Detection of Mycoplasma gallisepticum, Isolation, and Determination of Tylosin Susceptibility of Isolates from Commercial Chickens

Mycoplasma gallisepticum (MG) is a contagious avian pathogen that causes financial losses to the poultry industry. Isolation of the pathogen is difficult and time-consuming, and therefore, far from a routine method. Serological testing methods to detect antibodies resistant to MG are widely used in routine diagnosis. Tylosin is a class of macrolide antibiotics tremendously administered in veterinary medicine for the treatment of mycoplasmosis and prophylaxis. This study aimed to detect MG by immunoassay testing, culture, and polymerase chain reaction (PCR) in commercial poultry farms and to investigate the tylosin susceptibility of the isolates. To verify the presence of antibodies resistant to MG, 750 blood samples were randomly collected from 38 broiler farms from 2019 to 2022 in Mazandaran and Golestan provinces, Iran, and rapid slide agglutination (RSA) assay was performed. Positive results were analyzed by the enzyme-linked immunosorbent assay (ELISA) for further investigation. Here, 920 swab samples were collected from 38 non-vaccinated commercial farms for culture, and PCR tests were performed for the isolated strains. The activities of tylosin were tested in vitro against these isolates using the broth microdilution method. The lowest antibiotic concentration that resulted in a color change was considered the minimum inhibitory concentration (MIC) value. Twenty-four (63.1%) farms were positive in the RSA test, and 21 (55.2%) farms were positive in the ELISA test. Nine (23.68%) of the farms grew on culture media, and 8 (21.05%) were detected as Gallisepticum species by PCR. The geometric mean of MIC for tylosin was 5.75 µg/ml, MIC50 was 4 µg/ml, and MIC90 was 8 µg/ml. The results indicated that commercial farms were infected with MG. Considering the ability of MG to spread and the probable use of the RSA test as a rapid and cheap method, it can be argued that ELISA and RSA serological tests can be used to find MG in poultry flocks, and the positive result should be confirmed by standard microbiological tests or PCR. It was also found that the isolated parts of MG changed their sensitivity to tylosin, indicating the need for routine testing to optimize treatment dose and efficiency.