Mp has been recognized as a significant and common cause of pediatric CAP. In this study, it was found that 2.65% of hospitalized children with CAP experienced Mp positive, which is less than 19.1% reported in Wuhan, China . Such low incidence may be attributed to the different study populations and methods used for Mp testing. Previous studies [16,17,18] have indicated that Mp is detected year-round, and its seasonal peaks have been reported in various seasonal periods starting from the end of summer to winter. In this study, Mp positive was more common in autumn, which is consistent with the findings of Chiu et al. .
At present, it is widely considered that mixed detections with multiple pathogens are common in children with CAP. Furthermore, Mp infection is often associated with preceding or concomitant viral and bacterial infections in children [19, 20]. In the present study, we found that children with co-detection accounted for 38.18% of total cases with Mp positive. This proportion is lower than that in Taiwan population  and higher than that in Beijing population . However, the incidence of mixed detections with viruses was 27.27% in this study, which is higher compared to both Taiwan and Beijing populations. These differences may be due to the exclusion of HRV test in their studies. HRV is the most prevalent respiratory virus and is often associated with the common cold. Moreover, HRV may be associated with more severe lower respiratory tract infections in children, including bronchiolitis and pneumonia . The results of this study showed that HRV was the most common pathogen among children with Mp co-detection, at a frequency of up to 18.18%. Thus, researchers and pediatricians should pay more attention to Mp-HRV co-detection among children with CAP. Age is an important factor that can affect pathogen distribution. The incidence of Mp infection is highest among children aged 3–7 years, while respiratory viral infections are more common in children younger than 2 years [22, 23]. Likewise, in this study, the patients with Mp-virus co-detections were significantly younger, especially prevalent among those ≤3 years old.
Mp-infected children can present with fever, cough, chest pain and wheeze, along with non-respiratory symptoms such as arthralgia and headache . In the present study, we found that, similar to Mp mono-detection, fever, cough and sore throat were the three main symptoms of Mp co-detection, followed by non-respiratory symptoms, including rash, headache and abdominal pain. In addition, there were some differences in the clinical symptoms between patients with co-detection and mono-detection. First, compared to Mp mono-detection children, the duration of fever was significantly longer in children with Mp-virus and Mp-virus (excluding HRV) co-detection. Second, lack of appetite was more prevalent in children with Mp-virus co-detection. Third, runny nose was more common in patients with Mp-virus co-detection and Mp-HRV co-detection. However, the clinical symptoms are relatively non-specific, indicating that these parameters may not be helpful to distinguish Mp co-detection from Mp mono-detection.
A recent study shows that the clinical outcomes of Mp infection are heavily dependent on the co-infected pathogen . Pientong et al.  have reported that Mp may serve as an important co-infectious agent of respiratory viruses, which increases the severity of acute childhood bronchiolitis. However, no significant differences were noted in the incidence of severe CAP, duration of hospitalization, and noninvasive mechanical ventilation among various etiological groups in the present study, suggesting that mixed detection of Mp with other viral or bacterial pathogen does not contribute to the severity of CAP. Chiu et al.  demonstrate that no significant difference is observed between the patients infected with Mp and those co-infected with virus, as similar to that reported in our study. To avoid overestimation of Mp-HRV co-detection, we further investigated the differences in clinical outcomes between children with Mp-virus (excluding HRV) co-detection and Mp mono-detection. Similarly, there was no significant difference between the two groups. Then, we further explored the role of Mp-HRV co-detection in children with CAP. Notably, a similar distribution of most complications, underlying conditions and disease severity parameters were found between the two groups. These results indicate that Mp-HRV co-detection may have little influence on the clinical outcomes of CAP. Likewise, we found no significant difference in the clinical outcomes between Mp-bacteria co-detection and Mp mono-detection, except that Mp-bacteria co-detection was more likely to be associated with abnormal blood gases. However, our results might differ from with the findings [19, 20] of different regions of China, suggesting that S. pneumoniae co-infection can lead to greater disease severity in children with Mp infection compared to single infection.
With the use of molecular diagnostics, co-detection with other viral/bacterial pathogens has been commonly identified in Mp positive patients. Nonetheless, this study confirms that the clinical features and severity of Mp mono-detected patients are relatively similar to those co-detected with viral and/or bacterial pathogens. Thus, we speculate that a large proportion of CAP patients may be infected with a major pathogen of pneumonia (i.e. Mp) and tend to have a colonization of other pathogens in respiratory tract . Furthermore, it is also possible that the viral or bacterial pneumonia patients may have a serologic evidence of past MP infection (IgM positive) or PCR positive (colonization).
Nevertheless, this study has several limitations. The presence of co-infection should be confirmed by serologic tests at least 2 times during hospitalization or convalescent stage. However, it is very difficult to apply at clinical fields, owing to lack of available methods. In this study, due to the limits of current respiratory bacteria detection methods, the pattern of identified pathogens might not accurately represent the CAP, especially in young children. Moreover, the small sample size could impose restrictions on determining the association between Mp co-detection and disease severity. In addition, this single center study might not be representative of the entire Chinese pediatric population.