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The link between long journeys and chest infections has been recognised in horses for years.

Severe pleuropneumonia — sometimes called shipping fever or transit fever — can develop after travelling.

This condition often proves difficult to treat and in the worst cases can be fatal. Even if a horse recovers with intensive treatment, he may never again achieve previous levels of performance.

Because racehorses and sport horses tend to be transported over longer distances than pleasure horses and hunters, shipping fever occurs more often in elite athletes. Any horse or pony may succumb, however, when boxed on a long journey.

We have known for some time that one of the factors that increases the risk is tying a horse up tightly during transit so that he cannot lower his head to clear his airways. The stress of travelling is also thought to reduce resistance to infection, but what else contributes to the development of illness after arrival?

Identifying risks

Vets in Australia have published a major study looking at a whole range of factors that may affect a horse’s susceptibility to shipping fever. They kept 12 healthy adult thoroughbreds and standardbreds in individual stables, where their behaviour was recorded by video camera. Two days before the experimental journey, a tracheal wash was taken from the lower airway of each horse, at the entrance to his chest, using an endoscope.

The horses were examined on the day of the journey and blood samples were taken. They were then loaded into a commercial horsebox, in two batches of six, for an eight-hour trip.

The horses travelled in individual sideways-facing stalls, each tied with an elastic lead rope that allowed them to turn and lower their head to the level of their knees. The driver had a 25-minute rest break but the horses were not unloaded, nor were they fed or watered during the trip.

The air was sampled in the stables and transporter before the journey, and throughout the journey in the transporter when the horses were travelling. The air was tested for bacteria, gases, humidity, temperature and wind speed. The behaviour of the horses throughout the trip was recorded by individual cameras.

Once the horses were unloaded at the end of the journey, a further tracheal wash and blood samples were taken. Re-examination and blood sampling were repeated over the next five days.

Stress factors

Analysis of the data revealed some interesting findings. The increase in certain behaviours while the horses were travelling — especially during the first five hours and compared to their normal behaviour in the stable — was an indicator of stress. There was more sniffing, chewing and licking when the vehicle was moving and more yawning and pawing when it was stationary.

Movements to keep balance were pronounced and became more obvious in the last hour of the journey, when the horses were tired from travelling. There was much less lowering of the head during the journey than in the stables, even though the elastic lead ropes did not prevent this.

Six of the horses displayed increased respiratory noises when unloaded, but none developed a raised temperature during the next five days. Although heart rates and cortisol levels were increased by travelling, they returned to normal after 12 hours. This showed that lengthy journeys represent an acute, short-term stress to horses — even those accustomed to travelling.

Blood samples showed that white blood cells increased but components called globulins and fibrinogen decreased, an indicator of what is called an acute phase inflammatory response. In other words, the horses were reacting to an inflammatory challenge induced by the journey. The blood enzyme creatine kinase, which reveals muscle stress, was elevated for several days after.

When the tracheal washes were examined, it turned out that the six horses who displayed louder lung sounds after the journey had more mucus and bacteria in their lower airways before travelling — although the examining vets did not know this at the time. Crucially, the horses showing greater stress behaviour and less frequent lowering of the head during the journey had the most mucus and bacteria in their tracheal washes afterwards.

Traditionally, it has been thought that the source of the bacteria found in horses’ lungs after travelling was due to poor ventilation and a build-up of ammonia and other gases and dust in the air. This research indicates that bacteria found in the lungs of travelling horses originate in the mouth and throat, rather than the air.

While considered “normal” in the mouth and throat, these bacteria are contaminants and will cause inflammation and subsequent infection if they make their way to the lungs.

The comprehensive but complex results show that travelling can be stressful for all horses, and that those showing greater evidence of stress during the journey are more likely to be at risk of shipping fever. The study adds to a growing knowledge of the effects of long-distance travel — and highlights the need for meticulous care of horses both during and following transit.

Planning a long journey? Follow this six-point plan:

• Monitor your horse’s health prior to travelling. If his rectal temperature is raised or he shows other signs of being off-colour, don’t go.
• Optimise air quality by opening vehicle vents and windows during transit, but avoid causing a ferocious draught that dries out his eyes. Allow him room to stretch and lower his neck while travelling.
• Let him graze or feed from the floor before and after the journey — and during to promote airway clearance.
• Break the trip every two to four hours. Some horses will not eat, drink or pee while on board, but only unload them if it is safe to do so. Appreciate that a horse will require a rest after a long journey — do not expect him to perform immediately.
• Horses generate heat while travelling, especially those in the middle of a herringbone format, so avoid over-rugging and check humidity levels.
• Record his temperature for a further five days after the journey and seek vet advice if he shows signs of ill health.

Ref Horse & Hound; 17 May 2018