Aerosol-induced Mycoplasma synoviae arthritis: the synergistic effect of infectious bronchitis virus infection.

The effect of infectious bronchitis virus (IBV) infection (10(4.5) median embryo infective dose per chick) on the induction of Mycoplasma synoviae arthritis was investigated. Mycoplasma-free brown layer pullets, approximately 5 weeks old, were exposed to an aerosol dose of > or =10(2-3) colony-forming units (CFU) of M. synoviae alone or 3 days after inoculation of a field strain of IBV (D1466) by the ocular-nasal route. Chicks injected intravenously with 10(9) CFU M. synoviae served as positive controls. Thirty-one per cent of chicks receiving M. synoviae aerosol alone developed arthritis compared with 50% of those co-infected with IBV. Ninety-five per cent of positive control birds developed arthritis. In a repeat experiment an additional group was exposed to M. synoviae aerosol 3 days after infection with IBV M41 strain. These birds also received two aerosol doses of M. synoviae 14 days apart (> or =10(0-1) CFU/bird initially and > or =10(1-3) after 14 days). Chicks injected intravenously with 10(6) CFU M. synoviae acted as positive controls. Fifty per cent of the positive controls were re-inoculated intravenously (10(8) CFU) after 14 days. Twenty-one per cent of chicks infected with M. synoviae aerosol developed arthritis compared with 33% of those co-infected with IBV D1466, and 55% of those co-infected with IBV M41. After the single intravenous, low-dose M. synoviae inoculation 11% of birds developed joint lesions, whereas 70% of those receiving the second inoculation developed lesions. These results suggest that IBV may enhance M. synoviae arthritis after aerosol exposure and that the incidence of joint pathology depends on the strain of IBV.