A bovine model of a respiratory Parachlamydia acanthamoebae infection

This study aimed to evaluate the pathogenicity of Parachlamydia (P.) acanthamoebae as a potential agent of lower respiratory tract disease in a bovine model of induced lung infection. Intrabronchial inoculation with P. acanthamoebae was performed in healthy calves aged 2 to 3 months using two challenge doses: 108 and 1010 bacteria per animal. Controls received 108 heat inactivated bacteria. While challenge with 108 viable Parachlamydia resulted in a mild degree of general indisposition, 1010 bacteria induced a more severe respiratory illness becoming apparent 1 – 2 days post inoculation (dpi), affecting 9/9 (100%) animals and lasting for 6 days. Extent of macroscopic pulmonary lesions was as high as 6.6 [6.0]% (median [range]) of lung tissue at 2 – 4 dpi and correlated with parachlamydial genomic copy numbers detected by PCR, and with bacterial load estimated by immunohistochemistry in lung tissue. Clinical outcome, acute phase reactants, pathological findings and bacterial load exhibited an initial dose-dependent effect on severity. Animals fully recovered from clinical signs of respiratory disease within 5 days. The bovine lung was shown to be moderately susceptible for P. acanthamoebae exhibiting a transient pneumonic inflammation after intrabronchial challenge. Further studies are warranted to precise the pathophysiologic pathways of host-pathogen interaction.