Inhaling Fungi Increases Horses’ Risk of Inflammatory Airway Disease

We know the ubiquitous dust and pollen present in horses’ forage and bedding can trigger equine asthma and respiratory allergies. What about fungi, though—how prevalent is it in horses’ environments, and could it be contributing to inflammatory airway disease (IAD)?

Emmanuelle van Erck, DVM, PhD, Dipl. EBVS, ECEIM, and her team at Equine Sports Medicine Practice in Waterloo, Belgium, sought to find out. She described their study and findings at the 2019 Annual American Association of Equine Practitioners Convention, held Dec. 7-11 in Denver.

Van Erck included in the study 731 European racehorses, Warmbloods, and leisure horses that had been referred to her clinic from 2013 to 2016 for signs of respiratory disease or poor performance. As her team does with all respiratory cases, noted clinical signs such as coughing and nasal discharge;  performed an airway endoscopy, tracheal wash, and bronchoalveolar lavage (BAL); and evaluated each horse’s environment (bedding, forage, pasture access, etc.);. They found that:

  • 88% of the horses had inflammatory airway disease (mild asthma).
  • Nearly all horses (721) lived indoors most of the day.
  • 81% were positive for fungal elements on tracheal wash.
  • 55% had positive fungal cultures, mainly Aspergillus and Penicillium
  • Horses with fungal elements were twice as likely to have IAD than those without.
  • Horses with positive cultures were twice as likely to also have positive bacterial cultures—perhaps due to a high environmental burden, a stressed immune system, or microbiota disruption, van Erck hypothesized.
  • Signs of coughing and epistaxis (nosebleed) occurred more frequently in horses with fungal proliferation.

Overall, the researchers saw the highest prevalence of fungi in the airways of IAD horses. They did not, however, note particular clinical signs associated with fungal presence.

“We’re likely underestimating the proportion of horses affected because there are no specific clinical signs,” said van Erck, adding that these might be “failed” IAD cases that haven’t responded to treatment.

She and her team then looked at each horse’s environment. They found that horses had a higher likelihood of fungi in their airways if they were bedded on straw vs. shavings. Those consuming dry hay had the highest percentages of neutrophils (white blood cells that increase in response to infection) in their BALs. Those consuming high-temperature-steamed hay had the lowest counts.

“The type of bedding and forage represent significant risk factors for IAD and fungal contamination of the airways,” said van Erck, adding that straw bedding combined with dry hay put horses at most risk.

Manage these horses with inhaled corticosteroid treatment and environmental changes, such as steaming hay to significantly reduce its number of fungal contaminants.