17 July 2026
Recent field isolates of the widespread pathogen Mycoplasma synoviae (MS), belonging to the dominant European clonal complex, have been shown to severely impair broiler performance and are directly associated with the development of ascites and hydropericardium.
For decades, Mycoplasma synoviae has been recognized as a major avian pathogen, although its clinical and economic impact on broiler chickens has remained a matter of debate. A new study has now examined the pathogenicity and economic consequences of contemporary European field strains in modern commercial broiler lines.
In a controlled experimental study conducted at Royal GD in Deventer, the Netherlands, researchers evaluated the virulence of two Dutch MS field isolates. Both strains belong to Multilocus Sequence Typing (MLST) Clonal Complex 56 (STCplx 56), the predominant genotype recovered from the respiratory tract of Dutch commercial poultry over the past two decades.
Pathogenesis was evaluated in two bird models: specific pathogen-free (SPF) broiler-type chickens and commercial Ross 308 broilers. Birds were challenged intratracheally at 5 days of age. In the commercial broiler groups, the researchers also investigated the potential synergistic effects of administering a live, mild Infectious Bronchitis Virus (IBV H120) vaccine five days before the Mycoplasma synoviae challenge.
The study demonstrated:
- Severe growth depression
- Ascites and hydropericardium: The most significant and economically important finding was the development of clinical ascites accompanied by hydropericardium. Up to one-third of birds in the MS-infected groups developed these cardiopulmonary lesions, regardless of sex. In contrast, no cases of ascites were observed in any of the uninfected control groups.
- Severe airsacculitis
These findings provide compelling evidence that contemporary circulating MS field strains are far more than a minor upper respiratory pathogen. Their ability to induce ascites, hydropericardium, severe airsacculitis, and marked growth depression is likely to translate into higher condemnation rates at slaughter and substantial production losses.
Feberwee et al. (2026). 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. Avian Pathology
