Effects of a "two-hit" model of organ damage on the systemic inflammatory response and development of laminitis in horses

The role of endotoxemia in the development of laminitis remains unclear. Although systemic inflammation is a risk factor for laminitis in hospitalized horses, experimental endotoxin administration fails to induce the disease. While not sufficient to cause laminitis by itself, endotoxemia might predispose laminar tissue to damage from other mediators during systemic inflammation. In "two-hit" models of organ damage, sequential exposure to inflammatory stimuli primes the immune system and causes exaggerated inflammatory responses during sepsis. Acute laminitis shares many characteristics with sepsis-associated organ failure, therefore an equine "two-hit" sepsis model was employed to test the hypothesis that laminitis develops with increased frequency and severity when repeated inflammatory events exacerbate systemic inflammation and organ damage. Twenty-four light breed mares (10) and geldings (14) with chronic disease conditions or behavioral abnormalities unrelated to laminitis that warranted euthanasia were obtained for the study. Horses were randomly assigned to receive an 8-h intravenous infusion of either lipopolysaccharide (5. ng/kg/h) or saline beginning at -24. h, followed by oligofructose (OF; 5. g/kg) via nasogastric tube at 0. h. Euthanasia and tissue collection occurred at Obel grade 2 laminitis, or at 48. h if laminitis had not developed. Liver biopsies were performed at 24. h in laminitis non-responders. Blood cytokine gene expression was measured throughout the study period. Lipopolysaccharide and OF administration independently increased mean rectal temperature (P< 0.001), heart rate (P= 0.003), respiratory rate (P< 0.001), and blood interleukin (IL)-1β gene expression (P< 0.0016), but responses to OF were not exaggerated in endotoxin-pretreated horses. The laminitis induction rate did not differ between treatment groups and was 63% overall. When horses were classified as laminitis responders and non-responders, area under the blood IL-1β expression curve (P= 0.010) and liver and lung gene expression of IL-1β, IL-6, IL-8, IL-10, and tumor necrosis factor-α (P< 0.05) were higher in responders following OF administration. The results indicate that endotoxin pretreatment did not enhance responses to OF. However, systemic inflammation was more pronounced in laminitis responders compared to non-responders, and tissue-generated inflammatory mediators could pose a greater risk than those produced by circulating leukocytes.