necessary,” she says. “This is an important reason to be careful with the use and selection of appropriate antibiotics.”

Signs of Colitis in Horses

“Sometimes the horse may appear dull and not interested in eating, with a high heart rate and fever,” says Al Migliorisi, DVM. “The horse can appear bloated and even show signs of colic. Diarrhea is often, but not always, present. This is an aspect of colitis that creates confusion sometimes in horse owners: How can my horse have colitis if I have not seen any diarrhea?”

The reason is that the inflammation itself can cause different degrees of dysfunction. The intestines may become hyperactive, or instead may slow down. In addition, it’s important to remember that horse’s hindgut can store an immense amount of feed and water.

“Severe complications can also include laminitis, kidney disease and, sometimes collapse and death,” he explains.

When should you call your vet?

“Any non-explainable and sudden changes in attitude, such as lethargy or lack of interest in feed in a horse that is already receiving antibiotics should be considered a potential alarm bell for either a [recurrence] of the primary problem, or a newly developed complication,” Migliorisi says.

Take your horse’s heart rate, respiratory rate, and temperature, and have those numbers handy when you call.

“This will help your veterinarian plan the next steps for your horse, and if a diagnosis of acute colitis is made, this should be treated as a potentially life-threatening emergency that warrants immediate advanced treatment, often unavailable on a farm setting,” he explains.

Colitis Treatment and Prognosis

One of the main treatment methods is placing an intravenous catheter and administering fluids to support the horse’s cardiovascular function by replenishing all the fluids the animal is losing in its gut.

“Another main goal is to decrease the underlying inflammation, which often causes most of the damage,” says Migliorisi. “We control inflammation by using different drugs with anti-inflammatory effects. A horse with any type of colitis is at risk of absorbing bacteria and bacterial toxins from its gut. Therefore, another goal of treatment is to administer medications, by mouth or intravenously, that will bind and neutralize these potentially harmful products.”

Prompt veterinary care for a horse with colitis is key.

“Unfortunately, approximately 90 percent of horses with acute colitis who are left untreated either die or are euthanized,” Migliorisi continues. “Overall, a prompt and early initiation of treatment improves prognosis. Reported survival rates for horses that receive treatment have ranged from 70 percent to almost 90 percent. Sadly, it is still possible for any horse affected by colitis to deteriorate quickly and unexpectedly, despite initial positive response to treatment.”

Recovery can take one to two weeks. Long-term impacts of a colitis episode are not well understood, though in cases that cause laminitis, treatment must be addressed with an experienced farrier.

While antibiotics can cause life-threatening colitis in horses, they can also be lifesaving when used carefully.

“Antibiotics have been a game-changer in the management of numerous conditions in veterinary medicine, but their use should be justified and judicious,” says Migliorisi.

This article about colitis in horses appeared in the January/February 2023 issue of Horse Illustrated magazine. Click here to subscribe!