Experimental infection of specific-pathogen-free domestic lambs with Mycoplasma ovipneumoniae causes asymptomatic colonization of the upper airways that is resistant to antibiotic treatment.

Mycoplasma ovipneumoniae (M. ovipneumoniae) is a respiratory pathogen associated with mild to moderate respiratory disease in domestic lambs and severe pneumonia outbreaks in wild ruminants such as bighorn sheep. However, whether M. ovipneumoniae by itself causes clinical respiratory disease in domestic sheep in the absence of secondary bacterial pathogens is still unclear. The goal of our study was to better understand the role of M. ovipneumoniae as a respiratory pathogen in domestic sheep and to explore potential antibiotic treatment approaches. Therefore, we inoculated four 4-month-old, specific-pathogen-free lambs with fresh nasal wash fluids from M. ovipneumoniae-infected sheep. The lambs were monitored for M. ovipneumoniae colonization, M. ovipneumoniae-specific antibodies, clinical signs, and cellular and molecular correlates of lung inflammation for eight weeks. All lambs then were treated with gamithromycin and observed for an additional four weeks. M. ovipneumoniae inoculation resulted in stable colonization of the upper respiratory tract in all M. ovipneumoniae-inoculated, but in none of the four mock-infected control lambs. All M. ovipneumoniae-infected lambs developed a robust antibody response to M. ovipneumoniae within 2 weeks. However, we did not observe significant signs of respiratory disease, evidence of lung damage or inflammation in any of the infected lambs. Interestingly, treatment with gamithromycin, which blocked growth of the M. ovipneumoniae in vitro, failed to reduce M. ovipneumoniae colonization. These observations indicate that, in the absence of co-infections, M. ovipneumoniae caused asymptomatic colonization of the upper respiratory tract that was resistant to clearance by the host immune response and by gamithromycin treatment.