Both IAD and heaves are diseases caused by poor air quality. The more severe disease, heaves, has a clear allergic component. Although horses with mild-to-moderate heaves can appear normal on physical examination when they are kept in a dust-free environment, clinical heaves, with obvious breathing effort, cough, and nasal discharge, can easily be induced in these individuals by exposing them to moldy hay. Horses with IAD, on the other hand, often have no signs of allergies at all – rather, IAD in these horses is similar to what is called occupational asthma in people. Although we commonly think of asthma as an allergic disease in people, in reality, many people who work or live in dusty environments – such as pig, cattle, or chicken farms, cotton factories, wood-working shops or landscaping, or are agricultural workers, can develop asthma without any underlying allergic disorder. For these workers, the particulates, endotoxin, and beta glucan levels are so high that the airways naturally produce a profound inflammatory response. Unfortunately for horses, their occupations often include living in environments that are neither good for horse lungs nor for human lungs.
Even the best of barns is laden with endotoxin (bits and pieces of dead, gram-negative bacteria that still trigger profound inflammation in the lungs), beta-glucans from molds, mites and other insects, and ammonia gas from urine. Viable (still living) bacteria can be found in the breathable air in many indoor arenas. If you were to feed your horse the best quality hay that money could buy, it would still be full of mold spores that do not cause infection, but that are small enough to be inhaled into the tiniest airways in the lung where they trigger airway inflammation. In some barns, diesel engines are found in the tractors that pull manure or hay wagons – diesel fuel is particularly problematic because it the particulates that it contains cause oxidative damage to the lungs. Air pollution does the same thing – and unfortunately, because of the way that the winds blow, even apparently pristine areas of New England can be affected by pollution that originates, for example, in the Midwest.
If you have ever wondered what this is doing to your own lungs, you would be wise to be a little worried – and you should clean up your barn for the good of your horse and you. A study from our laboratory showed that people who spend 10 or more hours in a horses barn have a markedly increased (up to 10 times) risk of developing respiratory symptoms compatible with asthma. (Mazan 2009) The high particulate, endotoxin, beta-glucan, and ammonia level that hurts horse lungs also triggers inflammation in our lungs.
Recent work from our laboratory (Houtsma and Mazan 2015) as well as evidence from other researchers suggests that viral disease, similar to the situation in children, can trigger or worsen disease. It may be that certain horses have an innate susceptibility to IAD – in this case, anything that sets off inflammation, such as viral respiratory disease, can put in motion a vicious spiral that results in IAD or heaves.