Advances in the diagnosis of equine lung disease: sampling from the lower airways

Comparison of tracheal aspirates before and after high-speed treadmill exercise in racehorses

OBJECTIVE

To determine whether percentages of neutrophils in tracheal aspirate (TA) samples collected from racehorses are increased after exercise and whether interpretation of results from TA samples taken before and after exercise agree.

DESIGN

Case series of 40 young Thoroughbred and Standardbred racehorses in race training presented for evaluation of poor performance.

PROCEDURE

TA samples were collected endoscopically from racehorses presented for poor performance 24 h before and 1 to 2 h after high speed treadmill exercise testing. Aliquots of the retrieved fluid were cytocentrifuged and smears were stained with Diff-Quik. Mean neutrophil counts were expressed as percentages of the total number of inflammatory cells counted and subsequently were categorised as either above or below an accepted cut-off of 20%. Comparisons between percentages of neutrophils before and after exercise were made.

RESULTS

Percentage of neutrophils from TA samples obtained from racehorses after exercise was significantly higher than neutrophil percentages from TA samples collected from the same horse before exercise. In horses with TA specimens that were categorised as having < or = 20% neutrophils before treadmill exercise, the percentage of neutrophils in their TA specimens after exercise was, on average, significantly higher and was greater than the cut-off value of 20%.

CONCLUSION

Recent strenuous exercise may change the proportion of neutrophils in lower airways of racehorses and practitioners should be aware of this when collecting and interpreting the results from TA samples. The most practical time for collection of a TA sample to obtain the most diagnostically useful information might be after a suitable washout period of at least 1 to 2 h post-exercise.

Equine pleuropneumonia

Pleuropneumonia is a clinically important equine disease, predisposed by a number of identifiable factors. Successful management is largely dependent on early identification and prompt initiation of appropriate treatment strategies. Rapid resolution of the disease process is associated with appropriate treatment commenced within 48 h of the causative insult. Lower airway contamination by oropharyngeal organisms and subsequent extension into the pulmonary parenchyma results in respiratory dysfunction and systemic toxaemia. Acute disease is associated with the isolation of facultatively anaerobic organisms, especially beta-haemolytic Streptococcus spp. and Pasteurellaceae. Delayed or inappropriate treatment is likely to result in chronic disease characterized by the involvement of anaerobic bacteria and a poor response to therapy. The primary mode of treatment for anaerobic infection of the human thorax is surgical drainage and resection of necrotic tissue but whilst such techniques have been described for the management of equine pleuropneumonia, the size of the equine thoracic cavity hinders accurate diagnostic evaluation and successful completion of such intervention. The chronic nature and cost of ongoing treatment and limitations on choice of antimicrobial agents warrant a poor prognosis for survival and a poorer prognosis for return to athletic endeavour.

Inflammation and increased numbers of bacteria in the lower respiratory tract of horses within 6 to 12 hours of confinement with the head elevated

Confinement of horses with their heads elevated for periods up to 24 hours was used to evaluate the extent and the effects of bacterial contamination of the equine lower respiratory tract. Significant (P < 0.05) increases in bacterial numbers (up to 10(9) colony forming units/mL in transtracheal aspirate derived samples) occurred within 6 or 12 hours in most horses. Pasteurella/Actinobacillus spp and Streptococcus spp were most commonly isolated. Lowering of the head for 30 minutes every 6 hours to facilitate postural drainage did not prevent multiplication of organisms to levels equivalent to those achieved by horses where the head was elevated for 24 hours. When horses were released from confinement and heads were no longer maintained in an elevated position, clearance of accumulated secretions and bacteria occurred within 8 to 12 hours. Thus, confinement with the head elevated resulted in significant bacterial contamination and multiplication within the lower respiratory tract during a period often encountered in routine management procedures, such as transportation. The clearance of accumulated secretions occurred over a prolonged period after release from such confinement.