The Modern Equine Vet
October 2023
Vol 13 Issue 10 2023
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Cover Story | Gastroentrology

What to Do With an Elevated GGT?

By Marie Rosenthal MS

It can be tricky to figure out what is wrong with a sport horse that is a little “off,” but with no outward appearance of illness, especially if liver disease is suspected because the animal has an elevated gamma-glutamyl transferase (GGT) result.

“It’s a little bit of a blurry situation, and that is the difficulty that we face. We have horses that are not outwardly sick,” explained Emmanuelle van Erck-
Westergren, DVM, PhD, ECEIM, ECVSMR, the owner of the Equine Sports Medicine practice, in Belgium.

“These are athletic horses, so they can continue working, they can continue participating in competitions, and your job is to try and link whatever comes up in the blood sample to what the horse is actually expressing.”

Unfortunately, several things can elevate GGT, including stress, she said at the British Equine Veterinary Association Congress 2023, held in Liverpool, England.

Active equine athletes frequently have a mix of biliary or hepatocellular enzymes in their blood, so she recommended when working up a horse to have some idea of what the issue might be before taking a blood sample. “If you are going on a fishing expedition, it is not very valuable. It’s good to know what you’re expecting. So, in regular horses, not top-level athletes, if I have an increase in GGT for instance, I would think more of hepatobiliary disease, inflammation of the digestive tract, such as IBD [irritable bowel disease] or horses that have parasites,” she said. Horses with pituitary pars intermedia dysfunction also can have elevated GGT values.

GGT can be a marker of workload overload, too. Several papers have described the GGT syndrome, which is a mild to moderate increase of just the GGT that is not associated necessarily to an increase in other enzymes, and there is no histological evidence of liver damage. It seems entirely linked to how much exercise the horse is getting, a possible maladaptation to training and oxidative stress.

“Oxidative stress occurs when the body uses oxygen for metabolism. More than 80% of a body’s oxygen intake goes into fueling the body, but 2% becomes reactive-oxygen molecules, which are very unstable with a free electron tries to interact with the environment to stabilize itself. It can interact with cell membranes and destroy these cell membranes by damaging the lipids. They can also damage proteins or enzymes. They can cause DNA mutations, and they are responsible for aging processes,” she explained.

In horses, several studies have shown that the more intense the exercise or training and conditioning practices are, the higher the oxidative stress burden. Some researchers think that oxidative stress could be a marker of overtraining.

“And this is where GGT is interesting because there is a correlation between levels of oxidative stress and GGT values,” Dr. van Erck-Westergren said.

But how useful is this information in determining what is happening to the horse standing in front of you looking a little off.

She said one paper was interesting. Mann, et al. looked at horses that were on the same premises and took horses that had high GGT values above 50 IU/liter, which is not a signal necessarily of liver disease, but is high enough to be abnormal. They compared them with horses that were below 36 units. They looked at targeted metabolomics to if there were processes that were associated with these increases in GGT and then also tested for viral hepatic viruses. They found abnormal values even in young horses in training. Their markers of oxidative stress correlated with elevated GGT. There was also a mild indication of cholestasis and no association with either equine hepaciviral or parvoviruses.

In a second part of the study, they looked at other parameters like levels of vitamin B6 and selenium, which are natural antioxidants, and found the horses with high GGT had significantly lower values of B6 and selenium.

This makes sense when one looks at the current competition schedules, she explained. The competition seasons are longer. Often, these horses have no down time.

“I wouldn’t be able to go 2 years without having a holiday or having a weekend off, but some of the horses are constantly on the road going to competitions, and that is certainly some of the horses for whom we see an increase in oxidative stress markers, and maybe GGT,” Dr. van Erck-Westergren said.

She discussed horses that were having a bad year. They were younger horses that seem to be physically affected with duller coats and loss of muscle. They had low white blood cell counts, and most of the horses showed high GGT values.

The trainer talked to the owners about training too hard and giving the animals a break.

Outside of lameness, fungi are a major cause of poor performance. Check out the feed room and do some calculations to figure out what the horse needs.

They tested the feed for fungi and switched to a commercial feed from oats and switched the bedding to wood shavings instead of straw. She said, in addition, steaming the hay used for bedding is the best thing to do if fungi are a concern “because it eliminates both molds and bacteria.”

Also look at any supplements that trainers are giving. In this case, the trainer was giving up to 900 mLs of oil mixed in the food, which she described as a huge amount. “We took away the oil which he had given to increase the energy levels based on the recommendations of a friend. And that’s how we got to the bottom of it,” she said.

As a result, the owner almost tripled his prize money from in comparison to 2019.

“Other associations, which I’d like you to make is if your athletic horses have access to pasture, go and have a look at the pastures. You must see what kind of different plants you have in that pasture,” she said, adding ragwort is a big concern.

Look for signs of infection or infestation. And run some tests. “Unless we actually test for them, we won’t be able to know to what extent they’re affecting our horses,” she said.

“Think not just about the individual, think about the herd as well,” she said.

“If I have increased liver enzymes in the equine athlete, should I worry? Well, it is the sign that something is going on,” she said. Look at the individual horse, its schedule, environment, feed, as well as the herd to get a fuller picture. MeV

For more information:
Mann S, Abuelo A, Stokol T, et al. Case-control exercise challenge study on the pathogenesis of high serum gamma glutamyl transferase activity in racehorses. Equine Vet J. 2023;55(2):182-193.