Imagine the scenario: You've been training for the big competition all year. Your horse has never looked better, he's stayed healthy, and he's performing to the best of his abilities. You load him in the trailer for the cross-country commute in hopes of bringing home the blue ribbon and set off on your journey. When you unload your horse at your destination, however, something is seriously wrong. The show veterinarian tells you that your horse is suffering from a pulmonary disorder called shipping fever, and just like that, your competition season is over.

A primary risk factor for the development of broncho or pleuropneumonia (pulmonary disorders commonly known as shipping fever) in horses is long-distance transport. At the 2011 America College of Veterinary Internal Medicine Forum, held June 15-18 in Denver, Colo., Rose Nolen-Walston, DVM, Dipl. ACVIM, assistant professor in the department of clinical sciences at the University of Pennsylvania's College of Veterinary Medicine, described the risk factors associated with transport-associated respiratory disease and what preventive methods owners can take.

Nolen-Walston explained that specific travel-associated factors amplify the horse's disease risk including a continuously raised head position, which causes profound and adverse changes in the respiratory tract's ability to effectively clear debris and microorganisms. Trailer design and standard restraint methods (ties) prevent the horse from fully lowering his head.

Nolen-Walston discussed one study in which researchers evaluated the effects of keeping a horse's head raised for prolonged periods of time--similar to their position when being transported. Within six to 12 hours, microbes increased exponentially in tracheal fluid and abnormal lung sounds (heard through a stethoscope) developed. Additionally, the researchers discovered that allowing the horses to lower their heads for 30 minutes every six hours was not effective in eliminating debris and microorganisms from the airways: "At least 8-12 hours is necessary to clear airway contamination," she added.

To compound this problem, some horses with pre-existing airway disease (such as influenza, heaves, or inflammatory airway disease) are at much greater risk for developing pneumonia. In these cases, the mucociliary clearance mechanism (which removes debris from the lungs) is already compromised before the horse even loads on the trailer. Simply, the horse will have an even harder time clearing debris from his lungs during transportation.

Stress has a negative impact on a horse's immune system, and hauling is considered one of the most common stressors for horses. Nolen-Walston noted that researchers have learned that the immune functions of horses transported loose in the trailer stall were significantly improved compared to those tied in the stalls.

Nolen-Walston also emphasized that ambient (environmental) trailer conditions along with air "quality" can amplify disease risk. Airborne dust, ammonia, and pathogens all increase substantially in trailers, she noted, particularly when feeding from a hay bag (which might increase the amount of inhaled particles in a horse's breathing zone) and when manure and urine accumulate in the trailer.

"In most trailers, the air exchange rate (the rate at which outdoor air replaces indoor air) is less than half the acceptable rate for a normal horse environment," she said. Agricultural engineers often recommend four to eight air exchanges per hour; however, some equine researchers and barn builders recommend 10 air exchanges per hour.

Nolen-Walston said researchers have found that horses allowed regular rest periods along with frequent trailer cleaning (i.e., removing manure and urine-soaked bedding) had no mucopurulent (containing pus and mucus) material in their airways upon arriving at their destination.

"Respirable particulates and volatile gases are rampant in horse trailers," noted Nolen-Walston. "These features, along with the inability to lower the head, markedly increase a horse's risk of developing shipping fever pneumonia."

Although the risk of shipping fever can't be definitively eliminated, she recommended horse owners implement practices to improve the shipping environment and lessen disease risk. Such preventive measures include:

  • Improve air quality by maximizing ventilation;
  • Soak hay thoroughly if feeding en route to reduce the amount of dust present in the feed and thus inhaled by horses;
  • Take longer, more frequent rest stops to reduce stress;
  • Allow the horse to be loose in the box if and when possible; and
  • Clean the trailer regularly along the travel route to reduce the amount of mucopurulent material the horse could inhale.

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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