MOLECULAR GENETIC ANALYSIS OF MEASLES VIRUSES CIRCULATING IN BULGARIA DURING THE YEAR 2024

Background: Measles is a well-known fever-rash viral disease and remains a significant public health challenge, as cases have rapidly increased in the past few years. From 1 November 2023 to 31 October 2024, 30 EU/EEA Member States reported 18 044 measles cases, of which 13 863 (76.8%) were laboratory-confirmed. Aim: The study aimed to monitor the circulation of wild-type measles viruses (MeV) in Bulgaria in 2024. Materials and Methods: During the MeV outbreak in the country in 2024, 126 clinical samples (63 nasal swabs and 63 urine) from 63 patients with a possible measles infection were investigated. Viral RNA was extracted using real-time PCR and conventional one step RT-PCR assays, and the nucleoprotein (N) gene from the viral genome was detected. The MeV-positive samples were sequenced by direct Sanger sequencing. Phylogenetic analysis was performed using the software program MEGA v. 11. Results: Acute measles infection was proved in 26 out of 63 tested individuals (41%). The affected were mostly children aged 1-4 (12/26, 46%) and 5-9 years (6/26, 23%). Our studies have established nasal swabs as a more biologically applicable material for rapid PCR diagnostics of MeV, and in all patients confirmed clinically and epidemiologically, the virus was detected in their nasopharynx. The affected regions of the country were Varna (20/26), Sofia city (3/26), and single cases in Sofia district (1/26), Stara Zagora (1/26) and Burgas (1/26). In 2024, phylogenetic analysis of MeV sequences showed a predominant circulation of the D8 and B3 genotypes with imports of the viral strains from the United Kingdom, Germany, Romania, and Austria. Conclusion: Despite the wide availability of effective vaccines, measles outbreaks continue to occur due to imported cases and transmission of the virus among unvaccinated children in communities.