Mycoplasma synoviae (MS) infections in poultry are an important epidemiological and economic problem in poultry production all over the world. The differences between M. synoviae strains are related to the pathogenicity and the course of the disease. In recent years, the pathogenicity of M. synoviae strains has increased, and some of them are capable of causing serious infections. Both horizontal and vertical transmission routes play an important role in MS infection in flocks. The aim of the study was to determine the impact of infection with selected MS strains obtained from chickens showing a clinical form of MS infection on SPF chicken embryos. Ten strains of M. synoviae were used for this purpose. The strains were isolated from the respiratory tract and the oviduct of chickens with symptoms typical of infection with this pathogen. Genetic material isolated from liquid cultures of these strains was confirmed by molecular (PCR and LAMP) and microbiological methods. The selected M. synoviae strains belonged to six different genotypes. Significant differences in virulence between the strains were demonstrated. In nine infected groups of embryos, M. synoviae strains caused weight loss, and in seven groups they produced anatomopathological changes characteristic of mycoplasma infections. The most pathogenic for SPF chicken embryos turned out to be strains characterized as genotype F isolated from the chicken oviduct and strains of genotype C isolated from the respiratory tract. One strain of genotype H isolated from the respiratory tract showed no pathogenic effect on SPF chicken embryos. The study showed that infections with M. synoviae can have a significant impact on the production of chicken chicks in commercial hatcheries and the economy of the poultry industry.
