Most Horses With Inflammatory Airway Disease Exposed to Fungi
Researchers and veterinarians know there’s a link between environmental contaminants (such as dust particles and fungi) and the disorders under the equine asthma umbrella—inflammatory airway disease (IAD) and the more severe recurrent airway obstruction (RAO, often referred to as heaves). However, researchers have only recently discovered that breathing in various kinds of molds can cause a horse to develop IAD, said Emmanuelle van Erck-Westergren, DVM, PhD, Dipl. ECEIM, of Equine Sports Medicine Practice, in Waterloo, Belgium.

“These airway problems are underdiagnosed, and we should be more aware of how important the environment is for the health and performance of our horses,” she said.

Van Erck-Westergren and colleagues evaluated 731 sport horses, averaging 8 years old, referred to her practice for breathing difficulties, poor performance, or routine exams. In each case, the clinicians performed an airway endoscopy, a tracheal wash (flushing saline into the windpipe and drawing the “wash” back up with a syringe to evaluate respiratory secretions), and a brochoalveolar lavage (injecting sterile fluid into a horse’s lung before drawing it back out again to look for signs of inflammation) and asked the owners questions about their horses’ food and bedding.

They diagnosed 88% of the horses with IAD, she said. Of those, 81% had fungal elements visible in the tracheal wash; the tracheal wash was the most reliable method for detecting fungal particles in her study, she added. The team also found that horses with fungal elements in the tracheal wash were more than twice as likely to have IAD than those without.

Van Erck-Westergren said IAD diagnoses resulted mainly from signs of airway inflammation as seen in the bronchoalveolar lavage and less so from clinical signs such as coughing or nasal discharge. However, she added, IAD did seem to have a frequent association with poor performance.

Why horses react to fungus is unclear and requires more research, she said. It could be that the fungus sets off the IAD directly, or it might be that IAD horses have a reduced immune response and, therefore, react more to the fungi than other horses would.

“The inhalation of fungi is the cause of IAD, but we are not sure what the underlying pathological mechanisms are, as fungi can be infectious, allergenic, or plainly irritant,” said van Erck-Westergren. “Every horse will react differently, or more or less, depending on his sensitivity. The outcome is always the same: inflammation.”

Horse farms can harbor many mold species in bedding, on walls, and even in fields. These species’ effects on horses vary. “Black mold is very toxic and can lead to more than ‘just’ IAD, as horses or even humans who inhale black mold can develop pneumonia and neurological symptoms,” van Erck-Westergren said. “Aspergillus species are fairly ubiquitous in stables but, if inhaled in larger quantities, can lead to allergic forms of asthma (and) cause pulmonary bleeding. Other molds are more ‘neutral’ and do not trigger a strong reaction. But they’re recognized by the body as foreign, so inhaling them will inevitably challenge the immune system.”

The high rate of IAD in the study horses wasn’t surprising, she added, considering they were primarily sport horses housed in box stalls. That kind of management system lends itself to more exposure to mold and, thus, IAD. However, she did note trends regarding the kind of forage and bedding used and the levels of fungal particles in the tracheal wash.

“Straw and dry hay are the most likely to be contaminated by fungi, as they are harvested from the fields and are in direct with soil, which hosts bacteria and fungi,” she explained. “Our study showed that the use of wood shavings is protective against IAD.

Steaming hay at high temperatures also appeared to reduce horses’ risk of IAD, she added.

“For forage, dry hay represents the highest risk,” she said. “Soaking the hay promotes microbial growth and is therefore counterproductive. Haylage quality was so variable that it wasn’t protective.”

The study, “Fungi in respiratory samples of horses with inflammatory airway disease,” was published in the Journal of Veterinary Internal Medicine.