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body weight play roles that have not been fully elucidated (Bertin & de Laat, 2017 ). Chronic ID can result in pancreatic beta- cell fail- ure in people (Swisa et al., 2017 ). This has been poorly defined in horses but should be considered as a differential diagnosis in horses presenting with hyperglycaemia, polyuria, polydipsia and weight loss in association with ‘normal’ insulin concentrations (Bertin & de Laat, 2017 ; Durham et al., 2009 ). Obesity is an important component of EMS, and the rise in equine
obesity parallels the human obesity epidemic (Johnson et al., 2009 ; Owers & Chubbock, 2012 ). While genetics and other non- modifiable factors play a role, the provision of inappropriately high- energy diets to horses that are predominantly kept as pets and are phys- ically inactive has contributed to a rise in equine obesity (Furtado et al., 2021 ; Johnson et al., 2009 ). The World Health Organisation defines obesity as the excessive accumulation of adipose tissue that has an adverse effect on health (WHO, 2023 ) and can manifest as either generalised or focal, external or internal accumulations of adi- pose tissue. Using the 9- point Henneke scale (Henneke et al., 1983 ), horses with a body condition score (BCS) ≥ 7/9 are considered obese, and horses with a BCS of 5/9 are considered to be in healthy body condition (Rendle et al., 2018 ). Obesity is a common feature of horses with EMS but not all obese horses demonstrate hyperinsu- linaemia (Durham et al., 2019 ). Conversely, while increases in BCS and cresty neck score are a common feature of hyperinsulinaemia, not all cases with hyperinsulinaemia demonstrate this phenotype (Clark et al., 2023 ;Knowles, Elliott, et al., 2023 ; Knowles, Harris, et al., 2023 ). While some horses do not appear obese on the outside, ultrasonographic evaluation might identify visceral or retroperito- neal fat deposits (Silva et al., 2016 ). These horses display what is known as the ‘lean- EMS’ phenotype, and the assessment of ID may be overlooked but is still critical in high- risk breeds or animals pre- senting with laminitis (Knowles, Elliott, et al., 2023 ; Knowles, Harris, et al., 2023 ). A recent study also highlighted that while physical fea- tures such as body condition and cresty neck score might be useful to identify ponies at high risk for laminitis, limiting testing to equids with these risk factors will fail to identify all horses with hyperinsu- linaemia (Knowles, Elliott, et al., 2023 ; Knowles, Harris, et al., 2023 ). There is thought to be a genetic component to EMS as breed-
related differences in the insulin response to oral sugars exist (Bamford et al., 2014 ). Studies have shown the prevalence of hyper- insulinaemia is higher in British native breed ponies and Andalusians compared to other horses (Karikoski et al., 2011 ; Morgan et al., 2014 ; Treiber et al., 2006 ). In the authors ' clinical experience, hyperinsu- linaemia can be encountered in a wide cross- section of breeds in- cluding Quarter Horses, Paints, Australian/German Riding ponies, Walers, Brumbies and Stockhorses. While breed is a non- modifiable factor, it is important to recognise the genetic predisposition for some animals to develop hyperinsulinaemia and implement proac- tive management and laboratory testing before the animal develops HAL. In line with expert recommendations (EEG, 2022 ), the authors
routinely measure insulin concentrations in horses, ponies or don- keys presenting with:
SUNDRA ET AL.
• Laminitis • Divergent hoof rings • Chronic lameness • Generalised obesity • Regional adiposity • Infertility • Wellness and pre- purchase examinations in high- risk breeds or phenotypes
• Contemplation of intra- articular or systemic use of corticosteroids The choice of test and its diagnostic value depends on the clini-
cal question that is being addressed. There is no single test that will diagnose hyperinsulinaemia in all horses (Clark et al., 2023 ; Frank & Tadros, 2014 ). Intravenous tests that assess the pancreatic response and peripheral insulin sensitivity are considered the ‘gold standard’ for the diagnosis of ID (Firshman & Valberg, 2007 ) but are not rou- tinely performed in a clinical setting. It is important to note that insulin results vary according to the assay and analyser being used (Table 1 ), and reference intervals may vary between diagnostic labo- ratories (Delarocque et al., 2023 ). To mitigate this issue, researchers have developed an open- access web app that allows veterinarians to compare their results with previously published values (Delarocque et al., 2023 ): https:// www. equin e- insul in- conve rter. org . The most practical diagnostic tests to perform in first- opinion
practice are a measurement of serum insulin in a fed state and/or the oral sugar test (OST) (Bertin & de Laat, 2017 ). Despite its low sensitivity, measurement of basal insulin concentration has been shown to be a good predictor for the development and recurrence of laminitis (Carter et al., 2009 ; Knowles, Elliott, et al., 2023 ; Knowles, Harris, et al., 2023 ; Menzies- Gow et al., 2017 ). A study of UK native breed ponies identified a 6% incidence of laminitis within a 4- year period in ponies with a non- fasted insulin >21.6 μU/mL and 69% in- cidence of laminitis within the next 4 years if insulin was >45.2 μU/ mL (Knowles, Elliott, et al., 2023 ; Knowles, Harris, et al., 2023 ). Measurement of fasted insulin concentration is no longer recom- mended; instead, it is advised that the animal is in a fed state, with access to pasture or forage, but not grain (Durham et al., 2019 ). The results (Table 1 ) might be variable depending on the forage/pasture being consumed, but it is a useful indication of the insulin response (and hence laminitis risk) under current management conditions. The results can also be used to assess the response to dietary changes. Measurement of serum insulin concentration is very convenient, but it has low sensitivity, and if the horse returns a normal resting insulin result in the face of high clinical suspicion, a dynamic test should be performed (Bertin & de Laat, 2017 ). The oral sugar test (OST) is commonly performed in equine prac-
tice as it is standardised but mimics the natural sequence of events that occurs in response to the ingestion of a high carbohydrate meal, including digestion and absorption, hormone responses and insulin secretion from the pancreas (Bertin & de Laat, 2017 ; Schuver et al., 2014 ). The OST does, however, lack the prehension and chew- ing phase that occurs when the horse is fed a meal which could have some implications for gastrointestinal motility and absorption. To
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