Inflammatory Airway Disease in Performance Horses
Everyone knows a horse or two that coughs at the start of work. Many of us have ridden such a horse, that practically hauls us out of the saddle as he lowers his head readying himself for a good, satisfying cough – once, twice, three times – and then, after a few minutes, stops and goes on his way. If you ask most horse people, they'll say that such a cough is acceptable – as long as the horse stops coughing before too long.
For many years, veterinarians also accepted some coughing even in performance horses as normal, primarily because we lacked the diagnostic techniques necessary to probe further and diagnose the subtle cause, which is a significant abnormality of the lung. This abnormality can now be readily detected.
We now know that coughing horses with lung disease are extremely common, but they should not be viewed as normal. We also know that the coughing, otherwise healthy horse, has a lower airway disease, known variously as inflammatory airway disease (IAD), small airway inflammatory disease (SAID), small airway disease, and chronic obstructive pulmonary disease (COPD).
What distinguishes IAD from heaves (late-stage COPD) is the lack of obvious labored or increased breathing effort of horses at rest. In fact, IAD is usually not visible in horses at rest.
Many of us also failed to realize over the years, that some horses that cough intermittently will go on to develop heaves if improperly managed. Indeed, most horses with heaves have a history of more low-grade signs of coughing and exercise intolerance (signs of IAD), making the connection between IAD (early on) and heaves (with age) likely.
What to Watch For
- Increased respiratory rate
- Nasal discharge that is usually clear.
However, if your horse has an early stage of IAD, he may not show any of these classic signs.
Anatomy and Physiology
IAD is a disease of the airways which disturbs normal lung function (causing exercise intolerance) in addition to acting as a source of irritation (causing cough). To understand the impact of IAD on the lung, we need to review some physiology.
Oxygen is the essential fuel for the body – if O2 can't get to the bloodstream, then effectively, the horse is like a car out of gas. No matter how willing the horse is, if he can't get enough oxygen to the bloodstream and thus to the muscles, he won't be able to do his job. The job of the respiratory system is to bring adequate amounts of oxygen into the body.
- First, the horse draws air in through his nostrils. Unlike humans, horses can't breath through their mouths – no matter how stuffy their noses may be. Their anatomy simply doesn't allow it.
- Next, the air must go through the larynx (voicebox) and down the trachea (windpipe).
- The trachea then branches into two mainstem bronchi (large airways). Each of these bronchi then branches into smaller bronchi, and then into smaller and smaller bronchioles (small airways). There are hundreds of thousands of these small airways.
- Eventually, the smallest of the bronchioles end in alveoli (air sacs), where the oxygen (O2) in the air diffuses into the blood, and the gaseous waste product in the blood (carbon dioxide, or CO2) diffuses out of the blood and is transferred back into the outside air.
IAD is a blockage of the bronchioles. In fact, the bronchioles are swollen, sometimes shut, and this prevents air from reaching the maximum number of alveoli. Since the alveoli are the gas exchangers of the lung, IAD restricts oxygen uptake from inspired air, and therefore reduces exercise tolerance.
When a horse suffers from IAD, then his small airways are narrower than normal because of inflammatory secretions, excessive mucus and fibrous tissue. Just as a kink in a garden hose will constrict the flow of water, so too will this narrowing of the airways constrict the flow of air. With a kinked garden hose, you can, to a certain degree, keep delivering the same amount of water as long as you turn up the water pressure. After a certain point, however, even this approach won't get you enough water for your lawn.
With the airways, the equivalent of turning up the pressure is to breath harder. This increase in pressure results in a higher airflow, but it also increases the amount of work that the horse has to do to achieve each breath. Eventually, this mechanism also fails to result in a high enough airflow. The end result? Your horse is breathing harder and harder to get less and less fuel to his muscles.
Horses develop inflamed airways in response to allergens in their environments. Some animals (we don't know why some are affected and others are not) develop an immunologic response to common substances in their environments, such as plant pollens, and the molds and spores found in even the best of hay. The lung, thinking that it is repelling foreign invaders, mounts an attack. This attack takes the form of inflammation of the lung, with its full complement of cells and debris.
IAD may be triggered by a viral disease such as equine influenza. Certainly, the flu can cause your horse to have reactive airways for months after the actual disease is gone – you may note a persistent cough, especially during exercise.
A phenomenon known as airway reactivity exacerbates this whole process. Inflamed airways are more reactive, or 'twitchier' than the airways in normal horses. This means that the airways, which are already narrowed, constrict even more in response to a number of stimuli – these stimuli may be the same allergens that caused the airway inflammation in the first place, such as molds and pollens, or the stimuli may be cold air, or the gases and particles found in the air in highly polluted areas. It is truly a losing situation for the horse.
One thing that you might notice is that your horse has more difficulties on hot, humid days than on clear, cool days. Often the problems are seasonal as well. This is similar to people with asthma or other chronic respiratory diseases. When the health bulletins on the radio start warning people with respiratory diseases to stay inside and to use air conditioning, you may notice that your horse has difficulty breathing as well.
Some horses with IAD simply have mild signs of exercise intolerance. A jumper may start knocking down rails, or may not make the times that he used to do easily. Dressage horses often show a reluctance to come into a frame. Horses have enormous respiratory reserves, however, and dressage horses, hunters, and lower level jumpers don't work at peak aerobic levels despite being highly trained athletes. This is why they usually are diagnosed when they are at a more advanced stage of disease, showing classic signs of IAD as we noted earlier.
Horses who work at peak exercise, such as racehorses, polo ponies, or high level event horses, usually show signs at an earlier stage of disease. Racehorses may be a tenth of a second slower than previously – this doesn't sound like much, but it translates into being 2-3 lengths behind a horse that they could previously have beaten. Oftentimes, racehorses are described as having 'hit the wall' at the 3/4 mark. A high goal polo pony might not be able to ride off his opponent when he is nearing the end of a chukker. These signs may be subtle, and it is not immediately obvious that the culprit is the respiratory system.
History. Your veterinarian will start with a careful medical history. Questions that are important include:
- When did you first notice a problem?
- What signs and symptoms did you notice?
- How is your horse stabled?
- Is the housing new or old?
- Is there good ventilation?
- Has your horse moved recently?
- Does your horse live adjacent to an indoor arena?
- Where is the hay stored?
- Is the hay overhead or in a separate building?
- What does your horse eat?
- Has the hay source changed?
- Is the hay of good quality?
- When are your horse's symptoms the worst?
- Do you notice a problem more in the winter or summer?
- Do any other horses in the barn have the same symptoms?
- Has your horse had a respiratory virus in the last 3 to 6 months?
- Has your horse been treated for this problem before?
- If so, has he had a good response to any treatments?
Your veterinarian will conduct a thorough clinical examination, which should include the following:
- Breathing assessment. Your veterinarian will visually assess your horse's breathing while he is resting to look for any abnormal expiratory effort, increase in respiratory rate, or evidence of difficulty breathing. She will listen to your horse's respiratory system, from the trachea to the lungs, using a stethoscope. She should hear normal respiratory noises, but if your horse has a more advanced stage of IAD, she may hear musical sounds, especially on expiration, called wheezes.
- Temperature. In order to make sure that your horse doesn't have an infectious respiratory disease, your veterinarian will take your horse's temperature.
- Blood tests. She may also choose to do blood tests to look for any signs of infection.
- Endoscopic exam. Your veterinarian may choose to look at your horse's trachea using an endoscope – horses with IAD often have an excessive amount of mucous in their airways. You don't always see this externally, because horses are quite adept at coughing up this mucous and then swallowing it.
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.
- Less than 5 percent neutrophils
- Less than 2 percent mast cells
- Less than 0.5 percent eosinophils
There are three objectives to treatment for IAD:
- Bronchodilators. Initially, your horse is often suffering from spasms of constricted airways, known as bronchospasm. In order to open up these constricted airways, your horse may be given drugs known as bronchodilators, which are best given in the inhaled form.