There's a lot floating around in the air in a horse's environment, and it probably isn't surprising that it can have a negative effect on his health. At the 2008 American Association of Equine Practitioners Convention, held Dec. 6-10 in San Diego, Calif., Melissa Millerick-May, MS, PhD, assistant professor in the Division of Occupational and Environmental Medicine at Michigan State University, discussed multiple risk factors for airborne particulates in stabling and their association with respiratory problems.


The relationship between environmental particulate matter and airway inflammation in Thoroughbred racehorses

She said the syndrome of inflammatory airway disease (IAD) includes the accumulation of mucus and inflammatory cells in the airways in the absence of clinical signs of disease. In addition to the risks viruses and bacteria pose to the horse, environmental exposure to particulates dispersed from feed, bedding, footing materials, and other sources (such as diesel exhaust) all can lead to IAD. Even small increases in aerodynamic (airborne) small-diameter particles can worsen airway inflammation.

Sizes of particles of concern reported in this study are smaller than can be seen with the naked eye. The largest are comparable to a single grain of pollen, while the smallest particles are smaller than a red blood cell. Degree of exposure to these particles varies with stable and stall location, season, and the degree of activity around the area.

Millerick-May's research was done at Thistledown racetrack in Ohio, with 717 measurements taken in 81 stalls. One stable was of new construction that optimized ventilation. The other two stables were older solid brick buildings with large sliding doors at each end and small, high windows along the back of stalls. The stalls were constructed of solid wood fronts with high partitions.

During the early morning hours of busy activity and stall cleaning, large particles were suspended within the breathing zone and stayed suspended in the air for some time. The researchers found that July, with its wet, rainy weather, had the lowest concentration of particles. The heat and dryness of mid-September were associated with the largest concentration of airborne particles, while cold, snowy November weather also created high particle concentrations because the stables were closed up at night with no cross-ventilation to disperse the air.

While high activity levels in the barn in early morning aerosolized increased levels of particles, the researchers found that in the late afternoon and evening smaller particles were still present--they take longer to settle out due to their size and weight. They also noted reduced evening particle concentrations in the less-enclosed stable with good ventilation, but stalls in that barn closer to the road or track showed elevated concentrations. Similarly, increased particle concentrations were noted in stalls in proximity to the manure-handling building or to an air-moving fan or an area with increased foot traffic (human and horse).

On endoscopic exam and tracheal wash analysis, 67% of horses had visible tracheal mucus, with horses housed in enclosed stables having the greatest prevalence. There was no significant correlation between particle concentration and number or percent of inflammatory cells.

Recommendations to help improve airways and to lower risk of IAD include:

  • Improve ventilation, but also consider placement of air fans;
  • Use quality low-dust hay and bedding;
  • Use water as a dust suppressant during dry weather;
  • Minimize human activity, such as raking and sweeping, while horses are inside, and eliminate the use of leaf blowers within the stable.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her recent book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care (available at or by calling 800/582-5604). She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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