Cryptosporidium baileyi and Mycoplasma synoviae co-infection enhances M. synoviae colonization and aggravates tissue damage in chickens

Introduction Mycoplasma synoviae ( M. synoviae ) is an economically significant pathogen that causes respiratory infections, synovitis, and arthritis in chickens, inflicting substantial economic losses on the global poultry industry. Its frequent co-infection with other respiratory pathogens, often exacerbates the resultant pathogenic damage. As an important respiratory pathogen, it remains unclear whether Cryptosporidium baileyi ( C. baileyi ) can cause co-infection with M. synoviae in commercial large-scale poultry farms and what synergistic pathogenic pattern exists between them. Methods A total of 1,118 choanal cleft swab samples were collected from commercial chicken farms across eight regions in Guangdong province for the detection of M. synoviae and C. baileyi . The extracted DNA was analyzed by qPCR for M. synoviae and nested PCR for C. baileyi , respectively. Furthermore, a total of 90 one-day-old chicks confirmed free of C. baileyi and M. synoviae were randomly divided into six groups to establish a co-infection model and investigate the synergistic pathogenic effect pattern of the two pathogens. Results The overall positive rates of M. synoviae and C. baileyi were 41.32% and 17.80%, respectively, with significant regional, city-level, and age-related variations. Samples from Western Guangdong and chickens over 45 days old showed the highest infection risks for both pathogens. The co-infection rate was 10.55%, and a significant positive association was observed between the two pathogens (OR = 2.44, p 0.001). Co-infection risk increased markedly with age, especially in chickens older than 45 days. Chicken co-infection model was established to explore synergistic pathogenesis between C. baileyi and M. synoviae . Co-infection did not alter the prepatent period of C. baileyi , but significantly increased oocyst shedding peak and prolonged excretion time. Meanwhile, C. baileyi markedly elevated M. synoviae loads in the choanal cleft at multiple time points. Gross and histopathological examinations showed that co-infection aggravated laryngotracheal lesions caused by C. baileyi , and exacerbated footpad, joint and air sac lesions induced by M. synoviae. Overall, C. baileyi and M. synoviae exert mutual promoting effects on proliferation and pathogenicity in chickens. Discussion The findings in the present study confirm the high prevalence of M. synoviae - C. baileyi co-infection in commercial poultry flocks and demonstrate that co-infection synergistically enhances the pathogenicity of both pathogens. These results fill an important knowledge gap in co-infection research, provide novel insights into the interaction mechanisms of multiple pathogens in poultry, and offer key scientific support for addressing complex disease challenges in modern poultry production.