Since 2016, chronic inflammatory airway syndromes such as chronic obstructive pulmonary disease (COPD), inflammatory airway disease (IAD) and recurrent airway obstruction (RAO) have been classified under the umbrella term of equine asthma syndrome. Despite treatment with environmental management along with corticosteroids and bronchodilators, not all severe cases of equine asthma respond to these strategies.
Besides bronchospasm and mucus accumulation that typify asthma, airway obstruction and remodeling in equine asthma are thought to be, in part, due to neutrophilic inflammation. The macrolide azithromycin has immunomodulatory properties, including reduction of pulmonary neutrophilia and lessening of hyper-responsiveness in lower airways.
A study undertaken at the University of Montreal College of Veterinary Medicine used a blind, randomized, crossover design on six horses with severe asthma to test the immunomodulatory effects of azithromycin—10 mg/kg given orally once daily for five days, then every other day for two more doses [Mainguy-Seers, S.; Vargas, A.; et al. Randomized study of the immunomodulatory effects of azithromycin in severely asthmatic horses. Veterinary Record Aug 2019, vol. 195, no. 5, p. 143; doi: 10.1136/vr.105260].
The researchers evaluated bronchoalveolar lavage fluid for multiple indicators of lung function:
- Airway remodeling
- mucus accumulation
- luminal airway neutrophilia, and
- pro-inflammatory mediators, such as cytokines like interleukin and tumor necrosis factor.
Azithromycin reduced interleukin expression, but did not improve the other variables, including neutrophil activation and recruitment.
Ceftiofur (with no immunomodulatory properties) was also evaluated for the same parameters, but failed to demonstrate any beneficial effect.
No adverse effects were seen, such as diarrhea, from macrolide treatment over the 10-day therapy course.
In this study, each macrolide antibiotic was used as a stand-alone treatment and not in conjunction with conventional steroid or bronchodilator therapy.
The researchers suggest that in keeping with responsible use of anti-microbial medication, equine asthma should not be treated with antibiotics as a first-line therapy.
Significant reduction of neutrophilic chemoattractant interleukin shows promise in using azithromycin for refractory cases of severe equine asthma and more investigation is warranted.