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Hypertension and insulin resistance in a mixed-breed population of ponies predisposed to laminitis

Simon R. BaileyDepartment of Veterinary Basic Sciences, Royal Veterinary College, Royal College Street, London, NW1 0TU, England.

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Jocelyn L. Habershon-ButcherDepartment of Veterinary Clinical Sciences, Royal Veterinary College, Royal College Street, London, NW1 0TU, England.

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Kathryn J. RansomDepartment of Veterinary Basic Sciences, Royal Veterinary College, Royal College Street, London, NW1 0TU, England.

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Jonathan ElliottDepartment of Veterinary Basic Sciences, Royal Veterinary College, Royal College Street, London, NW1 0TU, England.

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Nicola J. Menzies-GowDepartment of Veterinary Clinical Sciences, Royal Veterinary College, Royal College Street, London, NW1 0TU, England.

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Abstract

Objective—To determine the metabolic phenotype of a group of laminitis-prone ponies when at pasture in summer, compared with when at pasture in winter.

Animals—40 ponies of various breeds predisposed to recurrent pasture-associated laminitis and 40 unaffected control ponies.

Procedures—Body condition score and size of the crest of the neck were assessed, blood samples obtained, and blood pressure measured by use of an indirect oscillometric technique, while ponies were kept on winter pasture (last week of November or beginning of December) and again on summer pasture (June). Serum insulin concentration and plasma glucose, triglyceride, uric acid, and ACTH concentrations were measured. Insulin sensitivity was calculated with proxies derived from basal serum insulin and plasma glucose concentrations.

Results—No significant differences were apparent between ponies predisposed to laminitis and control ponies during winter. However, in June, laminitis-prone ponies had increased serum insulin concentration and plasma triglyceride and uric acid concentrations, compared with control ponies. Also, laminitis-prone ponies were relatively insulin resistant, compared with control ponies. Mean blood pressure was significantly higher during summer in laminitis-prone ponies (median [interquartile range], 89.6 mm Hg [78.3 to 96.9 mm Hg]), compared with control ponies (76.8 mm Hg [69.4 to 85.2 mm Hg]).

Conclusions and Clinical Relevance—Summer pastures appear to induce metabolic responses in some ponies, leading to expression of the prelaminitic phenotype, which includes hypertension as well as insulin resistance. Signs of this metabolic syndrome may not be apparent in affected ponies during periods of grazing winter pasture. Understanding this syndrome may enable improved countermeasures to be devised to prevent laminitis.

Abstract

Objective—To determine the metabolic phenotype of a group of laminitis-prone ponies when at pasture in summer, compared with when at pasture in winter.

Animals—40 ponies of various breeds predisposed to recurrent pasture-associated laminitis and 40 unaffected control ponies.

Procedures—Body condition score and size of the crest of the neck were assessed, blood samples obtained, and blood pressure measured by use of an indirect oscillometric technique, while ponies were kept on winter pasture (last week of November or beginning of December) and again on summer pasture (June). Serum insulin concentration and plasma glucose, triglyceride, uric acid, and ACTH concentrations were measured. Insulin sensitivity was calculated with proxies derived from basal serum insulin and plasma glucose concentrations.

Results—No significant differences were apparent between ponies predisposed to laminitis and control ponies during winter. However, in June, laminitis-prone ponies had increased serum insulin concentration and plasma triglyceride and uric acid concentrations, compared with control ponies. Also, laminitis-prone ponies were relatively insulin resistant, compared with control ponies. Mean blood pressure was significantly higher during summer in laminitis-prone ponies (median [interquartile range], 89.6 mm Hg [78.3 to 96.9 mm Hg]), compared with control ponies (76.8 mm Hg [69.4 to 85.2 mm Hg]).

Conclusions and Clinical Relevance—Summer pastures appear to induce metabolic responses in some ponies, leading to expression of the prelaminitic phenotype, which includes hypertension as well as insulin resistance. Signs of this metabolic syndrome may not be apparent in affected ponies during periods of grazing winter pasture. Understanding this syndrome may enable improved countermeasures to be devised to prevent laminitis.

Contributor Notes

Dr. Bailey's present address is Faculty of Veterinary Science, University of Melbourne, Parkville, VIC 3010, Australia.

Supported by a Welfare Project Grant from the Home of Rest for Horses (now known as the Horse Trust).

The authors thank Carol Crawford, Trish Marks, and Drs. Hector Zerpa and Fernanda Sepulveda for their technical assistance.

Address correspondence to Dr. Bailey.