Discussion

Clinical observations suggest that fat horses (generalised obesity or horses/ponies with a 'cresty' neck) are more susceptible to laminitis. Furthermore, obesity (i.e., body condition score [BCS]
>7/9) and/or the presence of regional fat deposits (e.g. cresty neck
score [CNS] >3/5) are criteria for diagnosis of equine metabolic syndrome (EMS), a recently described condition that predisposes horses to laminitis (Frank et al. 2010). The results of epidemiological studies, however, provide conflicting evidence on the association between obesity and laminitis risk and there are no published studies demonstrating a mechanistic link between adiposity and laminitis. In the context of this presentation, therefore, it is more appropriate to ask the question: Are obese horses more prone to laminitis?
A number of case-control and cohort studies have explored the association between body condition (or regional adiposity) and laminitis. In a case-control study of 258 cases examined at 6 veterinary teaching hospitals, a cresty neck was present in significantly more acute and chronic cases than controls (Alford et al. 2001). In a prospective study on the severity and outcome of pasture-associated laminitis, the majority of affected animals (89 of 107 cases) were considered to be overweight/obese and there was a trend (P = 0.09) for animals with a higher body mass index (BMI, weight-to-height ratio) to have more severe laminitis (Menzies-Gow et al. 2010a). Baxter (1986) also reported that heavier horses were less likely to survive an episode of laminitis, although this study included only 12 animals. High body condition score (BCS >7/9) and/or a cresty neck (CNS >3/5) were significant predictors of incident laminitis in a cohort of inbred herd of ponies, although risk of laminitis may have been associated with insulin resistance or hyperinsulinaemia rather than obesity per se (Carter et al. 2009a). Measures of adiposity such as a high BMI were not a significant risk factor for laminitis in a retrospective study of pasture-associated laminitis in the South of England reported (Menzies-Gow et al. 2010b). Similarly, Slater et al. (1995) found no association between weight and risk of laminitis in a case-control study of laminitis in Texas. It is apparent that additional prospective studies are needed to clarify the association between obesity (or regional adiposity) and laminitis risk.
At least 3 theories have been proposed to explain the heightened susceptibility of obese horses or ponies to laminitis. First, in overweight animals there may be increased biomechanical strain on the suspensory apparatus of the distal phalanx, with potential for exacerbation of the detachment of epidermal laminar cells from the underlying basement membrane in the face of factors that initiate laminar pathology. Indirect support for this theory comes from the above mentioned studies that reported more severe laminitis or poorer outcome in heavier animals (Baxter 1986; Menzies-Gow et al. 2010a). The second theory links obesity and laminitis via insulin resistance (IR) and associated high circulating insulin that may directly induce laminar injury. In horses, as in other species, obesity has been associated with IR and an exaggerated insulinaemic response to glucose administration (Frank et al. 2006; Carter et al. 2009b). However, it is important to emphasise that not all obese horses are insulin resistant - and that IR can occur in nonobese horses. Similarly, up to 25 - 30% of obese adult humans remain metabolically healthy and insulin-sensitive (Bluher 2010), supporting the hypothesis that not all excess bodyweight (adiposity) carries equal risk. The same could be true in horses and ponies, and this may explain anecdotal observations that not all obese horses and ponies appear to be at increased risk of laminitis. Finally, a chronic inflammatory state in obesity has been proposed to play a role in susceptibility to laminitis. There is some evidence linking obesity in horses and ponies with a pro-inflammatory state (Vick et al.
2007; Adams et al. 2009), although a recent study by our group
found no difference in laminar inflammatory events of lean vs. obese, insulin resistant ponies subjected to a dietary carbohydrate challenge designed to mimic abrupt exposure to pasture rich in nonstructural carbohydrate (Burns et al. 2012).
The question of whether obesity renders horses and ponies more susceptible to laminitis is important given the apparently high prevalence of overweight/obesity in horse populations and the fact that obesity is for the most part modifiable through dietary and exercise management. Thus, there is a clear need for additional studies on the relationship between adiposity and laminitis risk.