Co-infection with macrolide-resistant and macrolide-susceptible Bordetella pertussis strains in an infant: a case report.

BACKGROUND

Bordetella pertussis infection causes severe respiratory illness, particularly in young infants. Although macrolides are recommended as the first-line treatment, macrolide-resistant strains have emerged in several countries. In Japan, a nationwide pertussis epidemic occurred in 2025, during which numerous infections were reported not only due to macrolide-susceptible B. pertussis (MSBP) but also due to macrolide-resistant strains (MRBP).

CASE SUMMARY: A 2-month-old female developed a persistent cough and was admitted to the pediatric intensive care unit (PICU), where she received frequent suctioning and short-term high-flow nasal cannula therapy. BioFire FilmArray Respiratory Panel 2.1 analysis of her nasal swab detected B. pertussis DNA; azithromycin was therefore administered for 5 days. Direct sequencing of the DNA extracted from her nasal swabs initially indicated an A2047G mutation in the 23S rRNA gene of B. pertussis, indicating MRBP infection, although the electropherogram was unclear. Trimethoprim-sulfamethoxazole was initiated on day 8 of her PICU stay following confirmation of MRBP infection. Subsequently, bacterial culture yielded two different B. pertussis strains, which were confirmed as MRBP and MSBP through antimicrobial susceptibility testing. Multilocus variable-number tandem-repeat analysis and whole-genome analysis demonstrated that the strains were genetically unrelated. The patient was discharged and subsequently fully recovered after a 14 day course of trimethoprim-sulfamethoxazole.

CONCLUSION

To our knowledge, this is the first confirmed case of co-infection with MRBP and MSBP strains. Such mixed infections may be missed by the diagnostic methods currently used in clinical practice, highlighting the importance of careful interpretation of test results.