The best trailer rides for horses end in something fun or exciting—maybe a ride with friends along the beach, or a show you’ve been working to qualify for all year long. But when stress and travel conditions combine to trigger a respiratory condition known as shipping fever, a trailer ride can take you to a destination you never want to visit—severe and even deadly illness.

Shipping fever can happen anywhere—at home and on the road—because it’s basically just bacterial pneumonia, explains Peter Heidmann, DVM, DACVIM. But it’s most often triggered by long journeys in a trailer. “Generally speaking, horses don’t acquire bacterial pneumonia without one or more risk factors,” he says. “One of the most common risk factors is long-distance travel, hence the term shipping fever.”

In addition to operating Montana Equine, a referral clinic with several locations, Heidmann is head of internal medicine at Palm Beach Equine Clinic in Wellington, Florida, a popular destination for competition horses from all over the country and the world. He sees firsthand the toll travel takes on horses and has a wealth of information and advice for reducing the risk of shipping fever.

The risks on the road

Bacterial pneumonia is an infection of the lungs caused by certain strains of bacteria. “There’s a long list of organisms that can be involved. But the vast majority of the time it is Streptococcus zooepidemicus,” says Heidmann. “S. zooepidemicus is a commensal organism. That means it can be present in the nasal passages off and on throughout a horse’s life. It generally doesn’t cause problems as long as it stays in the upper airway. But if it sets up shop in the lungs, it can cause illness.”

Normally the horse’s immune defenses keep S. zooepidemicus in check and out of the lungs. “We know this pathogen co-evolved with horses. And they are well adapted to it. Unless they get rundown and the immune system isn’t working as it should be,” says Heidmann. And that’s where trailer rides come in.

Horses in a moving trailer aren’t just standing there—far from it. They are constantly working to keep their balance, especially through acceleration, stops and turns. At the same time, they may be squabbling with fellow passengers and fretting about the friends they’ve left behind. In addition, poor ventilation and temperature fluctuations can make for environmental conditions inside the trailer that are stressful for horses.

All of which means that transport puts physical and mental strain on horses, raising their levels of cortisol, a stress hormone. Add in other health conditions and a horse’s immune system may be overwhelmed and unable to stop the bacteria from colonizing the lungs. “Things like uncontrolled Cushing’s or metabolic disease, which suppresses the immune system, can increase the risk for pneumonia,” says Heidmann. “If you add transport on top of that, the risk factors for developing bacterial pneumonia during shipping are substantially higher.”

Individual risk

These risk factors are specific to each individual. That’s why only one horse in a group being transported may develop shipping fever.

“Bacterial pneumonia does not transmit from horse to horse or nose to nose. It’s not a situation where multiple horses within the trailer will get it just because they are in close proximity to one another,” Heidmann explains. “If a horse has influenza or rhinopneumonitis or strangles, those organisms are highly contagious, but bacterial pneumonia generally is not.”

The severity of shipping fever can vary, again depending on factors related to the individual horse. Bacterial pneumonia is a serious illness, for example. But an even worse condition, pleuropneumonia, occurs when fluid and bacteria accumulate in the pleural space, the cavity between the lung and body wall.

Pleuropneumonia is much harder on the horse than pneumonia and more difficult to treat. Fluid in the pleural space can restrict the expansion of the lungs as the horse tries to breathe. Abscesses can form and rupture, leading to sepsis, a potentially deadly infection of the bloodstream.

“The worst cases are the ones that are not caught early and have other bacteria present,” says Heidmann. “There’s a long list of bacteria that can be involved. But the vast majority of the time it is S. zooepidemicus. And most of the time the infection is confined to the lungs— especially if it’s caught early and treated. If the infection progresses it can become life threatening. In cases where severely ill horses end up surviving, pulmonary fibrosis (lung scarring) can limit athletic performance, sometimes necessitating a change in the horse’s job description.”

Spotting the signs

A horse with shipping fever doesn’t walk off the trailer coughing. Instead, the signs of the infection generally take hours or days to develop. But when they do, it is usually obvious that the horse is ill.

Fever

Initially, most (but not all) horses will develop a fever (temperature greater than 101.5 F). Other signs include loss of appetite, low energy levels and depression. As the condition progresses, pleural (chest) pain causes shallow breathing, guarding and flinching when pressure is put on the chest. Overall, the horse may look uncomfortable and seem reluctant to move, cough or lie down. He may also develop bad breath or foul-smelling nasal discharge.

Lung sounds

A veterinarian can diagnose shipping fever based on a physical exam, including listening to the lungs for crackling sounds caused by fluid in the small airways, or wheezes, caused by bronchoconstriction. In addition, ultrasound imaging and radiographs will help determine the nature, location and extent of any fluid accumulation.

Fluid accumulation

If there is fluid in the pleural space, treatments to remove the fluid can help the horse in two ways. First, it will relieve some of the pressure on the lungs, making breathing easier…at least temporarily. More importantly, laboratory analysis of pleural fluid can identify the causal bacteria, so that the most appropriate antibiotic treatment can be prescribed. “If it’s a mild infection like S. zooepidemicus, several doses of appropriate antibiotics can be highly effective,” says Heidmann. “On the flip side, a gram-negative or mixed bacterial infection is not going to respond as well to a basic protocol,” because some of these bacteria have developed resistance to most antibiotics.

“If your horse has risk factors of concern, the single most important test is to get a sample of fluid and grow it in a lab to see which bacteria are there,” says Heidmann. “If you grow it, then you can test it against different antibiotics. That way you will know which medication or combination of medications will be your best choice for treatment.”

Even with the infection clears, shipping fever can leave a horse debilitated. “There are cases in which we clear the entire infection and it is resolved, but there is still so much inflammatory debris remaining in the lungs that they are permanently compromised,” says Heidmann. “In many of those cases we have to do surgery to take out some of the residue from the infection. This is why we em-phasize prevention.”

But, because shipping fever often progresses rapidly, treatment will commence even before tests come back. In most cases, a horse will initially receive a general antibiotic and supportive care, then his treatment regimen will be adjusted when the bacteria present are identified.

Five preventive steps

Research suggests that longer trailer rides—particularly those 100 miles or more—are more closely associated with the development of shipping fever than are short trips. But it’s wise to take steps to prevent the disease during a journey of any length. With that in mind, here are five precautions to consider every time you ship a horse.

1. Make sure the horse can lower his head to knee level during travel.

Multiple studies have confirmed that a high head position is a significant risk factor in the development of shipping fever. A lower head position allows any accumulated mucus or fluid to drain from the respiratory tract instead of ending up in the lungs. Experts recommend tying a horse loosely or not at all. If a horse fights with his neighbors or is otherwise fussy, try different combinations of traveling partners or even a muzzle, before resorting to tying his head high.

If you cannot avoid tying a horse’s head high on a particular trip, stop regularly and untie him so he can drop his head for a few moments and clear his airways, says Heidmann: “This is also good for their mental well-being, and this equals less stress and disease risk.”

2. Protect the air quality in the trailer.

Dust from bedding, hay and dried manure can irritate airways, triggering inflammation. To keep the air clear, clean your trailer before your trip, soak hay that will be fed en route, and dampen shavings (if you use them) before—and possibly again midway through—the trip.

Don’t forget dust that may come in from outside the trailer as well. “Minimizing highway dust is very important,” says Heidmann. Although generally you’ll want to encourage airflow through your trailer, you may need to close windows and vents in specific situations when the outside air quality is poor. “If you are traveling through an area where there are forest fires, for instance, you’ll want to shut all the windows on the trailer while passing through so there might be less smoke and ash coming into the trailer.”

Under some extreme circumstances, Heidmann says, you may need to cancel your trip entirely. “The competitive event you had your heart set on may not be worth risking the future health, or even the life, of your horse. It can be hard to see it that way when most of us want to get there and do the event, but if you will be traveling through regions that might have fires, look at the fire maps and weather predictions.

3. Reduce the stress of transport as much as possible.

This means shipping horses with friendly companions, providing snacks and taking frequent breaks.

“When we travel, we want to be in our comfort zone to the greatest extent possible,” says Heidmann, and horses are the same. “Having horses familiar with one another when traveling helps, like having familiar humans traveling together.” Try to place horse friends in spaces next to each other while traveling, splitting up those who aren’t as friendly with each other.

Heidmann adds that hay can play an important role in stress management on the road. “Most horses are more relaxed and less stressed if they can nibble while traveling,” he says. “It’s also physiologically healthy to keep food in the digestive tract. I like to use a small-mesh hay net that acts as a slow feeder, and the horses can keep nibbling hay. With the slow-feed hay net, they end up eating only about 60 to 70 percent of their normal diet while traveling, so we are not overloading them. This can help keep them eating, but eating slowly, and also minimizes dust from the hay itself.”

Shortening the duration of travel whenever possible is also helpful, says Heidmann. “Transport for a horse is similar to us getting on an airplane,” he says, “and no one likes a long flight.”

4. Ship only healthy, fully vaccinated horses.

A horse whose immune system is already stressed is more likely to develop shipping fever. If you have any questions about your horse’s health, or if he just seems rundown, postpone your travel plans.

Also, inquire about the health of any horses you may be shipping for others. “Health screening makes a big difference,” says Heidmann. “You definitely want to avoid having a snotty-nosed horse get in the trailer with your horse because you don’t know if that horse might have rhino or some other contagious disease.” In addition, make sure that your horse is up-to-date on all his vaccines at least two weeks before traveling.

“This can keep diseases such as influenza from setting the stage for a secondary bacterial infection,” Heidmann says.

5. Monitor the temperatures of horses after shipping.

“One of the simplest ways to catch problems early is to take temperatures,” says Heidmann. “Not every horse with pneumonia will get a fever, but the ones with a fever are the ones you want to diagnose, and if there is illness, begin appropriate treatment right away.” Heidmann recommends taking a horse’s temperature twice a day for at least a week after longer trailer rides, adding that many horses with mild infections won’t have fevers early in the morning, but will show them after 3 p.m.

There’s enough stress associated with trailering horses to also have to worry about illness developing after the fact. That’s why it’s important to take a few precautionary steps before and during a journey to prevent shipping fever. Not only will your horse arrive happier and healthier, but he’s more likely to stay that way.