Solving the Equine Asthma Riddle
Equine asthma affects a variety of horses, but with a correct diagnosis horses with asthma can be more easily managed. But what causes equine asthma and what is the best way to treat it? Respiratory issues are second to musculoskeletal complications in horses, says Julia B. Montgomery, Med Vet, PhD, Dipl. ACVIM (LAIM), associate professor in Large Animal Clinical Sciences at the University of Saskatchewan’s Western College of Veterinary Medicine.
“Every cough means something,” she says. “If your horse is occasionally coughing or a bit more sluggish, it’s important not to ignore it and consider pursuing further diagnostics.” Over 70% of horses will have at least one asthma episode in their lifetime; however, not every horse with mild to moderate asthma at one time in their life will develop severe asthma, she adds.
The Spectrum of Equine Asthma
Equine asthma includes a broad spectrum of respiratory diseases, explains Melissa Mazan, DVM, Dipl. ACVIM, professor and associate chair of the Department of Clinical Services at the Tufts University Cummings School of Veterinary Medicine’s Hospital for Large Animals in North Grafton, Massachusetts.
“In the horse world, we break it up into mild, moderate, and severe asthma,” she explains. “Severe asthma is what we used to call heaves or recurrent airway obstruction. It’s detectable to a veterinarian externally when a horse is in crisis or has recurrent episodes of respiratory embarrassment—meaning the horse is visibly struggling to breathe.”
Montgomery suggests the term equine asthma syndrome better differentiates between levels of asthma. Horses with severe asthma have clinical signs at rest, including respiratory difficulty or hypertrophy (enlargement) of abdominal muscles. Other signs include nostril flaring or severe exercise intolerance, she adds.
Genetics, respiratory or bacterial infection, seasonal allergies, and environmental factors such as dust, pollen, other airborne particulates, or stall ventilation can offer clues that a horse has asthma, but what if clinical signs aren’t severe?
“In athletes such as racehorses with equine asthma, they might never cough, and have no signs of respiratory disease at rest,” Mazan says. “However, these horses might be 10th of a second, or even hundredths of a second slower than they used to run. While they won’t cough at rest or have nasal discharge, after racing, they have a higher respiratory rate on recovery, which indicates equine asthma syndrome.”
In these athletes both the horse owner and veterinarian must consider even the slightest clinical signs. Even the most astute veterinarian or owner can miss signs of equine asthma, she adds, especially if the performance demands placed upon a horse are less intense.
Viral and bacterial respiratory infections or variations in the horse’s lung microbiome (the microbial community living there) potentially play a role in equine asthma. Blood testing differentiates equine asthma from infectious respiratory diseases such as pneumonia but, in the absence of clinical signs and conclusive blood results, diagnosing equine asthma might require the veterinarian to dig deeper to find an answer.
Advanced Diagnostics for Equine Asthma
Uncovering the extent of equine asthma entails identifying the degree of airway inflammation using bronchoalveolar lavage. Montgomery describes the procedure as a lung wash because it coats airways and alveoli with sterile saline. Inserting a tube through a horse’s nostril, the veterinarian retrieves cell samples from deep inside the lung to pinpoint inflammatory cells.
“On an inflammatory cell level, there are some distinctions,” Montgomery says. “Different types of inflammatory cells might be involved or contribute to lung inflammation, which is a very active area of research. The assumption is if there are different types of inflammatory cells, there are probably different signals that ignite the inflammation.”
Factors that exacerbate equine asthma vary depending on geographical region and time of year. Still, they often involve elements in a horse’s environment, including moldy hay, dust, poor air quality, or pollen. Seasonal triggers such as pollen often irritate asthmatic horses.
“Many horses will have asthma exacerbations, worsening their clinical symptoms seasonally,” says Montgomery. “The assumption is that it has to do with environmental factors like air quality or pollen. We see asthma exacerbations in the spring when the snow melts and in late summer or early fall when there is more harvest activity and dust in the air.”
Treatments for Helping Horses Breathe Better
Equine asthma can be treated but not cured. Our sources say treatment using a bronchodilator restores a horse’s ability to breathe—but only for a short time. Clenbuterol, an oral bronchodilator, relaxes and opens airways to provide relief within 30 minutes. However, veterinarians view bronchodilators as a “rescue” treatment.
Corticosteroids relieve airway inflammation and can be administered orally, by injection, or as an inhalant. However, systemic corticosteroids can carry risks for horses with equine metabolic syndrome or PPID, so cautious use is advised, notes Montgomery.
Medication isn’t a silver bullet for tackling equine asthma. “None of these treatments will help unless you improve the environment and remove the triggers,” says Montgomery. “This influences how quickly the horse responds to other treatments but, unfortunately, that’s sometimes the hardest thing to do.”
Environmental triggers remain at the core of respiratory illnesses like equine asthma. Mitigating stimuli in a barn or environment—controlling dust, keeping areas ventilated, and using bedding like recycled paper instead of shavings, for example—is helpful but not a permanent solution. Mazan adds that many barns are designed for people more than horses: “The bottom line is that even the best-designed barns are poorly ventilated and have a lot of dust and endotoxins.”
Myth of the Clearing Cough
A cough is not always just a cough, and the notion of the “clearing cough” is erroneous, Mazan says. “A cough clears things like mucus and inflammation,” she notes. “The normal physiologic response of airways is too narrow to prevent harmful things from entering.”
Horse owners should invest in proper diagnostics for an accurate equine asthma diagnosis and then chart a course of treatment with their veterinarian. “You will save yourself and your horse a lot of grief down the road,” Montgomery says.
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