The virulence of two recent Dutch Mycoplasma synoviae field isolates in different broiler types and the association with ascites and hydropericardium as a new finding.

In the present study, the virulence of two Mycoplasma synoviae (MS) field isolates was evaluated in specific pathogen-free (SPF) and modern-day commercial broilers. Both MS isolates belonged to Multilocus Sequence Typing (MLST) clonal complex (STCplx) 56, the most prevalent STCplx isolated from the respiratory tract of Dutch commercial poultry in the last two decades. Additionally, in this study, a well-defined histologic air sac lesion (ASL) score was combined with an in situ hybridization (ISH) test to further study the cellular reaction in the air sacs and tracheal tissue. Birds were challenged intratracheally with MS at 5 days of age. The synergistic effect of a mild infectious bronchitis virus (IBV) H120 vaccine given 5 days prior to MS challenge was also evaluated in the commercial broilers. The clinical impact of the MS strains was comparable to earlier studies with older isolates, as were the synergistic effects of IBV. The present study demonstrated a difference in effect on growth, macroscopic air sac lesions (mASL) and histologic (h)ASL between the two broiler types. A relevant new finding with economic impact for the poultry industry was that ascites was observed related to both isolates; however, the mechanisms underlying MS-associated ascites require further investigation. The hASL, combined with the MS ISH assay, revealed differences in cellular reaction in the air sacs compared to the trachea, explaining the lower MS detection rate in the air sacs in the chronic phase of MS infection. This also emphasizes the risk of misdiagnosing MS-related airsacculitis in broilers.