Inhalation of organic dusts and lipopolysaccharide increases gelatinolytic matrix metalloproteinases (MMPs) in the lungs of heaves horses.
Authors: Nevalainen M, Raulo S M, Brazil T J, Pirie R S, Sorsa T, McGorum B C, Maisi P
Journal: Equine veterinary journal
Summary
# Editorial Summary This 2002 study investigated how inhaled organic dust and bacterial lipopolysaccharides (LPS) trigger the release of matrix metalloproteinases (MMPs)—tissue-degrading enzymes—in the lungs of both healthy and heaves-affected horses. Researchers used bronchoalveolar lavage fluid to measure MMP activity in response to three different challenges: mouldy hay/straw exposure, hay dust suspension, and inhaled LPS, comparing responses between six healthy controls and six to seven horses with heaves (now termed recurrent airway obstruction). The key finding was that hay and dust challenges significantly elevated total gelatinolytic activity only in heaves horses, predominantly through increased pro-MMP-9 and active MMP-9, with activity peaking at 5–24 hours post-challenge; conversely, LPS inhalation increased MMP activity in both groups. A striking correlation existed between neutrophil counts and MMP-9 levels, suggesting that neutrophil-driven inflammation directly drives MMP release in the airways. For practitioners managing heaves-affected horses, these results demonstrate that seemingly minor dust exposure triggers a disproportionate enzymatic response that perpetuates tissue damage and airway dysfunction—reinforcing the critical importance of rigorous dust control, quality forage selection, and environmental management as primary preventive measures rather than relying solely on symptomatic treatment once inflammation is established.
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Practical Takeaways
- •Heaves-affected horses show heightened inflammatory MMP responses specifically to moldy hay/straw and hay dust, suggesting environmental management is critical to reduce lung damage and respiratory dysfunction
- •The strong correlation between neutrophil counts and MMP-9 activation indicates these enzymes drive tissue-destructive inflammation in heaves; monitoring dust exposure may be more effective than treating active disease
- •LPS inhalation affects both healthy and heaves horses similarly, suggesting endotoxin-related challenges pose universal respiratory risk regardless of baseline disease status
Key Findings
- •Hay/straw and hay dust suspension challenges increased bronchoalveolar lavage fluid gelatinolytic MMP activity only in heaves-affected horses, primarily comprising pro- and active MMP-9
- •LPS inhalation increased gelatinolytic MMP activity in both healthy and heaves-affected horses, unlike organic dust challenges
- •BALF neutrophil counts showed highly significant correlation (P<0.0001) with proMMP-9 and active MMP-9 levels, but not with MMP-2 levels
- •Gelatinolytic MMP activity peaked at 5-24 hours post-challenge in heaves horses and returned to baseline by day 4