Greater transport stress in horses linked to higher risk of developing lung disease

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Horses that show more outward signs of stress during a long journey are more likely to develop transport-associated respiratory problems than their calmer counterparts, researchers have found.

“Behavioural observation en route may identify animals at increased risk of transport associated respiratory disease,” Barbara Padalino and colleagues reported in the peer-reviewed journal PLOS ONE.

Respiratory disease is a common consequence of transporting horses.

The risk of it developing may be influenced by pre-existing inflammation of the airways, the length of the trip, the position the horse carries its head, the air quality inside the truck, the temperature, dehydration and transport-associated effects on immunity.

Transport-associated pneumonia in horses is not linked to a specific pathogen. Rather, a mixture of different bacterial species have been linked to transport-related lung infections, the researchers noted.

The study was designed to determine the effects of an eight-hour truck journey on various behavioural, clinical, environmental and respiratory parameters in horses, and to identify possible associations between them.

The researchers hypothesised that behavioural responses could be related to the mental and physiological stress of transportation.

Twelve healthy horses, half Standardbreds and half Thoroughbreds, were recruited for the study. They comprised 7 geldings and 5 mares, with an age range of 3 to 8.

The horses each underwent a clinical examination, respiratory endoscopy and collection of blood before and after their journey, as well as tracheal washes in which sterile saline solution is flushed through a portion of the lungs and collected for analysis, in this case for bacteriological evaluation and genetic microbiome investigation.

Behaviour was assessed in stables before transportation and throughout the journey, in which the horses travelled without food or water but within the thermo-neutral zone of between 5°C and 25°C.

Transportation caused mild, but significant, effects on fluid and electrolyte balance, and blood analysis pointed to an acute inflammatory response.

The proportion of neutrophils – white blood cells – in the tracheal wash was higher after the journey, with higher levels of Pasteurellaceae, a bacterial family normally found in the upper airways of mammals.

“Horse behaviour en route predicted clinical and respiratory outcomes,” the study team reported.

“The frequency of stress-related behaviours was greatest in the first hour of the journey, and balance-related behaviours were most common in the final hour of the journey.

“Horses which lowered their heads less frequently en route and showed more stress-related behaviours had higher physiological stress (serum cortisol and heart rate on arrival), increased tracheal mucus and inflammation scores, and higher tracheal bacterial concentration after the journey.”

Six horses with abnormal lung sounds heard through a stethoscope after the journey were found to have had higher tracheal inflammation scores before being loaded for the journey, as well as an overall higher concentration of bacteria in their tracheal wash. An increased percentage of neutrophils in tracheal washes was still evident five days after the journey.

“These horses may have been at increased risk for airway bacterial proliferation and contamination due to sub-clinical airway inflammation,” the researchers said.

“These observations emphasise the importance of thorough veterinary examination before and after transportation.”

While transport-related health problems involved many factors, the researchers suggested that a clinical examination, including listening for abnormal lung sounds, and an endoscopic inspection of the lower respiratory tract before and after a journey, as well as behavioural observation en route, could identify animals at increased risk of transport-associated respiratory disease.

Discussing their findings, the researchers said heart rate and serum cortisol concentrations were higher at unloading than at preloading, suggesting that transportation was a mental and physical stressor for horses.

“As both stress-indicators were restored to normal values within 12 hours after the journey, the transport event in the current study should be considered an acute stressor with relatively rapid recovery.”

The highest frequency of stress-related behaviours was registered during the first hour of transport, suggesting this to be the most stressful part of a journey.

“Based on our study and the findings of others, it seems that horses adapt to the journey and to the vehicle after approximately 5 hours.

“However, our behavioural responses might also indicate fatigue, evidenced by increased movements relating to balance after the fifth hour of transport, the observed peak in balance-related behaviour at the 8th hour.

“As social animals, anxiety and arousal can be socially transmitted to herd mates, a factor that should be taken into account when mixing horses with different temperament and travel experiences.”

The increased bacterial load in the tracheal wash of most of the horses immediately after transportation was associated with a preferential multiplication of Pasteurellaceae bacteria.

“These findings support the role of Pasteurellaceae as an early, opportunistic invader when pulmonary clearance mechanisms are compromised.

“There was no evidence of concurrent increases in other bacteria previously associated with pleuropneumonia in horses in this study, such as Enterobacteriaceae, Streptococcus spp or strict anaerobes, suggesting that these organisms may become part of the disease process at a later stage or under different circumstances.”

Examination of tracheal samples showed a return to near normal (preload) concentrations of respiratory bacteria 24 hours after the journey, along with a return to a more diverse microflora community.

“However, tracheal inflammation and mucus, tracheal wash colour and turbidity scores in our horses did not recover in the day after the journey, as had been demonstrated in previous studies, a discrepancy that might be related to differences in management post journey.”

The transported horses in the study were kept in stables after the journey, whereas other reported studies involved the horses being kept at pasture.

“Overall,” they said, “this multidisciplinary study showed that the level of stress experienced by the animal en route is manifested with increased frequency of stress-related behaviours and reduced duration with their head in the downward position.”

The study team suggested monitoring horse behaviour during the journey, a 24-hour rest period after a trip, as well as a clinical examination before and after the journey, would likely reduce the incidence of transport-related respiratory disease, as well as identify the animals at greatest risk.

Monitoring horse behaviour during the journey was the most important recommendation, Padalino told Horsetalk.

“Based on our findings, if you monitor horse behaviour you should be able to identify the horse at risk of pneumonia. It will be the horse which was showing more stress-related behaviour, spending the journey with an elevated head position.”

The study team comprised Padalino, Peter Knight, Pietro Celi, Leo Jeffcott and Gary Muscatello, all affiliated with the University of Sydney; and Sharanne Raidal, with Charles Sturt University.

Padalino is currently at the College of Veterinary Medicine and Life Sciences, City University of Hong Kong.

Padalino B, Raidal SL, Knight P, Celi P, Jeffcott L, Muscatello G (2018) Behaviour during transportation predicts stress response and lower airway contamination in horses. PLoS ONE 13(3): e0194272. https://doi.org/10.1371/journal.pone.0194272

The study, published under a Creative Commons License,  can be read here

For queries on the paper, contact Barbara Padolino at barbara.padalino@cityu.edu.hk.

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