The microflora of the lower respiratory tract of the horse: an autopsy study

A series of 31 thoroughbreds and 18 non-thoroughbreds was examined post mortem to assess what bacteria might be present in the lower respiratory tract, and to assess their significance by correlating the bacteriological findings with histological changes in the lung. The distal trachea and seven representative lung sites were swabbed in each case. Sixty-six per cent of the sites were sterile with remaining sites producing scanty growths of mainly coagulase negative Staphylococcus spp., E. coli, Bacillus spp. and Pseudomonas spp. Anaerobes, mainly Clostridium perfringens, Bacteroides fragilis and Bacteroides oralis, were only recovered on enrichment media and were isolated more frequently from the right lung than the left. More bacterial species were isolated from thoroughbreds than non-thoroughbreds. Significant inflammation was absent in the majority of lungs examined and this correlated well with the large percentage of sterile sites and low number of potential pathogens. The results indicate that the lower respiratory tract of horses is normally sterile or may contain small numbers of bacteria of low pathogenicity. Chlamydia and respiratory viruses were not isolated from any horse.