Mycoplasma gallinaceum is not among the most pathogenic mycoplasmas affecting poultry, but its continuous reisolation
from flocks in South Africa displaying typical signs of mycoplasmosis prompted us to revisit its role in respiratory disease.
Specific-pathogen-free white leghorn chickens were co-challenged with either M. gallinaceum (MGC) and QX-like infectious
bronchitis virus (IBV), or the more virulent Mycoplasm gallisepticum (MG) and IBV. No clinical signs were observed apart from
sneezing in chickens challenged with IBV, MGCþIBV, and MGþIBV. On postmortem examination, one bird each in the MGC
þ IBV and IBV groups developed peritonitis or airsacculitis, respectively. In the tracheas, the MGþ IBV group showed the most
severe ciliary damage with a mean ciliostatic score of 32.40 compared to scores of 26.83 and 20.4 for the MGC þ IBV and IBV
groups, respectively. Corresponding tracheal lesions were recorded. Quantitation of the challenge pathogens by quantitative realtime
PCR and real-time reverse transcriptase–PCR determined that MGC was shed in much higher titers from the trachea than
MG, when co-infected with IBV. Interestingly, the presence of both MG and MGC appeared to enhance IBV replication in the
tracheas of infected chickens, whereas the presence of IBV suppressed MG and MGC proliferation in the trachea. In general, the
nonpathogenicity of M. gallinaceum in chickens was confirmed, but it was able to aggravate respiratory disease and pathogen
proliferation with virulent QX-like IBV.