MONITORING OF DENGUE VIRUSES IN FIELD-CAUGHT AEDES-AEGYPTI AND AEDES-ALBOPICTUS MOSQUITOS BY A TYPE-SPECIFIC POLYMERASE-CHAIN-REACTION AND CYCLE SEQUENCING

Virologic surveillance for dengue through the detection of the prevale nt serotype(s) circulating in the human population during inter-and in tra-epidemic periods constitutes a reliable sentinel system for dengue outbreaks. We have applied a rapid and sensitive, semi-nested, revers e transcription-polymerase chain reaction (RT-PCR) assay using nonstru ctural protein 3 gene primers for the type-specific-detection of dengu e viruses in artificially infected and in field-caught adult Aedes mos quitoes. In laboratory experiments, the assay was sensitive enough to detect one virus infected mosquito head in pools of up to 59 uninfecte d heads. In a prospective held study conducted from April 1995 to July 1996, female adult Ae. aegypti and Ae. albopictus mosquitoes were cau ght from selected dengue-sensitive areas in Singapore and assayed by R T-PCR. Approximately 20% of 309 mosquito pools were positive for dengu e viruses. Of the 23 RT-PCR-positive Ae. aegypti pools (containing 1-1 7 mosquitoes each), 18 pools (78.3%) were positive for dengue 1 virus. There were 40 RT-PCR-positive Ae. albopictus pools (containing 1-33 m osquitoes each) of which 31 (77.5%) were positive for dengue 1 virus. The predominant virus type responsible for the current dengue epidemic since 1995 was also dengue 1. The geographic locations of the virus-i nfected mosquitoes correlated with the residences or workplaces of pat ients within dengue outbreak areas. A total of 43.5% of the positive A e. aegypti pools and 25.0% of the positive Ae. albopictus pools contai ned only a single mosquito. Both Aedes species showed similar overall minimum infection rates of 57.6 and 50 per 1,000 mosquitoes. Infected Ae. aegypti were detected as early as six weeks before the start of th e dengue outbreaks in 1995 and 1996. However, infected Ae. albopictus appeared later, when the number of cases was increasing. Virologic sur veillance by RT-PCR for detecting dengue virus-infected Aedes mosquito es in the field may serve as an early warning monitoring system for de ngue outbreaks.