Inflammatory Airway Disease in Performance Horses
Dr. Melissa Mazan
Lung function testing is a way to test the mechanical properties of the horse's lungs. Essentially, what we want to know is how much work does the horse have to do to achieve a certain airflow? We can determine this by measuring the resistance that the horse's respiratory system presents to airflow. Respiratory system resistance. Resistance is best thought of as the relationship between pressure and flow. Mathematically, we express resistance (RRS) as RRS = pressure/flow. If the respiratory system has to generate a high driving pressure to achieve the necessary flow of air, then we know that respiratory system resistance is high. Because narrow airways have a higher resistance than wide airways, we can thus deduce that the airways are narrowed.
Histamine challenge. Although measuring resistance at baseline is useful, it is still a somewhat crude way of determining whether airways are narrowed – there has to be a lot of airway obstruction, and, consequently airway narrowing, for it to be reflected as an increased RRS. In order to pick up subtle changes, we perform a second test known as a histamine challenge, otherwise known as bronchoprovocation.
Anyone who has suffered from hay fever knows that antihistamines can make the itch, burning eyes, and runny nose go away, or at least feel a whole lot better. That is because antihistamines counteract the effect of histamine, which is released from inflammatory cells called mast cells. Although antihistamines, unfortunately, do not work very well in horses, histamine itself does cause the airways to constrict.
In normal horses, as with normal people, it takes a very large dose of histamine to cause the airways to constrict measurably. Horses with airway inflammation and airway hyperreactivity rapidly double their airway resistance with even a very small dose of histamine. This is known as provoking the airways, and thus this test is also known as a bronchoprovocation test
We designate a doubling of the airway resistance as the provocatory concentration necessary to cause a 100 percent increase in airway resistance, or the PC100RRS. Horses with a PC100RRS of less than 6 mg/ml of histamine are designated as having hyperreactive airways. Horses with a PC100RRS of greater than 8 mg/ml histamine are considered normal.
Bronchoalveolar lavage (BAL). This is a method for sampling the cells that are deep within the airways. If we know what types of cells are involved in a particular horse's airway inflammation, then we can target more specifically the inflammation with our treatment.
When we perform a BAL, a long, sterile, soft tube is inserted from the horse's nostril all the way to the depths of one of the smaller bronchi. We infuse a sterile, warm solution into that portion of the lung, and then suction the fluid back out. With the fluid comes a sampling of the cells that dwell deep within the lung. Then, we examine these cells under a microscope.
Horses with IAD tend to have elevated levels of inflammatory cells such as neutrophils and mast cells. Occasionally, they may also have excessive numbers of cells known as eosinophils. Normal horses have:
Less than 5 percent neutrophils
Less than 2 percent mast cells
Less than 0.5 percent eosinophils