The term shipping fever is a general catch-all to describe respiratory disease in horses that tend to occur after they have been transported. The same kinds of disease can occur in other situations as well, but the stress of travel and the conditions horses experience while traveling can often lead to problems.
Shipping fever is a bacterial pneumonia, says Dr. Peter Heidmann, DVM, MPH, DACVIM, of Montana Equine Medical and Surgical Center and Palm Beach Equine Clinic in Florida. Pneumonia affects the lungs. Pleural pneumonia affects the chest cavity and the lungs. The severity of bacterial pneumonia can vary greatly, depending on which bacteria become established in the lungs or in the pleural space, despite the surveillance by the immune system. “The most common one is Streptococcus. Streptococcus equi causes strangles but S. zooepidemicus is present in most horses at some point in their lives, growing inside the respiratory tract,” Dr. Heidmann says. “This is considered a normal organism in horses, but if it sets up shop in the lungs, that will cause pathology.” It generally doesn’t cause any problems as long as it stays in the upper airway inside the nostrils.
The horse’s normal immune defenses keep those bacteria from doing any harm in that location. “What we know about this pathogen is that it co-evolved with horses and they are well adapted to it—unless they get rundown and the immune system isn’t as strong as it should be,” says Dr. Heidmann.
“The worst cases are the ones that are not caught early and have other bacteria present. There’s a long list and most of the time it is confined to the lungs—especially if it’s caught early and treated,” Dr. Heidmann says.
