RESURGENCE OF PERTUSSIS IN NORTHEASTERN BULGARIA DURING THE 2024-2025 EPIDEMIC: A MOLECULAR STUDY ON INCIDENCE AND MACROLIDE SUSCEPTIBILITY

Background
Despite being vaccine-preventable, pertussis is often detected, resulting in outbreaks, which is the case in many European countries, including Bulgaria, in 2024.

Material and methods
Nasopharyngeal swabs from symptomatic individuals from 6 regions (R1-R6) were tested by PCR. The DNA was extracted using the SaMag DNA Extraction Kit. Quantitative PCR was used to detect the macrolide resistance determinant and was further confirmed with melt curve analysis.

Results
149 samples were tested at the Clinical Microbiology laboratory of the University Hospital “St. Marina”, Varna, Bulgaria, between May 2024 and May 2025. Of them, 23 (15.4%) were positive for Bordetella DNA, and in 22, Bordetella pertussis infection was confirmed. Most of the positive cases were from R1 and were detected in the warmer months (n=18; 78.2%). Positive cases were prevalent in females (n=15, 65.2%), and the median age of positive individuals was 9 years (IQR: 1-11). The PCR and following melt curve analysis did not detect any macrolide-resistant cases.

Conclusions
This study demonstrates a rise in cases of pertussis (15.4%) in Northeastern Bulgaria, which is consistent with the national and European epidemiological reports. The most common mutation, associated with macrolide resistance in B. pertussis was not detected. Our results also confirm the seasonality of the disease and identified individuals between 9-10 years of age as the main risk group for contracting and developing this symptomatic disease.