Objectives: This study was carried out to define the types of influenza viruses circulating among humans and to understand the seasonality of influenza virus activity. Such information is essential for deciding on influenza vaccination strategy and on the appropriate time for delivering influenza vaccination, if such a vaccination policy was decided to be a priority.
Method: During the period July 2003 - August 2004, 300 nasopharyngeal aspirates (NPA) were obtained from a systematic sample of patients reported to Out-patient Department, Colombo North Teaching Hospital, Ragama with < or =4 days history of acute respiratory tract infection (ARTI). The clinical signs and symptoms of the patients were prospectively recorded. Isolation of the influenza virus was carried out by inoculating in Madin Darby Canine Kidney cell line (MDCK). The isolates were identified by immunofluorescence assay and characterised by haemagglutination inhibition test. RT-PCR was carried out on all NPA samples. Genetic sequencing and phylogenetic analysis of the haemagglutinin gene of representative viruses were carried out.
Results: Twenty three influenza A and nine influenza B viruses were isolated by cell culture methods. Influenza A H3N2 Panama/2000/99-like viruses were isolated in 8% of patients with ARTI and influenza B/Sichuan/ 379/99-like viruses were isolated in 3%. Twenty eight influenza A virus infections were identified by the RT-PCR method. Phylogenetic analysis was carried out with data from other H3-subtype viruses isolated worldwide. The Sri Lanka viruses are antigenically and genetically similar to those in the northern and southern hemispheres.
Conclusions: Influenza viruses circulate at different times of the year and is the aetiological agent causing 11% of all ARTI. Influenza activity corresponded to a peak in rainfall; however the correlation of influenza virus activity with rainfall is not invariable. The Sri Lankan isolates of 2003-4 were genetically related to the influenza A viruses circulating around the globe.