Severe equine asthma leads to raised blood pressure in the pulmonary arteries (pulmonary hypertension) which, if sustained over a long time, can result in right sided heart failure.
Lung inflammation and narrowing of the airways reduces the amount of oxygen the horse is able to absorb into the bloodstream. This reduced blood oxygen concentration causes constriction of the pulmonary blood vessels (hypoxic vasoconstriction) which increases pulmonary blood pressure.
However, supplementing oxygen only partly reverses this pulmonary hypertension and therefore other factors are likely involved.
A recent study (1) investigated remodeling of the pulmonary arteries in severe equine asthma and its reversibility following long-term antigen avoidance strategies and inhaled steroid use.
The total wall area of pulmonary arteries from different regions of lung obtained post mortem from 12 asthmatic horses (6 in remission, 6 in exacerbation) and 6 controls was measured. The smooth muscle mass of pulmonary arteries was also estimated.
Reversibility of vascular changes in 11 asthmatic horses was also assessed after 1 year of antigen avoidance alone (5 horses) or treatment with inhaled steroids for 6 months followed by allergen avoidance for 6 months (6 horses).
Pulmonary arteries of asthmatic horses in both exacerbation and remission showed 7-12% increased wall area in the most apical (closest to head) and caudodorsal (back upper lung) regions compared to control horses. The smooth muscle mass of the pulmonary arteries was similarly increased.
Both allergen avoidance and inhaled steroid reversed this increase in pulmonary artery wall area but were closer to normal values with antigen avoidance. This agrees with other studies which show long-term antigen avoidance is superior at controlling airway inflammation compared to inhaled steroids (2).
The authors conclude that severe equine asthma is associated with remodeling of the pulmonary arteries and an increase in smooth muscle mass.
The resulting narrowing of the artery could enhance hypoxic vasoconstriction, contributing to pulmonary hypertension. In this study emphasises the importance of antigen avoidance for controlling severe equine asthma.
1. Ceriotti S, Bullone M, Leclere M, Ferrucci F, Lavoie J-P (2020) Severe asthma is associated with a remodeling of the pulmonary arteries in horses. PLoS ONE 15(10): e0239561. Available HERE
2. Leclere M, Lavoie-Lamoureux A, Joubert P, Relave F, Setlakwe EL, Beauchamp G, et al. Corticosteroids and antigen avoidance decrease airway smooth muscle mass in an equine asthma model. Am J Respir Cell Mol Biol. 2012;47(5):589–96.