Do Circulating RSV-Genotypes Affect Established Biennial Epidemic Periodicity in Zagreb Region?

Objective: The biennial epidemic pattern of respiratory syncytial virus (RSV) circulation in Croatia has been preserved and could not be related to climatic factors and the predominant RSV subtypes. The possibility that the circulation of different RSV genotypes affect the outbreak cycle in children in Croatia (Zagreb region) over a period of 3 consecutive years was explored in the paper. Methods: The study group consisted of inpatients, aged 0 - 10 years, who were hospitalized with acute respiratory tract infections caused by RSV, in Zagreb, over the period from 1 January 2006 to 31 December 2008. The virus was identified in the nasopharyngeal secretion using direct immunofluorescence method. The virus subtype and genotype was determined by real-time PCR and sequence analysis, respectively. Results: RSV infections identified in 731 children. RSV subtype A caused 399 infections, and subtype B 332. Two subtype A genotypes (NA1 and GA5) and three subtype B genotypes (BA7, BA9 and BA10) were found. During persistent RSV biennial cycles namely four succeeding outbreaks, the new genotype from the previous smaller outbreak persevered into the upcoming larger outbreak. Conclusion: Our molecular-epidemiology study of RSV subtypes and genotypes during calendar months demonstrates that the biennial RSV cycle cannot be fully explained by the dynamic of the predominant circulating genotype of RSV. Other unknown factors account for the biennial cycle of RSV epidemics in Croatia.