Abstract: Background – Asthma symptoms can vary during the menstrual cycle in women. Similarly, airway obstruction is attenuated during the luteal phase in mares affected by severe asthma (SEA), a phenomenon that could be related to the relaxing effect of progesterone on bronchial smooth muscle. Objectives – To evaluate the effects of progesterone on airway obstruction and inflammation in SEA. Animals – 4 mares and 4 geldings with SEA from a research herd. Methods – In a randomized blinded cross-over study with a 3-week washout period, the effect of a 10-day daily oral altrenogest treatment (0.044 mg/kg) was compared to that of dexamethasone (0.06 mg/kg). Lung function, tracheal mucus accumulation and airway neutrophilia were assessed before and after treatment. Data were analyzed with two-way ANOVAs and Dunnett’s corrections. Results – Both treatments improved pleural pressure, pulmonary resistance and elastance overtime (p< 0.0001), although dexamethasone was more effective (treatment group difference (p< 0.05) and time-treatment group interaction (p< 0.05); pleural pressure reduction from 30.7 ±7.7 to 21.9 ±8.8 cm H2O with progesterone (p=0.02) and from 30.0 ±11.3 to 8.5 ±1.5 cm H2O with dexamethasone (p< 0.0001)). The response was similar between mares and geldings. The mucus accumulation and airway neutrophilia remained unchanged. Conclusions and Clinical Importance – The partial relief of airway obstruction by progesterone suggests its potential use as an adjunct therapy in SEA mares that display respiratory distress fluctuation during the estrus cycle. Whether altrenogest modifies the lung function and performance of healthy and mild asthmatic mares require investigation.
