Bordetella pertussis is a common pathogen in infants hospitalized for acute lower respiratory tract infection during the winter season.

Introduction: There is some evidence that Bordetella pertussis (B. pertussis) can co-infect with viral respiratory infections in young infants.

Methods: B. pertussis infection was studied by culture, polymerase chain reaction (PCR), and loop-mediated isothermal amplification (LAMP) from nasopharyngeal swabs (NPSs) in 49 infants < 12 months of age, who were admitted for lower respiratory tract infections during the winter season. Seven other possible viral pathogens were documented by antigen detection or PCR in NPSs. The clinical feature of infants with mixed infection of B. pertussis and respiratory viruses were examined.

Results: Overall, B. pertussis infection was found in 10 (20.4%) cases, nine were less than 6 months of age and seven were unvaccinated. Viral etiology was found in 41 (84%) cases and pertussis-viral co-infection was present in eight patients, five of whom had mixed infection with respiratory syncytial virus. Only the presence of staccato coughing, cyanosis, and lymphocytosis were significantly different in B. pertussis-positive cases compared with B. pertussis-negative cases. Of the 10 pertussis cases, only the culture-positive cases showed the typical symptoms and laboratory findings of pertussis in addition to virus-associated respiratory symptoms with severe hospital course, whereas cases identified as DNA-positive lacked the characteristics of pertussis and their clinical severities were the same as B. pertussis-negative cases.

Conclusion: In the absence of typical paroxysmal cough and lymphocytosis, we should carefully consider diagnosis of pertussis in young children hospitalized for presumed viral respiratory illness according to local epidemiological surveillance.