The temporal sequence of influenza H1N1 and Mycoplasma pneumoniae co-infection causes disease severity in Syrian hamster models

Introduction Influenza H1N1 virus is one of the most prevalent subtypes among influenza viruses, and co-infection with Mycoplasma pneumoniae (Mp) is frequently documented in clinical respiratory infections. However, the pathological mechanisms underlying the temporal sequence of H1N1-Mp co-infection remain poorly characterized, and relevant animal models are lacking. Methods In this study, we established a model of influenza H1N1 and Mycoplasma pneumoniae co-infection in Syrian hamsters and infected two pathogens in interval of 72 hours. Clinical manifestations, body temperature, body weight, pathogen loads in nasal, pharyngeal, and anal swabs, as well as blood cytokine profiles were dynamically monitored over 14 days post-infection (dpi). Additionally, tissue pathogen loads, histopathological changes, routine blood parameters, and blood biochemistry indicators were evaluated at 7 and 14 dpi. Results The results demonstrated that hamsters first infected with H1N1 followed by Mp (F-M group) exhibited significantly more severe histopathological lesions (assessed by HE staining), higher pathogen loads, and dysregulated cytokine responses compared to other infection groups. Conclusion Our findings highlight the critical role of infection order in determining the severity of H1N1-Mp co-infection, providing novel insights into the temporal dynamics and pathogenic mechanisms of respiratory co-infections.