Researchers found that glucose and insulin test results should be interpreted differently for donkeys than they are for horses.
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Albeit from the same genus, donkeys and horses have distinctly different physical characteristics, from their ears to their hair coats. But they differ internally, too.
Researchers recently determined, for instance, that when it comes to diagnosing endocrine and metabolic disturbances, such as insulin resistance, metabolic syndrome, and pituitary pars intermedia dysfunction (PPID, or equine Cushing’s disease), donkeys and horses have significantly different test parameters.
In the past, relatively little research has been conducted on donkeys. But because these hardy animals play an important role in developing countries’ economies, researchers have begun to evaluate how owners and veterinarians can better care for them, including testing for and diagnosing endocrine and metabolic conditions.
Francisco Mendoza, DVM, PhD, MSc, Dipl. ECEIM, and colleagues from University of Córdoba, in Andalusia, Spain, recently took a closer look at how donkeys regulate blood glucose (blood sugar, the body’s most important fuel molecule). Specifically, they aimed to establish normal glucose-insulin values in healthy adult donkeys, as well as evaluate the efficacy of two tests that are commonly used to diagnose insulin resistance in horses:
The intravenous glucose tolerance test (IVGTT), in which the veterinarian administers glucose intravenously, then measures the animal’s blood glucose levels over the next several hours; and
The combined glucose-insulin test (CGIT), in which the veterinarian administers both glucose and insulin intravenously and measures both levels over the next several hours.
The team employed 16 adult nonpregnant female Andalusian donkeys consuming a hay-only diet. The researchers carried out the CGIT on 10 of the donkeys and performed the IVGTT on 10 donkeys (four of which were also tested with the CGIT) during the spring.
“It has been demonstrated that autumn influences these kind of tests,” said Mendoza, a professor and equine internal medicine specialist at the university’s Animal Medicine and Surgery Department. “For this reason we carried out these tests in spring in order to avoid some kind of bias.”
Upon examining the results, the team found that the two tests appear to function differently in donkeys than they do in horses. For example, the donkeys’ glucose concentrations fell below the baseline after 180 minutes on the IVGTT, but then returned to normal at 300 minutes. These findings have not been described in horses, Mendoza said.
“The explanation for this phenomenon is not clarified yet,” he explained. “We are working (to better understand) this issue, (but) likely it is due to a further previous insulin release from B-pancreatic cells (a rebound phenomenon).”
The donkeys’ CGIT results also differed from those of horses: Both glucose and insulin concentrations took longer to return to baseline following the test. As such, Mendoza recommended delaying sampling compared to what is recommended for horses (collecting a sample about 45 minutes after administration) and beginning sample collection 60 to 75 minutes following glucose administration.
The team also evaluated additional predictors for insulin resistance. They found that while donkeys’ and horses’ insulin sensitivities appear to be similar, the donkeys in the study had higher modified insulin-to-glucose ratio (MIRG) values than horses.
“MIRG is a parameter for pancreas function, (indicating) both pancreatic responsiveness and insulin secretion, but it is not an insulin sensitivity indicator,” Mendoza explained.
“Differences found in CGIT and IVGTT curves compared with horses indicate that it is necessary to make a specific interpretation for donkeys in order to avoid risk of misdiagnosis in this species,” the team concluded.
The study, “Characterization of intravenous glucose tolerance test and the combined glucose-insulin test in donkeys,” was published in The Veterinary Journal.
About the Author
Casie Bazay, NBCAAM
Casie Bazay holds a bachelor of science degree in education from Oklahoma State University. She taught middle school for ten years, but now is a nationally certified equine acupressure practitioner and freelance writer. She has owned Quarter Horses nearly her entire life and has participated in a variety of horse events including Western and English pleasure, trail riding, and speed events. She was a competitive barrel racer for several years and hopes to pursue the sport again soon.
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