Molecular characterization of respiratory infection with Mycoplasma bovis in feedlot cattle

Mycoplasma bovis is a pathogen of the bovine respiratory disease complex (BRD) and other well-recognized clinical syndromes in cattle throughout the world. Utilization of newly developed DNA-based molecular techniques has markedly increased the understanding of the epidemiology and pathogenesis of mycoplasmal infections in humans and domestic animals. This study investigated temporal changes in the prevalence of M. bovis infection, and whether M. bovis concentration and 'genotype may influence the disease outcome of naturally occurring cases of respiratory disease in feedlot cattle. The study population was 130 heifers from a single feedlot in Ontario, Canada. Bronchoalveolar lavage fluid (BALF) and serum were collected upon arrival from 60 heifers. Subsequently, heifers were sampled at the onset of acute respiratory disease (up to two weeks) and 60 days post-arrival. In parallel, heifers not showing signs of respiratory disease (clinically healthy controls) were sampled at both times. Overall, 186 BALF and serum samples were collected. The prevalence of infection with M bovis based on culture increased from 1.7% on arrival to 72% and 86% at 15 and 60 days post-arrival, respectively. Similarly, serological evidence of infection increased from 33% on arrival to 74% and 100% at 15 and 60 days post-arrival, respectively. Using real-time PCR, M. bovis concentrations were determined in 92 BALF samples obtained on arrival, and at 15 and 60 days post-arrival, and ranged from 1.87x10E3 to 1.03x10E8 cfu/ml. There were no significant differences in concentration between respiratory disease cases and clinically healthy controls. Using amplified-fragment length polymorphism (AFLP) and based on 95% similarity, 13 genotypes were obtained from 62 M. bovis field isolates. The AFLP genotypes gathered into two clusters (A and B) with 73% similarity between clusters, Cluster A comprised 74% of the strains with four distinct genotypes. Cluster B comprised 26% of the strains with nine distinct genotypes. Different AFLP genotypes were present in samples obtained from individual heifers at different sampling times. There was no association between AFLP genotype and disease status. These results suggest that M. bovis concentration in BALF and AFLP genotypes do not correlate with disease outcomes in naturally occurring cases of respiratory disease in feedlot cattle.