MEDICINE II: ANTI-INFLAMMATORIES, CATARACTS, AND ENDOCRINE DISEASE


clarify here, when referring to fed vs. fasted states, the definition of “fed” describes an animal that has access to some kind of forage, whether it be pasture or hay, but typically no grain, or at theminimumgrain iswith- held 4hours prior to testing. The “fasted” state refers to the animal being fasted at theminimumof 3 to 6hours but oftentimes overnight with no access to grain or for- age of either pasture or hay.While fasting is no longer recommended when assessing resting (basal) insulin concentrations, fasting (3–6hours) has been recom- mended when performing the OST, even though few studies have been conducted to support this fasting rec- ommendationinID horses. A studyconducted inmeta- bolically normal ponies has shown that fed vs. fasted state does impact the insulin responses to the OST. This study evaluated ponies tested with the OST right off of pasture or after an overnight fast ;10hours and showed that the fasted ponies had significantly higher insulin responses to the OST when compared to the fed state ponies.2 However, these researchers conducted another study to investigate the fed vs. fasted state of the OST in distinguishing between previously laminitic vs. nonlaminitic ponies and found there was no differ- ence in insulin responses to the OST in different fasting conditions of the ponies.3 A very recent study has been conducted in ID horses that evaluated similar condi- tions of fed vs. fasted states in response to the OST with similar outcomes.4 Resultsfromthisstudyshowed that while ID horses sampled in a fasted state of 3hours had significantly lower resting insulin concen- trations, no differences were found between fed vs. fasted states for insulin responses to the OST.4 In this study, investigators also evaluated these same responses in non-ID horses and found no difference in insulin responses to the OST in fed vs. fasted non- ID horses.4 These results are supported from two earlier studies that examined the effects of fed vs. fasted on OST responses in non-ID horses and found no differences in insulin responses.5–6 Overall, these studies indicate that while there can be differences in the fed vs. fasted state insulin responses to the OST in ponies, this does not seem to be the case for horses. Regardless, fed vs. fasted state results should not be used interchangeably, and when monitoring ID status over timein thesameanimal, oneshouldusethe same method of fed vs. fasted state. If the fed state is used for the OST, it is important to take into consider- ation the type of forage, and ideally the equid ismain- tained on a low-nonstructural-carbohydrate forage at thetimeof sampling. Itmay alsobeworthwhiletocon- sider evaluating the insulin responses to feedstuffs as recent research has shown significant differen- ces in postprandial insulin responses of ID horses compared to non-ID horses; this testing could also help withmanagement decisions.7


3. OST – Dose and Type of Corn Syrup


The OST is used to try and mimic or reflect the natu- ral, postprandial responses to various feedstuffs in horses, which is useful to study and identify horses


with excessive responses of the entero-insular axis to oral sugar. Thus, Karo light corn syrup is used as the sugar challenge in the OST, and this brand formula- tion is composed of maltose and glucose. However, Karo is not available in all countries, and thus the type of corn syrup and differences in composition can potentially impact insulin results.A recent study eval- uated the insulin responses of horses receiving either Karo light corn syrup or Crown Lily white, a corn sy- rup available in Canada, and found no significant dif- ferences between insulin responses to the OST when using Karo or Crown corn syrups in horses.6 The dose of corn syrup to use in theOST is oftentimes question- able as to whether to use the low dose (0.15mL/kg) or the high dose (0.45mL/kg) of corn syrup. Research in ponies has shown that when using the higher dose, stimulated insulin responses to the OST were better able to distinguish between previously laminitic ponies or nonlaminitic ponies,while they found no dif- ference in responses to the low dose of corn syrup.3 A recent study conducted in both non-ID and ID horses comparing the low- vs. high-dose OST found no differ- ence in 60-minute insulin responses between the two doses in either non-ID or ID horses.4 Thus, these results indicate that a low-dose OST at this time is appropriate for diagnosing ID in horses, but a high- dose OST may be useful when equivocal cases are detected and beingmonitored.


4. OST – Season


It is known that season impacts the hypothalamic-pi- tuitary-adrenal axis; thus, there is a need for sea- sonal hormone reference ranges, for example, for adrenocorticotropic hormone when diagnosing PPID; however, very little is known about the effects of sea- son on insulin responses to the OST. A recent study performed OSTs across seasons on a group of horses from Finland and found no differences or seasonal patterns in insulin responses to the OST; however, they found insulin dysregulation status varied throughout the year.8 The authors indicated several limitations of this study that could have influenced the study results.8 Even more recently, a study con- ducted in Kentucky evaluated the effects of season on insulin responses to the OST in ID and non-ID horses over a 2-year period and found significant effects of season on insulin responses to the OST.9 This study found that for non-ID horses, season had no effect on basal or resting insulin concentrations, but it also found that OST insulin values were higher in the spring vs. summer. Further results from this study showed that season affected both basal insulin and insulin responses to the OST in ID horses, with both being higher in spring than fall and summer, and winter OST responses were higher than fall.9 In this study, horses changed their ID categorization across season, and relying only on basal insulin, ID status across seasons was confirmed only 21.1% to 63.6% of the time, while OST insulin responses across season confirmed ID 84.2% to 100% of the


AAEP PROCEEDINGS / Vol. 68 / 2022 233


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