Insulin resistance (IR) is a major factor in the susceptibility of ponies to pasture-associated laminitis yet epidemiological research to date is limited. The aim of this study was to determine the prevalence and risk factors of hyperinsulinaemia as an indicator of IR in ponies.
Pony studs within a 100 km radius of Gatton, SE Queensland, were identified via internet listings and relevant publications, contacted and visited on consent. Blood samples were obtained and analysed for serum insulin (DSL RIA), plasma ACTH (Immulite 1000), serum triglyceride and plasma leptin concentrations. Ponies were evaluated for body condition score (BCS, 1-5 scale), increased fat deposition (cresty neck, increased supraorbital fat) and history or evidence of laminitis. Hyperinsulinaemia was defined as serum insulin > 20 µIU/ml. Equine Cushing's Syndrome (ECS) was defined as ponies > 15 years of age with ACTH >50 pg/ml and used as an exclusion criterion.
Of the 26 pony studs or traders identified, 23 were able to be contacted and 22 available for visit (response rate 96%). The study population consisted of 208 ponies; 70 Australian Ponies, 67 Welsh Mountain Ponies or Cobs (WMP), 51 Connemaras and 20 Shetlands. Mean BCS was 4.0 ± 0.5; mean age 10 ± 7.1 years (median 9; range 1-34 years). The majority of ponies were currently used for breeding (53%), followed by young and dry stock (36%) and showing or riding (11%). 81% (152/188) of ponies were kept entirely on pasture, the other 19% being supplementary fed.
Twenty ponies (9.6%) were excluded based on suspicion of ECS. Of the remaining 188 ponies, the prevalence of IR was 27.7% (52/188) (95% CI: 21.3-34.1%). Mean BCS was greater in ponies with IR (4.2 ± 0.6) vs. those without (3.8 ± 0.5) (P<0.001). There was a mild correlation between serum triglyceride and both serum insulin (Spearman's r = 0.36; P<0.001) and plasma leptin concentrations (Spearman's r = 0.39; P<0.001). There was a moderate correlation between plasma leptin concentration and body condition score (Spearman's r = 0.56; P<0.001). Univariate analysis showed an increased risk of IR in ponies provided supplementary feed (OR 3.68; 95% CI 1.6-8.5). There was a significantly increased risk of IR in WMP (OR 2.5; 95% CI 1.3-5.0) and decreased risk in Connemaras (OR 0.2; 95% CI 0.1-0.5). Neither gender nor current use affected risk of IR, but risk was increased if owners reported a history of laminitis (OR 7.4, 95% CI 2.6-21.0), or if laminitic rings were detected on physical examination (OR 3.1, 95% CI 1.5-6.3). There was also an increased risk of IR in ponies with a cresty neck detected on physical examination (OR 4.6; 95% CI 2.3-9.1).
In conclusion, hyperinsulinaemia is prevalent in ponies in this population and associated with both indicators of obesity and historical or current laminitis. The WMP breed appears to be at increased risk in this population.