Emergency Vector Control in a DENV-2 Outbreak in 2002 In
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Jpn. J. Infect. Dis., 60, 271-279, 2007 Original Article Emergency Vector Control in a DENV-2 Outbreak in 2002 in Pingtung City, Pingtung County, Taiwan Hwa-Jen Teng*, Tzay-Jinn Chen1, Shu-Fen Tsai, Chiung-Pin Lin, Horng-Ying Chiou, Min-Cheng Lin, Shih-Yan Yang, Yi-Wun Lee, Chi-Chieh Kang2, Ho-Cheng Hsu3 and Niann-Tai Chang4 Centers for Disease Control, Department of Health; 1Department of Health, Executive Yuan, Taipei; 2Bureau of Health and 3Bureau of Environmental Protection, Pingtung County Government; and 4National Pingtung University of Science and Technology, Pingtung, Taiwan (Received January 9, 2007. Accepted June 6, 2007) SUMMARY: This paper reports the strategy and effectiveness of an emergency control program conducted in Pingtung City, Taiwan in response to dengue outbreaks. In our control strategy, we carried out 3 insecticide space sprays with an interval of 6-7 days and 2 source reductions to cover the entire duration of dengue virus exposure in humans and mosquito vectors. The control effect was demonstrated by a significant reduction in the Breteau (51.1%) and larval (80.0%) indices, but no such effect was demonstrated by alterations in the adult index (54.9%), house index (45.0%), container index (33.8%), or by indoor (15.8%), outdoor (31.2%), or total water-filled containers (22.7%) per 100 premises examined. The contribution made by the reduction in the number of positive containers was primarily in the outdoor (77.2%), and not the indoor containers (–6.0%). This reduction attributed to an overall reduction of 96.0% Aedes albopictus larvae and 71.0% Aedes aegypti. Therefore, 4 weeks after this extensive emergency control measure, the number of dengue cases dropped to one. Finally, due to both the decrease in temperature resulting from the upcoming winter, and to the sustained effort toward source reduction, the transmission cycle of DENV-2 in Pingtung City was interrupted at the beginning of 2003, and no additional cases were identified in late 2003. vectors of dengue in Taiwan. The distribution of the former INTRODUCTION species is geographically limited to areas south of the northern Dengue fever is a travel-related disease in Taiwan, as trav- Tropic of Cancer (9,10), and this species is more important elers can carry dengue virus (DENV) from endemic areas than the latter in the epidemiology of dengue in southern into the island. After being transported to the island, this Taiwan, which includes Pingtung City. Ae. albopictus indi- virus is passed on to Aedes mosquitoes, which can cause an viduals are distributed throughout the island. This mosquito outbreak of dengue fever. Several historical outbreaks were is responsible for epidemics of dengue, e.g., small outbreaks documented before World War II, and the last one was recorded in Chungho, Taipei County in 1995 (162 cases), Tung-Hai in 1942-1943 throughout Taiwan. The virus had been silent University in Taichung in 1995 (8 cases), and Taipei City in for 37 years, until a DENV-2 outbreak occurred in 1981 on 1996 (14 cases) that have occurred in areas without Ae. aegypti a remote islet of Hsiao-Liuchiu. Fortunately, the dengue (11,12). Therefore, due to the presence of these two efficient outbreak did not result in an epidemic in Taiwan proper. dengue mosquito vectors and the introduction of DENVs, Later, a DENV-1 epidemic exploded in 1987-1988 in south- Taiwan has experienced small to moderate dengue outbreaks ern Taiwan. In 1987-2002, the epidemic patterns of dengue from time to time. Two major outbreaks with DENV-1 (4,389 in Taiwan cycled with small-scale outbreaks occurring al- cases) and DENV-2 (5,336 cases) occurred in 1988 and 2002, most every 3 years, and large-scale epidemics occurring nearly respectively, because the transmission cycle was not interrupted every 10 years (1). Most local outbreaks began in June-July, during the previous year. To terminate virus transmission peaked in October-November, and ended in December to within 1 year in Taiwan, it is important to prevent dengue January - February of the next year, depending on control endemics from developing into epidemics and to reduce the efforts. However, with the exception of two large outbreaks chance of acquiring dengue hemorrhagic fever (DHF)/dengue in 1988 and 2002, cases originating from these 2 years were shock syndrome (DSS) cases. found year-round. A previously developed dengue vaccine is Studies have suggested that the severity of dengue at the promising, but remains at the clinical trial stage (2-6). There- level of secondary infection depends on the interval between fore, countries with vector mosquitoes should have a control the initial DENV-1 infection and the secondary DENV-2 strategy and emergency infrastructure in place to stop the infection (13), as well as on the type of virus, especially in the transmission cycle during an outbreak. A prevention strategy case of DENV-2 (14,15). The transition from dengue fever to that maintains mosquito density at or below a 1% house DHF and DSS in Taiwan was seriously challenged in 2002 index can effectively reduce the threat of epidemics (7,8). by a DENV-2 outbreak with an interval of 14 years since the Aedes aegypti L. and Aedes albopictus Skuse are the main DENV-1 endemic in 1988. In 2002, there were 20 fatalities among 240 DHF cases (8.75% case fatality) (16). Multiple *Corresponding author: Mailing address: Centers for Disease circulating serotypes were also detected in the small outbreaks Control, Department of Health, 161 Kun-Yang Street, Taipei, of 2003 (types 1 and 2), 2004 (types 1 and 4), and 2005 (types Taiwan 11561. Tel: +886-2-26531385, Fax: +886-2-27822319, 2 and 3). Therefore, it has been essential to build and main- E-mail: [email protected] tain an efficient emergency infrastructure that would enable 271 a rapid response to epidemics in order to protect high-risk year, with the rainy season lasting from May to October. This populations in southern Taiwan. Unfortunately, since the last city experienced in DENV-1 outbreaks in 1987, 1988, and outbreak, no optimal example of emergency preparedness 1995; no outbreaks occurred in the period from 1996-2001. has yet been identified (17). To date, most publications on The case distribution in 79 wards of Pingtung City in an out- dengue outbreak have dealt with epidemiology and vector break occurring in 2002 is presented in Fig. 1B. Both dengue surveillance (7,18-21). vectors, Ae. aegypti and Ae. albopcitus, can coexist in the In order to provide an example of emergency vector control same city; however, Ae. aegypti is found primarily in the center in response to dengue outbreaks, we outline here the strategy of the city, while Ae. albopictus inhabits primarily the city for and effectiveness of an emergency control program con- boundaries (Fig. 1C). ducted in Pingtung City, Pingtung County, Taiwan. The present Control strategy: Financial and human resources reserved control strategy entailed spreading 3 rounds of insecticide for dengue control in response to outbreaks will always be spray and implementing 2 rounds of source reductions to associated with difficult allocation issues. Emergency con- cover the entire durations of potential exposure to DENVs in trol measures should focus on areas in which dengue vectors humans and vectors. The present approach included inhibit- thrive (in our case, first-floor premises and basements). In ing the ability of mosquitoes to lay eggs on less preferable addition to their presence of infectious mosquitoes, viruses are water-holding sites in their struggle for survival, as well as also found in patients during the viremic phase, which lasts prevention of partial failures during the execution of control for 4-5 days (Fig. 2) (22,23). Emergency insecticide spray measures. The methods used for the evaluation of this pro- for killing infected adult mosquitoes, should be applied up to gram are also discussed below, in order to facilitate improve- three times, intervals of 7 days, to cover the intrinsic incuba- ments to such programs to prevent future outbreaks. tion (4-7 days) and infectious (0-5 days) periods affecting patients with dengue (23) who are within the flight range of mosquitoes. In order to interrupt the transmission cycle, it MATERIALS AND METHODS is critically important to reduce the probability of a dengue Description of the control area: The program described patient being bitten by a female vector. In addition to health here, implemented to control the transmission of a dengue education campaign regarding personal protection, two series outbreak in 2002, was conducted in Pingtung City, which is of source reduction should be applied at an interval of 7 days located at 22°43´-22°36´N and 120°26´-120°32´E (Fig. 1A). in order to remove mosquito larvae from the premises of Pingtung City is a medium-sized city (approximately 65.1 infected patients; this measure aims to prevent the spread of km2 in area) with 65,750 households (215,520 inhabitants) disease due to dengue patient mobility. Effective coverage in Pingtung County, Taiwan. The average temperature is 25°C by insecticide spray and source reduction are both critical for and the total annual precipitation is approximately 2,000 mm/ the success of a control program. In our program, we included Fig. 1. The control city in a Taiwan map (A), the number of dengue confirmed cases in each ward of Pingtung City (B), and the distribution of Aedes aegypti in Pingtung City (C) based on a 2003 survey; black areas indicate that either Ae. aegypti larvae or adults were collected at least once, gray areas indicate that no Ae. aegypti were collected, and white areas indicate that no surveys were conducted.