An Explosive Epidemic of DENV-3 in Cairns, Australia

Scott A Ritchie,Andrew F Van Den Hurk,Andrew Day,Rebecca C Christofferson,Christopher N Mores,Sonja Hall-Mendelin,Duane J Gubler,Alyssa T Pyke,Shannon N Bennett,Nikos Vasilakis

doi:10.1371/journal.pone.0068137

Abstract

From November 2008-May 2009 Cairns Queensland Australia was struck by an explosive epidemic of DENV-3 that exceeded the capacity of highly skilled dengue control team to control it. We describe the environmental, virological and entomological factors associated with this outbreak to better understand the circumstances leading to its occurrence. Patient interviews, serological results and viral sequencing strongly suggest that the imported index case was infected in Kalimantan, Indonesia. A delay in notification of 27 days from importation of the index case until Queensland Health was notified of dengue transmission allowed the virus to amplify and spread unchecked through November 2008. Unseasonably warm weather, with daily mean temperatures exceeding 30°C, occurred in late November and would have shortened the extrinsic incubation period of the virus and enhanced transmission. Analysis of case movements early in the outbreak indicated that the total incubation period was as low as 9–11 days. This was supported by laboratory vector competence studies that found transmission by Aedes aegypti occurred within 5 days post exposure at 28°C. Effective vector competence rates calculated from these transmission studies indicate that early transmission contributed to the explosive dengue transmission observed in this outbreak. Collections from BG sentinel traps and double sticky ovitraps showed that large populations of the vector Ae. aegypti occurred in the transmission areas from November – December 2008. Finally, the seasonal movement of people around the Christmas holiday season enhanced the spread of DENV-3. These results suggest that a strain of DENV-3 with an unusually rapid transmission cycle was able to outpace vector control efforts, especially those reliant upon delayed action control such as lethal ovitraps.

Highlights

Dengue is the leading arboviral cause of morbidity worldwide, with an estimated 390 million infections occurring annually [1]
We retrospectively examined routine Ae. aegypti surveillance data collected by Queensland Health (QH) staff, consisting of Biogents Sentinel (BGS) traps at 13 fixed locations [31] and sticky ovitraps (SOs) set at ca. 70 locales
Heavy rains in late Sept. - mid Oct. (121 mm from 4 events) could have hatched Ae. aegypti eggs leading to the rapid escalation in Ae. aegypti numbers collected in BGSs traps and SOs in November (Fig. 1)

Summary

Introduction

Dengue is the leading arboviral cause of morbidity worldwide, with an estimated 390 million infections occurring annually [1]. The primary vector of urban dengue, the mosquito Aedes aegypti, utilizes artificial containers for immature development while adults prefer to harbor within man-made premises. The control of Ae. aegypti typically involves ‘house-to-house’ treatment of water-holding containers and interior residual spraying (IRS) of premises [2,3]. A delay in recognition of a dengue epidemic can exponentially increases the total number of cases and total cost to the community [5]. Despite a relatively low mortality rate, the large number of cases cumulatively creates relatively high disability-adjusted life year (DALYs) values for dengue. A mean loss of 658, 465 and 127 DALYs per million individuals annually has been reported in Puerto Rico [6], Thailand [7] and Latin American+Caribbean regions [8], respectively

Methods

Results

Conclusion