An observational study of the temporal dynamics of natural Mycoplasma (Mesomycoplasma) ovipneumoniae infection in a range flock of domestic sheep (Ovis aries)

Abstract

Objective. To characterize the dynamics of Mesomycoplasma ovipneumoniae colonization and serum antibody responses in a rangeland flock of domestic sheep (Ovis aries). Methods. A cohort of 40 female lambs was repeatedly tested for M ovipneumoniae from birth to 25 months of age. Nasal swabs were analyzed for M ovipneumoniae infection by PCR, and a competitive ELISA was utilized to determine antibody levels. Body weights and body condition scores also were determined. Results. M ovipneumoniae infection was rapidly acquired, with 80% of the study animals testing PCR positive at 2 months of age and 96.2% testing PCR positive at 4 months of age. The PCR positivity rate then fluctuated between 35% at 10 months and 94% at 25 months. Antibody levels expressed as percentage of inhibition were 57.9% in the dams and 52.7% in their offspring within 24 hours after birth, fell to 18.7% in the lambs at 2 months, and then remained variable, ranging from 36% to 64.2%. No significant association between M ovipneumoniae colonization and antibody levels or daily weight gains was found. Conclusions. In a flock with high M ovipneumoniae infection rates, infections were acquired early in life and then fluctuated over time with no clear trends. Importantly, the serum antibody responses that we measured did not appear to enable sustained pathogen clearance. Clinical Relevance. Our study indicates that, once infected, sheep do not develop effective sterilizing immunity to M ovipneumoniae, and serum antibodies cannot be considered a reliable correlate of protection in individual sheep.