Pulmonary and systemic effects of inhaled endotoxin in control and heaves horses.
Authors: Pirie R S, Dixon P M, Collie D D, McGorum B C
Journal: Equine veterinary journal
Summary
Inhaled endotoxin (lipopolysaccharide from gram-negative bacteria) has long been suspected as a trigger for airway inflammation in stabled horses, particularly those with heaves (recurrent airway obstruction). Pirie and colleagues exposed six healthy horses and seven asymptomatic heaves-susceptible horses to escalating doses of aerosolised endotoxin (20, 200, and 2,000 µg) and monitored pulmonary inflammation, lung function, and clinical responses, whilst also measuring endotoxin concentrations during natural hay and straw exposure. Both groups mounted dose-dependent neutrophilic airway inflammation, but heaves-susceptible horses demonstrated greater sensitivity, with inflammatory responses detectable below 20 µg compared to the 20–200 µg threshold in controls; moreover, only heaves horses showed measurable lung dysfunction, and only at the highest 2,000 µg dose. The findings suggest that whilst inhaled endotoxin clearly drives airway inflammation and dysfunction in susceptible individuals, it acts as one component of a multifactorial disease process rather than a sole causative agent—meaning that stable management strategies targeting dust and endotoxin reduction will benefit both at-risk and healthy horses, but clinicians should not expect endotoxin control alone to resolve clinical heaves. For farriers, vets, and coaches advising on respiratory health, this reinforces the importance of addressing the complete stable environment (ventilation, forage quality, bedding type) alongside any targeted interventions.
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Practical Takeaways
- •Heaves-susceptible horses are significantly more sensitive to inhaled endotoxin than healthy horses; stable management to reduce dust and endotoxin exposure is critical for these animals
- •Poor stable environments with high airborne endotoxin can trigger airway inflammation in normal horses, making ventilation and bedding management important preventive measures for all horses
- •Heaves is multifactorial; controlling endotoxin exposure alone may not resolve clinical signs but is an important component of comprehensive management alongside addressing other environmental triggers
Key Findings
- •LPS inhalation induced dose-dependent neutrophilic airway inflammation in both control and heaves-susceptible horses
- •Heaves-susceptible horses showed lower response thresholds (<20 µg for airway inflammation; 200-2,000 µg for lung dysfunction) compared to control horses (20-200 µg for inflammation; >2,000 µg for dysfunction)
- •2,000 µg LPS inhalation induced detectable lung dysfunction only in the heaves group
- •Inhaled endotoxin likely contributes to heaves pathogenesis in concert with other inhalants rather than as a sole causative agent