Exercise intolerance can be a frustrating condition to diagnose as the symptoms may not always be consistent and the manifestations can vary as much as the cause(s). For example, a horse may be unwilling to work or work to his/her full potential due to discomfort. One would think that a lameness could be identified, but not always. Perhaps the issue is saddle fit, the way the horse is ridden or the rider. These can each be a challenge to identify as a cause. What about subtle respiratory conditions? Maybe the horse doesn't cough or have an elevated respiratory rate, yet this can't be immediately ruled out. Perhaps ther is a nutritional component, where the horse is trying to work with an inadequate amount of calories to generate enough energy for the work, or possibly the problem could be related to EPSM - a syndrome where horses have an inability to properly metaboilize sugars in the feed which results in muscle problems.
In this article I will address the respiratory component, the other issues can be explored through the following articles: See article on Neuromuscular Diseases.
In this photograph there is a "heave line" that runs diagonally across the horse's chest and abdomen. The indented area represents a place where muscles are being used to push air out of the lungs due to constricted airways. While this is quite apparent in this phtograph it can be noticed especially in horses with longer haircoats.
A horse flaring their nostrils and exhibiting exercise intolerance may be more easily identified as one that is having an issue related to their respiratory system. Of course not every horse will flare their nostrils this much, but when they do or when they have a slower than normal recovery with respect to their post-exercies respiratory rate then an issue with their airways should be suspected.
Endoscopy has been an invaluable tool for many years and when a respiratory issue is suspected this may be one of the first procedures performed. Visualizing the pharyngeal area may provide information on structures that could partially block the upper airway. Typically a problem in this area will involve some type of noise that indicates air flow is affected.
Once the upper airway has been ruled in or out considering involvement of the lower airway is the next step. There are generally two main categories for this region: infection or inflammation. For the purposes of this discussion I will focus on inflammation and the role it plays with allergic airway disease. As the name implies "allergic airway disease" refers to a reaction from an allergic factor. What exactly causes the reaction can vary from certain feeds, weeds in the hay, a particular grass and/or molds. While it would be ideal to identify and eliminate the allergens from the environment, the reality is that their identification is almost impossible. There are tests that help with the identification, but usually more emphasis is placed on the treatment and management of horses with these sensitivities.
Attempts to de-sensitize the horse to the offending agent(s) has mixed success. There is a lab that will analyze the blood to determine what environmental factors: feed, dust, plants/trees and molds, that are involved then provide an anti-serum which is used over time as an injection series to lower the horse's response to the identified factors. For some I have found this to be beneficial, but for others the treatments have not been as successful.
Tufts University Veterinary School has a very complete respiratory work-up program, including the evaluation of the respiratory tract while the horse is being exercised on a treadmill. In addition to these tests a cardiac work-up may be important for some horses that also have murmurs and/or arrhythmias.
Treatments can be the use of environmental controls (better ventilation, decreased dust, wetting the hay), systemic medications (steroids, anti-histamines) and inhaled medications. While environmental management is always importand and the use of systemic medications will often be necessary, the best traetment for longer term management involves the use of inhaled medications as is often done with people.