Taiwan - Tainan City’s implementation of area-based chemical control for dengue fever

Issues and Objectives

Starting in April 2015, a dengue fever outbreak occurred in Tainan City. As the number of cases rose rapidly, more and more cases couldn't be controlled in time. At this point, it wasn't feasible to continuously chase after new cases; instead, priority should be given to controlling outbreaks in specific areas, such as those with a particularly high number of cases. Area-based control is crucial. If outbreaks aren't addressed first, other sporadic cases could develop into clusters, and eventually become concentrated areas of cases. Once the hardest-hit areas are prioritized for control, the number of cases will not surge, and the outbreak in nearby areas can also be stabilized somewhat.

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Improvement Measures Taken

Block selection:

Based on topography, land features, roads, etc., the area with the highest number of cases within 14 days of onset within a unit area is defined by extending 50 meters outward from the outermost case area, and then adjusting the covered alleyways and lanes. An area is approximately 400 households or less (in one morning). For areas with more than 400 households, prevention and control measures must be carried out in both the morning and afternoon sessions, or multiple site controllers are required to ensure the progress of on-site spraying.

The sorting after the frame is defined:

1. Prioritize those with the most cases.
2. The epidemic is rising rapidly.
3. Based on the trends of the epidemic over the past 4/2/1 weeks and on the current day, we can predict the future trend.
4. Community diagnostic results, population concentration, etc.

Control of gun usage and the number of households affected (divided into 4 stages):

1. From August 11th to September 3rd, 30 spray guns (for the military) and 20 spray guns (for PCOs) will be available daily for application in each district.
2. From September 3rd to September 17th, 80 spray guns will be used daily (50 from the National Army and 30 from PCO), which is estimated to be effective for approximately 1330 households per day.
3. On September 9, the Secretary-General received a briefing on the progress of prevention and control. The city government purchased 50 guns (41 of which arrived on September 17, and the remaining 9 will be put into use on October 1).
4. The Central Epidemic Command Center was established on September 15. The Centers for Disease Control purchased 100 guns (15 of which arrived on September 25, and the remaining 85 will be put into use on October 7).

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Results

From the start of implementation until October 12th, a total of 180 smoke guns were deployed (excluding spares), capable of serving 3,600 households per day. By this time, the epidemic had been declining for three consecutive weeks. The Eastern District had the highest number of daily cases in the city, so the capacity was increased in the Eastern District. From October 8th, about 2 maps were executed per day in the Eastern District, and about 4 maps were executed per day on October 19th. However, there were still more than 10 maps pending each day. From October 22nd to 28th, this increased to 8-12 maps (about 1,200-1,600 households).

On November 2nd, the daily new cases were 80, and the military reduced its support to Tainan. From November 16th onwards, the daily new cases were below 30. Except for the East and South districts, other administrative districts planned their own prevention and control measures. Based on the past dengue fever outbreak trends, the epidemic enters a rapid increase period from September, peaking in November, and then gradually declines as the weather gets colder (18 degrees Celsius for 3 consecutive days). Due to the optimal allocation of manpower and equipment, and the suppression of the increase through human effort, the epidemic was successfully reversed in week 37, and the next stage is to maintain the effectiveness. Chemical control is an emergency measure, especially during severe outbreaks, to kill virus-carrying mosquitoes in time. At the same time, breeding stock eradication must be carried out in a two-pronged approach to maintain epidemic control.

Due to climate change, the future climate will be more favorable for the survival of disease-carrying mosquitoes, which may lead to the normalization of large-scale dengue fever outbreaks in the south. Therefore, this case demonstrates that block-based prevention and control combined with the elimination of breeding sites is the only truly effective strategy. Incorporating this strategy into the routine procedures of future epidemic prevention can be regarded as an adaptation strategy to strengthen the public health and epidemic prevention system in response to future climate change.

Glossary

Climate change risk assessment: Using effective assessment tools to understand the medium- and long-term impacts of climate change on various sectors, and to propose corresponding adaptation strategies and actions to reduce the risks brought about by climate change.

Case type:

Structural and physical options: engineering techniques or reinforcement of existing structures, integrated technology development

Social options: learning and exchanging disaster knowledge in the community, and developing and applying early warning information.

Institutional options: Establishing institutions at the economic, policy, and legal levels

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References

Southern Regional Control Center, Taiwan Centers for Disease Control (2017). A review of the 2015 dengue fever outbreak. pp. 6-13

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Reference Websites

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