Approximately 80% of stabled horses show evidence of airway inflammation, and a recent survey performed by the Tufts Cummings School of Veterinary Medicine shows that at least 25% of horses show overt signs of inflammatory airway disease, such as cough or exercise intolerance.
Approximately 80% of stabled horses show evidence of airway inflammation, and a recent survey performed by the Tufts Cummings School of Veterinary Medicine shows that at least 25% of horses show overt signs of inflammatory airway disease, such as cough or exercise intolerance. Many causes have been promoted for such a high prevalence of airway inflammation and signs of disease, including viral infection, low-grade bacterial bronchitis, and allergic disease.
What most do not consider is the astonishingly high particulate load presented by the cleanest of barns. Particulates alone have been shown to cause airway inflammation both in humans and in laboratory animals; and barn dust, considered to be organic dust contains many components that increase its ability to cause the type of airway inflammation that characterizes IAD in horses.
Particulates refer to any particulates suspended in the air. They can be further subdivided by size, as size determines whether particulates can actually enter the lower airways, or will be largely excluded in the nose. PM10, or particulates smaller than 10 microns in diameter, can be inhaled into the nose and largest airways, and are considered inhalable, but are excluded from the lower airways. PM5, or particulates smaller than 5 microns in diameter, can be inhaled into the lower airways, and are considered respirable. These are the particulates that can, therefore, cause lower airway inflammation.
People working in industries with high levels of respirable organic dust, such as in swine or poultry barns, have been shown to have increased prevalence of lower airway diseases, such as chronic obstructive pulmonary disease and asthma, and their lung function deteriorates more quickly than in people who are not exposed to this sort of organic dust load. Indeed, It takes a very low level of respirable dust to induce airways dysfunction in people – on the order of 0.3 mg of respirable dust per cubic meter.
In contrast, a very well-kept horse barn, with all doors and window open for maximum ventilation, can have respirable dust levels as high as 1.0 mg/m3. When horses are eating hay, respirable dust levels in the horse's breathing zone may be as high as 12 mg/m3. Compounding the effects of stabling, many horses are primarily ridden either in indoor arenas or in dusty rings. Respirable dust levels in indoor arenas have been shown to be as high as 60 mg/m3 – a truly shocking level.
Particulates alone – regardless of their composition – have been shown to cause airway inflammation in both laboratory animals and people. This is due to the ability of particulates to act, essentially, as foreign bodies in the lung. When the horse breaths in particulates, the macrophages arrive promptly to engulf these foreign bodies, and the particulates can be seen on light microscopy inside macrophages.
This triggers a series of events, including increased oxidative stress, increases in inflammatory cytokines, and eventual airway neutrophilia. Eventually, multiple bouts of airway inflammation result in remodeling of the lower airways, with increased goblet cells, increased mucus production, epithelial hyperplasia, and even increased smooth muscle production.
Organic dust, however, contains more than just inert particulates. The organic dust in horse barns contains an extraordinarily high level of respirable endotoxin and mold. Endotoxin is well-recognized as one of the most important components of organic dust in causing airway neutrophilia in people exposed to the so-called ‘dusty trades' such as the cotton industry, and has been shown to cause pulmonary neutrophilia when given to horses by nebulization. Its primary action is to cause an upregulation of inflammatory cytokines such as tumor necrosis alpha and IL-1, which, in turn, results in upregulation of IL-8, a chemokine that results in airway neutrophilia which in turn contributes to eventual remodeling.
Mold cell walls are characterized by their content of beta-D-glucan, which, like endotoxin, is well-recognized as contributing to airway inflammation in people exposed to moldy housing and to other sources of organic dust. One's nose alon can attest to the high levels of ammonia present in many barns which potentiates airway inflammation by presenting a toxic to the airway epithelium. Finally, recent work in woodworker's asthma indicates that pine dust alone is capable of inducing airway inflammation – and pine dust is ubiquitous to the equine environment due to the use of pine shavings and sawdust.
The important thing to remember is that even well-maintained and well-cleaned barns contain these high particulates loads. To compound matters, many owners and trainers have cleaning and maintenance routines that increase particulates levels. The use of blowers to clean barn aisles is one of the most iniquitous practices. Not only does it cause a high level of particulates in the air, but, if a gas-powered blower is used, combustion particulates are also suspended in the air.
Vigorous sweeping of dusty aisles also contributes to the dust levels. Many grooms have a practice of throwing pitchforks of shavings and manure at the wall, rather cleverly relying on the heavy manure to fall closer to the wall than the lighter, clean shavings. However, the down side is that this practice results in a high particulate load in the air. The traditional New England method of storing hay overhead results in a continual sprinkling of hay dust to the horses below. Stalls surrounding an indoor arena participate in the high dust load kicked up by working horses.
What can be done about this? The first principle in decreasing dust is to wet down the offending substance. Aisleways should be sprinkled with water before sweeping. Indoor arenas should be kept dampened – many products exist for promoting decreases in dust from indoor arenas and other dusty areas. Horses should be kept outside while stalls are cleaned, and preferably for at least two hours thereafter, as higher particulate levels can be documented in the barn up to two hours after cleaning.
If hay must be kept overhead, then a tarp should be placed under it to decrease contamination of the air beneath. If possible, stalls should not be in proximity to indoor arenas. If possible, alternate bedding, such as pelleted bedding or paper should be used in preference to sawdust or shavings. Certainly, baked hay products such as Denge will decrease the particulate, mold, and endotoxin load, as will ensiled hay.
Users of ensiled hay must be vigilant to use reliable products, however, as poorly prepared haylage can promote growth of botulism spores, with disastrous consequences. Although it may be difficult to remediate existing barns, every effort should be made to improve ventilation, and a concerted effort should be made to change a mindset that horses need to be kept in a warm environment during the winter. If clients are contemplating building a new barn, a barn architect or engineer should be employed, as proper use of vents and insulation, and a thorough understanding of how to achieve good ventilation can result in far warmer barns with better ventilation.
In summary, even the best of horse barns present a high particulate load to the stabled horse. The high endotoxin and mold levels in organic dust cause the organic particulates to be especially effective in inducing airway inflammation, neutrophilia, and eventual remodeling. It will be impossible to counter the high levels of airway inflammation in stabled horses until we decrease the particulate load that they face on a daily basis.