Sentinel surveillance for influenza in Senegal, 1996-2009

Data on influenza in tropical and resource-limited countries are scarce. In this study we present results from 14 years of influenza surveillance in Senegal, one of the few tropical countries in Africa from which longitudinal data are available.From 1996 to 2009, we collected respiratory specimens from outpatients presenting with influenza-like illness at 13 facilities in order to investigate the epidemiology of seasonal influenza and the characteristics of the circulating influenza viruses. Specimens were tested for influenza using viral isolation and/or reverse-transcription polymerase chain reaction (RT-PCR).From 1996 to 2009, specimens were obtained from 9176 patients; 1233 (13%) were influenza-positive by virus isolation and/or RT-PCR. Among positive samples, 958 (77%) were influenza A, 268 (22%) influenza B, and 7 (1%) influenza type C; of influenza A viruses, 619 (65%) were A(H3) and 339 (35%) A(H1), of which 13 (1%) were identified as H1N2. The proportion positive was similar for children15 years, young adults 16-35 years, and adults 36-55 years (15%), but lower for persons55 years (9%). Although influenza A(H1), A(H3), and B all circulated during most years, influenza A(H3N2) predominated during 9 of the 14 years. Influenza activity consistently peaked during the rainy season (July-September). Phylogenetic analysis showed that viruses circulating in Senegal were similar to contemporary viruses circulating elsewhere in the world.Our data confirm that influenza is prevalent in Senegal, occurs in seasonal epidemics, and contributes to the burden of respiratory diseases in all age groups.