Clinical characteristics of pediatric parapertussis: A single-center case-control study

Parapertussis in children is often regarded as a disease similar to pertussis but remains underrecognized in Japan, where it is not a notifiable disease. This study aimed to elucidate its clinical and epidemiological features. A retrospective study was conducted at a pediatric center in Tokyo from October 2020 to November 2023. Patients under 18 years of age who tested positive for Bordetella parapertussis using the FilmArray® Respiratory Panel 2.1 were included. Data on demographics, clinical features, and laboratory results were collected from electronic records. Patients were classified into mono-infection (B. parapertussis only) or co-infection (with other pathogens). Fisher's exact and Mann-Whitney U tests were used for group comparisons, as appropriate. Among 11,384 tests, 45 were positive for B. parapertussis, first detected in August 2021. By 2023, the detection rate per test was 1.0 % (40/3941). The median age was 3.7 years, with 51.1 % male. Of the cases, 82.2 % had co-infections, most commonly with human rhinovirus/enterovirus (59.5 %). Symptoms were mild, with cough in 71.1 % and fever ≥38 °C in 46.6 %, and no patients required mechanical ventilation or intensive care. Azithromycin was administered to 64.4 % of patients. Lymphocyte counts were significantly lower in mono-infected patients than in co-infected patients (median [interquartile range]: 1065/μL [626-1611] vs. 5289/μL [3637-9655], respectively, P = 0.02). This study shows that parapertussis generally follows a mild clinical course but is often found with other pathogens. Given the lack of nationwide surveillance for parapertussis in Japan, more data are needed to assess the disease burden and develop management strategies.