Entomologic and virologic investigation of Chikungunya, Singapore

Abstract

Local transmission of chikungunya, a debilitating mosquito-borne viral disease, was first reported in Singapore in January 2008. After 3 months of absence, locally acquired Chikungunya cases resurfaced in May 2008, causing an outbreak that resulted in a total of 231 cases by September 2008. The circulating viruses were related to East, Central, and South African genotypes that emerged in the Indian Ocean region in 2005. The first local outbreak was due to a wild-type virus (alanine at codon 226 of the envelope 1 gene) and occurred in an area where Aedes aegypti mosquitoes were the primary vector. Strains isolated during subsequent outbreaks showed alanine to valine substitution (A226V) and largely spread in areas predominated by Ae. albopictus mosquitoes. These findings led to a revision of the current vector control strategy in Singapore. This report highlights the use of entomologic and virologic data to assist in the control of chikungunya in disease-endemic areas.

Figure 1

Geographic and temporal distribution of 123 indigenous chikungunya cases in Singapore. Shading indicates the 7 cluster areas where entomologic investigation was carried out. Data include cases reported through September 2008. The arrows in the timeline shown below the map indicate the months of occurrence of the local outbreaks from the beginning of January to the end of September 2008. BSP, Bah Soon Pah Road; FR, Farrer Road; KW, Kranji Way; LI, Little India; ME, Mandai Estate; QS, Queen Street; SK, Sungei Kadut; TE, Teachers’ Estate.

Figure 2

Phylogenetic analysis of the chikungunya virus (CHIKV) envelope 1 (E1) gene. The maximum-likelihood method was used to construct the phylogenetic tree by using 1,002 nucleotides of the sequence of the E1 gene from codons 91 to 424. The tree included 17 isolates detected in Singapore (shaded), 5 Sri Lankan isolates sequenced at the Environmental Health Institute, and 17 global sequences selected to represent all known phylogenetic lineages. In the tree, all sequences are labeled with GenBank accession numbers and country of origin, and are isolated by year/month. In addition, all locally acquired and imported Singapore isolates are labeled with the reported area and country of origin, respectively, within parentheses. Only the bootstrap values >70 are shown on branches. Scale bar indicates nucleotide substitutions per site. ECSA, East, Central and South African genotype; SG, Singapore.