Laminitis is a major cause of morbidity and death in horses. Although still incompletely understood, it is likely that the pathogenesis of laminitis is multifactorial, with a combination of local and systemic events leading to altered hoof blood flow and thromboembolism of capillary beds. Altered hemodynamics and inflammation-induced damage eventually lead to laminar separation, necrosis, and irreparable damage to the hoof laminae.1,2 Acute laminitis has been associated with a variety of clinical diseases, including carbohydrate overload, retained fetal membranes in mares, proximal duodenitis-jejunitis, and colitis, in which systemic inflammatory events precede local damage in the hoof. Diagnosis of laminitis is based on detection of clinical signs (eg, lameness and reluctance to move) and physical examination abnormalities (eg, increased hoof temperature and prominent digital artery pulses); in some cases, radiographic views of affected limbs may reveal divergence of the hoof wall and cranial aspect of the third phalanx. However, because these findings usually indicate disease after anatomic alteration has occurred, a reliable earlier indicator of impending laminitis is needed; such an indicator could alert clinicians that aggressive treatment should be instituted before functional or life-threatening damage occurs.
Several environmental, toxic, and metabolic factors have been implicated as causes of laminitis in horses.3 Intake of excess dietary carbohydrate is a known risk factor for naturally occurring laminitis, and colonic overloading with nonstructural carbohydrate (oligofructose) is an established experimental method of inducing laminitis in horses.4 The precise mechanism whereby dietary oligofructose contributes to damage of hoof laminae is unknown.4 However, increases in laminar vascular permeability that are indirectly linked to carbohydrate administration may alter hoof microcirculation, leading to laminar cell damage.1,5
Insulin has also been implicated in the development of laminitis in equids. Plasma insulin concentration increases 12 to 48 hours after experimental induction of laminitis with oligofructose in horses, and hyperinsulinemia induces laminitis within 48 hours in clinically normal ponies.4,6 Additionally, insulin-resistant horses (ie, horses with pituitary pars intermedia dysfunction or metabolic syndrome) are at increased risk of developing laminitis.7 However, insulin dynamics during the prodromal stages of experimentally induced laminitis have not been thoroughly investigated, to our knowledge, and it is unclear whether hyperinsulinemia consistently precedes development of clinical signs of laminitis.
An increase in UP concentration is an early consequence of renal vascular damage, either as a result of primary glomerular diseases or systemic inflammatory diseases that cause secondary glomerular vascular damage.8 Inflammation at any body site may result in intravascular antigen-antibody complex formation, followed by deposition of complexes within the glomeruli, local induction of inflammation and endothelial cell damage, and passage of excess protein into the glomerular ultrafiltrate. Measurement of UP concentration can therefore be used as an early but nonspecific method for detection of disease in dogs, cats, and people.9 We recently evaluated daily UPE in healthy horses and confirmed that UP:C ratio (measured in a randomly collected urine sample) could be used as a surrogate for 24-hour UP measurement in equids.10 However, whether UPE increases in horses with naturally occurring or experimentally induced systemic inflammatory diseases such as laminitis has not been determined.
Unfractionated heparin has been used for prevention and treatment of laminitis, particularly in horses deemed at risk, despite conflicting evidence of its efficacy11,12 Heparin exerts anticoagulant activity via potentiation of anti-thrombin III and may limit thrombus formation in small vascular beds.12 In addition, heparin has in vitro anti-inflammatory and indirect vasodilatory effects on vascular endothelium; such effects may prevent the stagnation of blood flow that ultimately results in thrombus formation and may slow or modify the local immune processes that occur in response to and contribute to progression of laminar cell necrosis.13,14 Interestingly, low-molecular-weight heparin has also been shown to reduce the magnitude of proteinuria in rats with experimentally induced glomerulopathy15
The purpose of the study reported here was to investigate the effects of unfractionated heparin administration on UPE during the developmental stages of experimentally induced laminitis in horses. The hypothesis was that UPE and serum insulin concentration would increase in horses with oligofructose-induced laminitis prior to onset of lameness. In addition, we hypothesized that treatment with heparin prior to and after experimental induction of laminitis would decrease the magnitudes of change in UPE and hyperinsulinemia in affected horses.
Urine protein excretion
BENEO-Orafti P95 oligofructose powder, BENEO-Orafti, Tienen, Belgium.
Heparin sodium for injection, 10,000 USP U/mL, Abraxis Pharmaceutical Products, Schaumburg, Ill.
Silkolatex Rusch Gold Foley catheter, Teleflex Medical, Bannockburn, Ill.
Vitros Crea quantitative color test, VITROS Chemistry, Ortho-Clinical Diagnostics, Rochester, NY.
Bio-Rad Microprotein dye-binding test, Bio-Rad Laboratories, Hercules, Calif.
RIA Gamma Counter 7000, Organon Teknika, Durham, NC.
Vitros Fusion, version 5.1, Ortho-Clinical Diagnostics, Rochester, NY.
SAS, version 9.1, SAS Institute Inc, Cary, NC.
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