Isolation and Identification of Influenza A Virus Strains Circulating in Malaysia, 2002–2006

Abstract

Background: The national influenza surveillance programme in Malaysia plays an important role in preparing for, and responding to epidemics and pandemics. This study aims to evaluate the annual incidence and to identify the circulating viral subtypes of Influenza type A virus in this country for a period of five-year, 2002–2006. Methods: Specimens were received from participating hospitals and clinics designated at various sentinel sites throughout the country from patients presenting with Influenza-like Illness (ILI). A total number of 5463 samples were received from 1 January 2002 to 31 December 2006 of which 277 were reported positive for Influenza A virus. Most of the specimens received were throat swabs and/or nasal swabs. Viral isolation was conducted using Madin Darby Canine Kidney cell lines (MDCK) and identified by indirect immunofluorescence method using commercially available type-specific (A or B) monoclonal antibodies. Type and subtype identification was performed by haemagglutination inhibition assay (HAI). Positive isolates were sent to the WHO Collaborating Centres in Melbourne, Australia for further analysis. Results: Our study shows that one or more Influenza A virus subtypes and strains circulated throughout the year, with distinctly higher peaks towards the second half of the year. The predominant circulating strain in 2002 was A/Moscow/10/99(H3N2)-like which represented 100% (9/9) of the total positive samples. In 2003, A/Fujian/411/2002(H3N2)-like (58%) was found to be the predominant circulating strain, A/Wellington/1/2004(H3N2)-like (32%) for 2004, A/California/7/2004(H3N2)-like (53%) for 2005 and A/New Caledonian/20/99(H1N1)-like (47%) for 2006. Conclusion: The Influenza A virus strains circulating in Malaysia were antigenically similar to the strains isolated world wide during the same period and to strains recommended by the WHO for inclusion in the vaccine formulations for use in the Southern Hemisphere winter season. This study emphasizes the importance of local influenza surveillance programme as early warning for upcoming epidemics and potential pandemics.