Respiratory problems in sport horses

Frequent coughing, increased respiratory effort, and nasal discharge are obvious signs of pulmonary (lung) disease in horses. But not all signs are so apparent. In fact, said Renaud Léguillette, DVM, MSc, PhD, Dipl. ACVIM, ACVSMR, the less discernable clinical signs are what most frequently accompany equine lower airway disease, making it challenging for veterinarians to diagnose.

Léguillette, an associate professor of equine internal medicine and Calgary Chair in Equine Sports Medicine at the University of Calgary Faculty of Veterinary Medicine, in Alberta, Canada, reviewed tools veterinarians can use to diagnose pulmonary disease in sport horses at the 2017 American Association of Equine Practitioners Convention, held Nov. 17-21 in San Antonio, Texas. He focused on noninfectious inflammatory conditions but noted the importance of distinguishing between those and infectious conditions in daily practice.

Respiratory issues are some of the most frequently diagnosed conditions in sport horses evaluated for poor performance, he said, and one of the leading causes of training disruptions.

“Small decreases in lung function can have a dramatic impact on performance,” he added.

Signs of Respiratory Problems in Sport Horses

Knowing the clinical signs associated with lung pathologies of the athletic horse allows veterinarians to collect the most relevant information from the owners to increase diagnostic accuracy, Léguillette said. Clinical signs can be subtle, he said, but they’re usually a good indicator of respiratory problems in sport horses.

Common signs of an upper airway issue (nostrils, nasal passages, pharynx, and part of the larynx) include:

  • Abnormal respiratory noise;
  • Exercise intolerance; and
  • Coughing.

Lower airway problems (affecting everything from the back of the larynx to the lungs) are often characterized by:

  • Coughing;
  • Nasal discharge, usually bilateral (from both nostrils) and mucousy;
  • Exercise intolerance; and
  • Labored breathing at rest.

“Studies have shown that owners are a good and reliable source for observing changes in their horses’ respiratory signs,” Léguillette added.

Specifically, researchers found that owners are better than veterinarians at detecting changes relating to coughing, he said, so he encouraged veterinarians to ask questions about coughing and described a study that showed:

  • Horses that cough frequently or constantly and during exercise are more likely to have severe airway inflammation; and
  • Horses that cough more frequently at the start of exercise are likely to have moderate to severe inflammation.

“I think it is therefore important to not ignore coughing observed at the beginning of exercise,” Léguillette said. “It is not normal, contrary to what many horse owners think.”

Diagnostic Testing

Once the veterinarian has evaluated the clinical signs, he or she can use diagnostic testing to pinpoint the exact problem.

Endoscopy Static (at rest, performed in a standing horse) and dynamic (during movement, also called over-ground endoscopy, performed in an exercising horse) upper airway endoscopy can help diagnose static and dynamic obstructions, Léguillette said. Such issues can include laryngeal hemiplegia (roaring), dorsal displacement of the soft palate, and pharyngeal collapse, among others.

Upper and lower airway endoscopy can also show evidence of exercise-induced pulmonary hemorrhage (EIPH, seen as blood in the airways or lungs), mucous accumulation (which can point to lung inflammation), and pharyngitis (inflammation of the pharynx).

Thoracic ultrasound Most useful for assessing the lung surface and diagnosing pneumonia and related issues, ultrasound of the horse’s thoracic region, or chest, can also reveal evidence of EIPH, Léguillette said.

Radiographs Although X rays are difficult to perform on full-sized horses in the field, due to the lungs’ large size, they can help veterinarians see the deeper lung structures, along with evidence of EIPH and pulmonary nodular fibrosis, he said. However, it lacks the sensitivity (the probability that results will be positive when run on a group of patients with the disorder) necessary to diagnose inflammatory disease.

Sampling to detect inflammation Veterinarians can collect a variety of samples from the respiratory tract that can point to inflammation, Léguillette said. These include tracheal wash and/or bronchoalveolar lavage samples (which involve flushing saline into the windpipe and drawing the “wash” back up with a syringe., or injecting a horse’s lung with sterile fluid before drawing it back out again, respectively). Practitioners examine both types of samples under a microscope to detect signs of inflammation. Léguillette said the BAL analysis is the more reliable of the two techniques for detecting lung inflammation. Although BAL is useful for diagnosing and monitoring horses’ lung condition, he added, it isn’t always clear-cut, because some horses maintain persistent inflammation even after treatment.

Lung function testing These tests are difficult to perform in the field and are most commonly used in research settings, he said. They include bronchoprovocation tests (which allow the practitioner to assess the respiratory system’s response to agents that cause bronchoconstriction, such as histamines) and measuring VO2max (the horse’s maximum oxygen consumption rate during maximal intensity exercise).

Take-Home Message

“Diagnosing pulmonary pathologies in athletic horses can be challenging and should start with a good documentation of clinical signs,” Léguillette said. “The interpretation of ancillary tests can be difficult if a good technique is not used properly.”

Nonetheless, veterinarians have tools at their disposal, ranging from imaging to sample analysis, that can help pinpoint the cause of respiratory problems in sport horses.