The pathogenesis of chronic laminitis remains unclear. There is growing evidence, however, of a link between this condition and the development of certain systemic diseases, such as kidney disease, that involve small blood vessel damage. There might also be a link between routine vaccination and acute episodes of chronic laminitis. This has led to the theory that chronic laminitis produces changes in the immune system that lead to hyperreactivity (heightened response)–to vaccines. Researchers from Texas A&M University recently conducted a study to investigate this theory in chronically laminitic horses.

Intradermal skin testing (IDT) was performed on chronically laminitic and healthy horses using 70 different antigens. The injection sites were evaluated at 15 and 30 minutes, and four and 24 hours after injection. Results demonstrated that chronically laminitic horses reacted significantly more positively than control horses at 15 and 30 minutes, and four hours. Chronically laminitic horses had a peak reaction at four hours, with a mean total reaction score of 62.9 (±11.5), compared to a peak score of 35.3 (±17.2) for control horses. Several horses from both groups were later biopsied at injection sites. Neutrophilic (of the neutrophil immune cell) infiltration was identified, which, when combined with the IDT findings, suggests type-III hypersensitivity in chronically laminitic horses. Type-III hypersensitivity is characterized by local inflammation and fluid swelling, vascular damage, lack of clotting ability, local hemorrhage, and ultimately the destruction of tissue in other species. Some of these factors fit what is seen in the chronically laminitis horse. Also, type-III hypersensitivity seen clinically with chronic disease and associated with systemic vascular disease (such as kidney disease) mimics the human condition systemic lupus erythematosus, often associated with the vascular condition Raynaud’s syndrome.