Effects of road transport on indices of stress in horses

Abstract

Stress associated with road transport is believed to be a significant contributor to the pathogenesis of post transport respiratory disease in horses. To determine the effects of road transport on pulmonary function, pulmonary aerosol clearance rates were measured in 4 horses 24 h before, and immediately after, 24 h of road transport by delivering aerosolised 99mtechnetium-labelled diethylenetriaminepentacetate (99mTc-DTPA) to the lungs and monitoring its washout. Each horse was transported twice, once while the trailer was equipped with a leaf-spring suspension and bias-ply tyres (trailer's original equipment, smooth ride) and once while the trailer was equipped with a torsion-bar suspension and normal pressure radial tyres (rough ride) in order to generate different ride characteristics. Before transport, blood was drawn from each horse for haematology and measurement of serum cortisol concentration; 24 h rates of hay and water intake and faecal output were recorded for each horse. Horses were then transported, 2 at a time, over a 128 km circular route of predominantly rural freeways at a constant speed of 72 km/h for 24 h. Horses were rested by stopping the trailer every 3.75 h for 0.25 h. During transport, heart rates (continuous 1 min averages), rates of hay and water intake and rates of faecal output were measured. Ammonia (NH3) and carbon monoxide (CO) concentrations were measured within the trailer and temperatures (wet bulb [WB], dry bulb [DB] and black globe [BG]) within the trailer were recorded each minute. Immediately after each experiment blood was drawn for haematology and measurement of pulmonary aerosol clearance rates were measured. For control studies, horses were housed in their stalls while heart rates were measured for 24 h. Slopes calculated from the 99mTc-DTPA clearance curves for pretransport horses were not significantly different from post transport clearance slopes. Pretransport mean 99mTc-DTPA clearance half-lives (T50, left lung mean +/- s.d. 41.7 +/- 15.8 min, right lung 44.6 +/- 19.1 min) were not significantly different from post transport T50 (left lung 53.5 +/- 14.0 min, right lung 52.0 +/- 11.6 min). Heart rates during transport were not affected by suspension type or trip order (the horse's first or second transport experiment) and were not significantly different from stall controls after the first 120 min of the experiment. Horses had increased red blood cell count, packed cell volume, haemoglobin, plasma protein and cortisol concentrations, and decreased body weights immediately post transport, indicating slight dehydration. Water and hay intake rates were significantly lower during transport than pretransport. Temperatures within the trailer were highest in the midafternoon and lowest in the early morning hours, but all temperatures measured in the trailer were within the comfort zone for large homeotherms. Ammonia and CO concentrations in the trailer during the transport period were within acceptable limits for human exposure. However, respirable articulates in the atmosphere were elevated above safe concentrations for human exposure.