Entomological investigation and control of a chikungunya cluster in Singapore

Abstract

In August 2008, a team from the National Environmental Agency conducted an entomological investigation of a chikungunya cluster in Singapore, with the primary aim of identifying the vector responsible for the outbreak and to assess the vector control operation. A total of 173 adult mosquitoes were caught using both the sweep-net method and the BG Sentinel Traps in and around the affected workers' quarters. Of these, 120 (69.4%) were Aedes albopictus and the rest were Culex quinquefasciatus. More than 2700 Ae. albopictus larvae were also collected from 33 breeding habitats detected. No Aedes aegypti was found. During the preintervention period, 6 (8.4%) out of 71 adult female Ae. albopictus were found positive for the chikungunya virus (CHIKV). Vector control measures resulted in a 90% reduction of adult Ae. albopictus caught by BG Sentinel Traps. Postintervention surveillance revealed the presence of CHIKV-positive mosquitoes. These findings led to continued intensive vector control operation in the affected area that further reduced vector population and interrupted the transmission of the disease. The E1 gene sequence of the CHIKV was identical to those of CHIKV isolated from human chikungunya cases working in the affected area, and contained the A226V mutation. The incrimination of Ae. albopictus as a major vector involved in the transmission of A226V CHIKV had led to the revision of chikungunya control strategy in Singapore. This study suggests the benefit of a vector control program that includes the evaluation of control measures in conjunction to virological surveillance in vector population.

FIG. 1.

Number of chikungunya cases, number of Aedes albopictus caught with BG Sentinel Traps, and vector control activities at the study site (July–September 2008). In response to a notification of a cluster on day 0, adult mosquito surveillance using sweep net and search and destroy of breeding habitats were concurrently conducted in the afternoon. By 18:00 h of day 0, BG Sentinel Traps were set up and indoor ULV misting was carried out. On day 1, mass outdoor thermal fogging was conducted after removal of the traps. Finding of continued presence of infected mosquitoes in the morning of day 2 triggered more vector control activities on days 3, 4, and 5. Meanwhile, search and destroy of breeding sites continued daily for a month (not shown).

FIG. 2.

Phylogenetic tree based on the E1 envelope glycoprotein genes of chikungunya produced by the neighbor-joining method. Sequences highlighted in bold are obtained from Ae. albopictus and those underlined are from human cases detected in Singapore. Whenever available, all sequences are labeled with GenBank accession numbers and country of origin and isolated by year/month. In addition, Singapore isolates are labeled with the reported area or country (imported human cases). Figures on the branches are bootstrap percentages based on 1000 replicates and only those above 70% are shown. SG, Singapore.