PPID horse with long hair
Experts say the most reliable tests for insulin dysregulation require veterinarians to feed horses and ponies a specific dose of glucose and take blood samples to see how their bodies react to the sugar.

However, glucose-rich foods don’t always contain the same amount of glucose, and they might trigger different insulin responses. Establishing an international consensus for the readily available glucose stimulants for oral glucose tests (OSTs), therefore, is critical, says Tobias Warnken, DVM, Dr. med. vet., PhD, Dipl. ECEIM, EBVS European Veterinary Specialist in Equine Internal Medicine of Boehringer Ingelheim Vetmedica, in Ingelheim am Rhein, Germany.

Better yet, having a standardized glucose test stimulant in the form of a specially designed pelleted snack could harmonize testing worldwide, says Melody de Laat, PhD, BVSc, of the Queensland University of Technology, in Brisbane, Australia.

The Trouble With Karo

In the U.S., veterinarians usually administer corn syrup to their equine patients for OSTs, a form of oral glycemic challenge (GC), Warnken says. OSTs provoke a post-meal-like insulin response, which is a more reliable indicator of insulin dysregulation (ID) than a basal insulin test, taken before an OST (or at any other time), because many insulin-dysregulated horses test normal without the GC, he says.

While it might seem simple enough to just give the horse or pony a bit of sugar before taking a blood test, the standard OST is based specifically on Karo brand light corn syrup, manufactured in Memphis, Tennessee. That means all the test result criteria—whichindicate whether a horse has ID—are based on what happens when the animals consume Karo light corn syrup, specifically.

Unfortunately, Karo syrup—or any corn syrup—is difficult to find in most parts of the world outside the United States, Warnken says. To make things more complicated, not all horses and ponies like corn syrup, so they refuse to swallow it.

Plus, scientists still do not agree on the correct Karo light corn syrup dose to give patients, he adds. Doses currently range from 0.15 mL to 0.45 mL per kg of the animal’s body weight—with a higher preference for larger doses.

When Karo Isn’t an Option

Northern Europeans generally offer a sugar cane syrup alternative, and Canadians sometimes use a different kind of corn syrup, Warnken says. But how much glucose do those syrups contain compared to Karo? We just don’t know, he says. “The main drawback of OSTs based on corn syrup (or likely other sugar syrups available for human food consumption) is that the exact sugar composition is not disclosed by the manufacturer and may vary with different batches,” Warnken and his colleagues state in their recent publication in the Journal of Veterinary Internal Medicine.

Regardless of location, veterinarians might try to get pickier eaters to consume various amounts of dextrose powder—which horses aren’t particularly fond of, either—mixed into chaff to make it more appealing, says de Laat. Again, the sugar content can vary in this scenario, meaning the range of acceptable insulin response becomes questionable, she says.

When all else fails, vets can forcefully administer dextrose powder through a nasogastric tube—which usually pleases neither the horses nor their owners, Warnken explains. But it offers the advantage of giving veterinarians full control of the amount of sugar given and the time at which it was consumed, he says.

A Standardized Worldwide Test Product: Pellets?

“(A) widely available glucose product that is administered in a uniform manner to horses undergoing a glycemic challenge test is needed,” state de Laat and her co-authors in their recent publication in the Journal of Veterinary Internal Medicine.

The answer might be in the form of a pharmaceutical pelleted snack, de Laat says. Her group teamed up with Warnken and his fellow researchers at Boehringer Ingelheim to custom-produce carbohydrate pellets for GC in horses. The pellets, which might be a promising option for worldwide OST standardization, are made up of oligosaccharides, based on glucose units, linseed flour, maize germ flour, and highly refined plant oils and extracts, de Laat says.

They resemble regular pelleted horse feed and are dosed according to the horse’s body weight. “Most of the time it’s like a small meal,” Warnken says.

Testing the Pellets

To measure the blood glucose and insulin responses the pellets might trigger and see if horses find them palatable, de Laat and her fellow researchers tested 92 horses and 65 ponies ranging from 4 to 40 years old, representing different breeds, sexes, and metabolic health, over 12 months in Australia, Germany, Sweden, the U.K., and the U.S.

The animals had access to hay but no feed or grass the night before testing. In the morning the researchers took blood samples before providing the custom pellets at a dose of 0.5 grams of carbohydrate per kilogram of body weightThey then took blood samples one, two, and three hours later.

More than 84% of the equids—especially the ponies—readily accepted the pellets, consuming them within four minutes on average, she says. The remaining animals either took longer than 10 minutes—the researchers’ established cut-off time—to eat the pellets or refused to finish eating them. None of the animals experienced apparent health issues related to consuming the pellets.

Interestingly, equids in Australia and Sweden were more reluctant to consume the pellets than in the U.S. or elsewhere in Western Europe, says de Laat. While horses and ponies generally enjoy sweet treats, “it is possible that some animals also found the pellets to be overly sweet,” she explained. Horses in the U.S. ate the pellets the fastest—on average, in only three minutes.

The pellets also led to an accurate diagnosis in ID horses and ponies when the researchers used a diagnostic threshold of 83 μIU/mL of insulin 120 minutes after eating, with a standard laboratory test known as the IMMULITE 2000XPi immunoassay, says de Laat. That’s essentially the same threshold used in standard OSTs—also known as OGTs, for oral glucose test—using dextrose powder (1 g/kg body weight) mixed in with feed, she says.

“Given the prevalence of ID and the need for ongoing dynamic testing in animals with ID, an OGT format where the carbohydrate is simple to administer, as demonstrated here, is ideal,” de Laat and her colleagues reported. “Voluntary intake of the pellets used in this study was about four minutes, which … could be advantageous for field-based testing.”

Testing New Kinds of GC Stimulators for OST

On his end, Warnken and his fellow researchers in Germany studied the effects of the new pellet, two syrups they designed, and dextrose mixed in chaff on 34 horses and ponies in various metabolic states. The carbohydrate-per-kilogram dose was the same as in de Laat’s experiment, he says.

The first syrup was a salt-flavored one made of 30% glucose and maltose, 37% oligosaccharides, 2% salt, and water. The second was an apple-flavored syrup made of 28% glucose and maltose, 37% oligosaccharides, 15% apple juice, 5% apple sugar extract, 0.05% apple aroma, and water.

On average, the equids were reluctant to finish eating the entire dose of pellets, syrups, or dextrose powder mixed into chaff when offered in a feed bucket for free intake, he says. This was not surprising, he adds, as he and other practitioners have noticed horses and ponies don’t easily accept glucose mixed in with their feed.

However, the animals readily accepted both syrups when administered via oral syringe, “with only minor and acceptable losses,” and they ate the whole pellets alone within five minutes, Warnken says.

The team found that the customized pellets and syrup were both palatable and readily acceptable by horses and ponies, he says. Plus, these treatments led to glucose and insulin responses comparable to the “gold standard”—tube-fed glucose—for assessing ID in horses and ponies.

“This design is a main advantage compared to syrups commercially available for human food consumption, with sometimes unknown composition or which might be subject to changes without notification,” he and his colleagues state.

An additional advantage is that with the pellets the insulin response to the glucose appeared to reach a rapid and prolonged plateau, meaning blood tests taken anytime within the two-hour window (starting one hour after glucose administration) have the potential to provide equivalent results. This could provide more leeway during field exams, he says.

In general, post-glucose insulin responses to the new pellets were somewhat higher than those to other triggers, possibly due to the added chewing effect, which might spur higher insulin rates—although the theory needs further investigation, Warnken says.

While the syrups worked well, the team decided to focus on the pellets in their ongoing work, “due to the ease of use and the simplified procedure,” he adds.

Take-Home Message

As managing metabolic issues in horses continues to take precedence, researchers are striving to improve testing methods for more accurate diagnoses and, therefore, treatment and better welfare. Owners should not hesitate to get their animals evaluated if they suspect an insulin disorder, says de Laat. “Whatever format of the test they choose to use, your vet needs to make sure that they use the appropriate insulin cutoff value for the test format and laboratory assay,” she says.