Environmental heat and airborne pollen concentration are associated with increased asthma severity in horses

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Reason for performing study:

Clinical exacerbations of severe equine asthma (formerly recurrent airway obstruction [RAO]) are more frequently reported during winter when horses are exposed to airborne dusts during stabling. However, we have also observed a worsening of clinical signs on days during a heatwave.


To investigate the association between environmental temperature and humidity and clinical signs of asthma in horses during clinical exacerbation of the disease.

Study design:

Retrospective longitudinal study.


Historical data on 14 severe asthmatic horses exposed to a dusty environment and evaluated using a previously validated clinical scoring system were analysed. Barn temperature and relative humidity values were obtained and air enthalpy (h) calculated. Correlation tests were used to study the relationship between mean daily clinical scores of horses and environmental variables. Lung function parameters recorded at 4 day intervals during hot (25°C) and warm (18°C) barn conditions were compared using a paired t test.


Significant positive correlations were observed between the mean daily clinical score and temperature (r = 0.58, P = 0.01) and air enthalpy (r = 0.55, P = 0.02). Maximal daily temperature correlated with airborne pollen concentrations (r = 0.51, P = 0.0002). In the absence of changes in the management of horses, higher barn temperature and enthalpy were associated with increased transpulmonary pressure (P = 0.005), pulmonary resistance (P = 0.008) and elastance values (P = 0.005).


Providing a cold environment may help attenuate the severity of airway obstruction in uncontrolled exacerbations of severe equine asthma. Furthermore, variations in environmental heat and associated pollen concentrations should also be taken into account when evaluating the response to therapy in clinical or research settings.

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