No Boost from Azithromycin for Asthmatic Horses

Severe equine asthma threatens quality of life, and veterinarians rely heavily on corticosteroids for medical management. In some cases, asthmatic horses are resistant to life-saving corticosteroids, leaving a significant void in therapeutic approach. Researchers recently paired azithromycin with corticosteroids in an attempt to help asthmatic horses.

Horses with a particular form of asthma characterized by a high proportion of neutrophils—specific white blood cells—in their airways are known to develop corticosteroid resistance. These patients suffer severe manifestations, including acute worsening of disease and airway obstruction secondary to remodeling of the airways, defined broadly as structural changes caused by asthma that lead to thickened airway walls and narrowing of airways.

In human medicine, add-on therapy with azithromycin can improve quality of life and minimize disease progression. Preliminary studies in horses, however, were not successful and use of this drug in asthmatic horses is not recommended.

Because there are few treatment options for neutrophilic asthma in horses, that same research team took another stab at the problem, giving azithromycin a second look. This time, 12 horses previously diagnosed with severe asthma were all administered 2,500 micrograms of inhaled fluticasone twice daily for five months. Six of those horses were also treated with azithromycin at 10 mg/kg by mouth once daily for five days then once every other day for the duration of the study.

Lung function, airway inflammation, and airway remodeling were each evaluated intermittently during the five-month study period.

“The data showed that airway inflammation decreased in the azithromycin-treated horses; however, no difference in lung function or airway smooth muscle mass was observed. This means that the addition of azithromycin did not potentiate the clinical effects of fluticasone,” explained Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research.

In other words, azithromycin does not appear to be a viable add-on medication for severely asthmatic horses with neutrophilic-based disease and resistance to inhaled corticosteroids.

“Severely asthmatic horses therefore remain reliant on current recommended strategies, including environmental management focused on decreased environmental dust in the horse’s breathing zone, corticosteroids, bronchodilators, and omega-3 fatty acid supplements,” Crandell said.

“Omega-3 fatty acids, particularly docosahexaenoic acid or DHA, have anti-inflammatory properties. The recommended dose of marine-derived EO-3, for example, provides sufficient DHA to meet the latest equine asthma guidelines,” Crandell said.^**

^*Mainguy-Seers, S., R. Boivin, S.P. Dogaheh, F. Beaudry, P. Helie, A.G. Bonilla, J.G. Martin, and J.-P. Lavoie. 2022. Effects of azithromycin on bronchial remodeling in the natural model of severe neutrophilic asthma in horses. Scientific Reports 12(1):446.

 ^**Couëtil, L.L., J.M. Cardwell, V. Gerber, J.-P. Lavoie, R. Leguillette, and E.A. Richard. 2016. Inflammatory airway disease of horses: Revised consensus statement. Journal of Veterinary Internal Medicine 30:503-515.