Sentinel surveillance of influenza-like illness in two hospitals in Maracay, Venezuela: 2006-2010

Abstract

Background: Limited information exists on the epidemiology of acute febrile respiratory illnesses in tropical South American countries such as Venezuela. The objective of the present study was to examine the epidemiology of influenza-like illness (ILI) in two hospitals in Maracay, Venezuela.

Methodology/principal findings: We performed a prospective surveillance study of persons with ILI who presented for care at two hospitals in Maracay, Venezuela, from October 2006 to December 2010. A respiratory specimen and clinical information were obtained from each participant. Viral isolation and identification with immunofluorescent antibodies and molecular methods were employed to detect respiratory viruses such as adenovirus, influenza A and B, parainfluenza, and respiratory sincytial virus, among others. There were 916 participants in the study (median age: 17 years; range: 1 month--86 years). Viruses were identified in 143 (15.6%) subjects, and one participant was found to have a co-infection with more than one virus. Influenza viruses, including pandemic H1N1 2009, were the most frequently detected pathogens, accounting for 67.4% (97/144) of the viruses detected. Adenovirus (15/144), parainfluenza virus (13/144), and respiratory syncytial virus (11/144) were also important causes of ILI in this study. Pandemic H1N1 2009 virus became the most commonly isolated influenza virus during its initial appearance in 2009. Two waves of the pandemic were observed: the first which peaked in August 2009 and the second--higher than the preceding - that peaked in October 2009. In 2010, influenza A/H3N2 re-emerged as the most predominant respiratory virus detected.

Conclusions/significance: Influenza viruses were the most commonly detected viral organisms among patients with acute febrile respiratory illnesses presenting at two hospitals in Maracay, Venezuela. Pandemic H1N1 2009 influenza virus did not completely replace other circulating influenza viruses during its initial appearance in 2009. Seasonal influenza A/H3N2 was the most common influenza virus in the post-pandemic phase.

Figure 1. Phylogenetic trees of influenza viruses circulating in Maracay.

This figure shows the phylogenetic relationship of the HA gene segment within influenza A/H1N1 (A), influenza A/H3N2 (B) and influenza B (C) viruses. Phylogenetic trees were constructed by the neighbor-joining method and bootstrap analysis to determine the best-fitting tree for the gene. For the comparison, we have included strains reported from GenBank. Only bootstrap values over 90% are shown.

Figure 2. Influenza-like illness detected by passive surveillance in Maracay, Venezuela: October 2006–December 2010.

Figure 3. Monthly distribution of acute febrile respiratory viral infections by different viruses detected through a passive surveillance in two health centers of Maracay.

Figure 4. Map of study sites in Venezuela.