Pathogenicity of Mycoplasma imitans in mixed infection with infectious bronchitis virus in chickens

Mycoplasma imitans (Mim) has been isolated from ducks, geese and partridges, and is closely related to Mycoplasma gallisepticum (Mg). The pathogenicity of Mim for chicks was investigated in single and mixed infections with infectious bronchitis virus (IBV) by giving IBV strain M41 at 1-day-old and Mim 2 days later. Single infections with IBV or Mim were also performed. No clinical signs or gross lesions were seen in chicks infected with Mim or uninfected control chicks, but they were seen in the other two groups. Clinical scores were consistently higher in birds with mixed infections than those infected with IBV alone, and were significantly higher (P < 0.05) between days 7 and 14. More birds developed sinusitis, tracheitis and airsacculitis (with greater severity) in the mixed than the single IBV infections. Mim was recovered more frequently and in greater numbers from the respiratory tract of birds with mixed than single infections. It was recovered from the lower trachea, air sacs and lungs only in mixed infections. Seroconversion to Mim occurred by day 14 in mixed infections, but not until day 21 in single infections. It appears that Mim can act synergistically with IBV in young chickens in a similar manner to Mg, although Mg may act as a primary pathogen under some circumstances.