Learning to Breathe Easier
- Dec 1, 2006
When your horse has heaves or inflammatory airway disease, you often feel utterly helpless. As he stands there with his head low, huffing and puffing, what can you do to quickly help him breathe easier?
Sarah Gardner, DVM, PhD, Dipl. ACVIM, associate professor in Equine Medicine at North Carolina State University (NCSU), is trying to find out. Her research investigates the pathophysiology of heaves, treatments, and delivery devices for those treatments. Her current work on a novel delivery device and medication for heaves isn't ready to be discussed yet, but her insight into heaves and inflammatory airway disease from her past research, clinical work, and pulmonary (lung) function testing service can help horse owners better understand the disease. And she's part of an ongoing discussion among veterinarians about a possible link between airway disease and anhidrosis (failure to sweat; more on that shortly).
Heaves and Inflammatory Airway Disease
Both of these conditions compromise a horse's breathing, although in different ways. Heaves is generally more severe and affects older horses, while inflammatory airway disease (IAD) affects younger horses and is often mistaken for a poor physical response to training. Luckily, the more severe heaves condition is less common.
"Abattoir (slaughter horse) studies have shown the prevalence of heaves to be up to 12%," says Gardner. "Other studies in young Thoroughbred and Standardbred racehorses have shown the prevalence to be 22-50% for IAD; it's quite high.
"We don't know as much about IAD as we do heaves, because it's much more recently recognized," she adds. "But it has a lot of similarities to heaves. Probably allergens are involved, as well as other dust particles that cause a nonspecific inflammatory response or airway hyperreactivity. There's probably a lot more IAD out there than we recognize, but it's just seen as exercise intolerance and cough. If a horse is working at a low level, his condition might not be noticed, but when he starts working harder, then his pulmonary limitations show up.
"In clients' horses here (at NSCU), primarily what we see is IAD," she reports. "We don't often see outright heaves; usually the horse we see only has signs of exercise intolerance and/or cough. Those horses we're able to tease out (find) with our pulmonary function testing service."
Pulmonary Function Testing
When a horse undergoes a pulmonary function test at NSCU, his pulmonary resistance--the ease with which air flows in and out of his lungs--is measured. "In a normal horse with nice, wide-open airways, resistance is very low, whereas in a horse with bronchoconstriction or airway obstruction associated with pus, mucus, or fluid within his airway walls, the airways will be small and have increased resistance to airflow," explains Gardner.
Once a horse's baseline pulmonary resistance is measured, he'll receive a dose of histamine (which causes bronchoconstriction and increased pulmonary resistance) via nebulizer (a medication inhalation device), then he'll be tested. Then he'll get a double dose and be retested; this process repeats until his airway resistance doubles. The entire test takes about 30 minutes.
"A normal horse will double his baseline pulmonary resistance at a concentration of histamine greater than 6 mg/mL," she says. "A horse is defined as having airway hyperreactivity if he doubles his baseline resistance below 6 mg/mL.
"If a horse has an elevated initial baseline resistance (i.e., has heaves), we don't do a histamine challenge test," she continues. "Instead we'll do a bronchodilator challenge test (using medicine that opens airways). That gives us an idea about the reversibility of the bronchoconstriction and is also an indicator of how he'll respond to that bronchodilator if you prescribe it.
"For a horse with heaves, pulmonary function testing can help you monitor response to therapy, and for horse with IAD, it can help you diagnose him and monitor his response to therapy," she explains.
Additional testing includes brochoalveolar lavage or "washing" a part of the lung and collecting the fluid to evaluate inflammatory cells (and other cells) present in the lung. Horses with IAD will have an elevated level of inflammatory cells. Radiographs might be done to evaluate lung structure, and a complete blood count might also be done to rule out infection.
"If I was buying a decent upper-level adult performance horse, I would want (Gardner) to test him in the prepurchase examination," says Dick Mansmann, VMD, PhD, clinical professor and director of NSCU's Equine Health Program. "You're going to put years of investment in him, and you're hoping to go to Rolex, the Olympics, or whatever high-level competition, and the last thing you need is a horse that has hyperreactive lungs and can't be around hay with all the traveling and training."
What tells you to consider calling a veterinarian to evaluate your horse's lung function? Gardner suggests:
- Labored breathing;
- Flaring nostrils at rest;
- Increased movement of abdomen during respiration;
- Elevated respiratory rate at rest;
- Nasal discharge;
- Coughing up mucus and/or pus;
- Exercise intolerance or poor performance.
These respiratory diseases are very hard to manage, primarily due to lack of owner compliance or inability to comply based on management situation, says Gardner. She recommends the following for affected horses:
- Do everything you can to decrease organic dust in the environment.
- Remove hay from the horse's diet and get an alternate source of fiber.
- If the horse has classic heaves, keep him outside 24/7 with access to a run-in shed.
- If the horse has pasture-associated pulmonary disease, keep him in a dry lot or in a barn with minimal dust.
- If the horse has both classic heaves/IAD and pasture-associated pulmonary disease, remove hay from the diet and keep him on a dry lot in the summer.
- Maintain proper ventilation in the barn; do not shut it up tight so you can stay warm.
"Many times, if these horses' environments are managed, they don't need to be treated with drugs," says Gardner. "That's always my goal. In some cases, management doesn't allow that, and pasture-affected horses usually need some type of treatment (generally a steroid, and possibly a bronchodilator). "
Airway Disease vs. Anhidrosis?
"When most people see a horse that's breathing hard, they think pulmonary disease, but in fact he might not be sweating (partially or completely anhidrotic) and he's just trying to cool down," says Mansmann. "And his heart rate is going to be high. This is certainly a major problem in the South. Some people believe these (anhidrotic) horses have a higher incidence of heaves and other respiratory disease, and that there might be an actual tie between the mechanisms of nonsweating and some form of chronic respiratory disease."
"Several horses I've seen had both IAD and partial anhidrosis," comments Gardner. "When I take a horse's history, I really question the owner about how much the horse sweats. The sedative we give during a pulmonary function test induces sweating, so it's a nonspecific test as to whether the horse sweats. If he doesn't sweat during his test, I'm always suspicious of anhidrosis. We have a hard time determining whether these horses' (respiratory) problems are due to partial anhidrosis (panting to cool down) or IAD (panting to get enough air), especially during summertime."
Is there really a link between respiratory disease and anhidrosis? No one knows yet, but perhaps research in respiratory labs like Gardner's at NCSU will one day provide an answer.
About the Author
Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.
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