The Role of Imported Cases and Favorable Meteorological Conditions in the Onset of Dengue Epidemics

Dengue/dengue hemorrhagic fever is the world's most widely spread mosquito-borne arboviral disease and threatens more than two-thirds of the world's population. Cases are mainly distributed in tropical and subtropical areas in accordance with vector habitats for Aedes aegypti and Ae. albopictus. However, the role of imported cases and favorable meteorological conditions has not yet been quantitatively assessed. This study verified the correlation between the occurrence of indigenous dengue and imported cases in the context of weather variables (temperature, rainfall, relative humidity, etc.) for different time lags in southern Taiwan. Our findings imply that imported cases have a role in igniting indigenous outbreaks, in non-endemics areas, when favorable weather conditions are present. This relationship becomes insignificant in the late phase of local dengue epidemics. Therefore, early detection and case management of imported cases through timely surveillance and rapid laboratory-diagnosis may avert large scale epidemics of dengue/dengue hemorrhagic fever. An early-warning surveillance system integrating meteorological data will be an invaluable tool for successful prevention and control of dengue, particularly in non-endemic countries

Establishment of a Better Prediction System for Dengue Epidemics in Taiwan: Temporal Analyses of Epidemiological and Meteorological Factors of Dengue during 1998-2007

受登革病毒感染的旅客經常成為散播病毒至其他地區的重要途徑，甚而引發他國的流行；然而這些感染的旅客入境後和當地氣候、病媒以及本地疫情之間的互動關係並不清楚。由於境外移入病例與本地登革疫情關係的議題長期受到忽略，本研究即在探究臺灣地區的境外移入登革病例和氣候因子，對於本地疫情發生之影響，進而可讓基層與中央衛生人員將危險層級資訊馬上應用於疾病防控策略。 我們使用羅吉斯（logistic）和普瓦松（Poisson）迴歸模式分析1998至2007年間臺灣南部地區經實驗室診斷證實的登革確定病例，以區辨在氣候因子的作用下，境外移入和本地登革病例的時序相關性。結果發現本地登革疫情的發生與境外移入病例數（2至14週前）、高溫（6至14週前）及低濕度（6至20週前）之間，具有延遲的相關性。此外，境外移入病例數和本地登革病例數僅在流行被引發的「初期」階段，才有明顯數量上的相關性；一旦流行持續發生，此種關係即不復見。另外，根據單變項分析的結論，挑選出的重要氣候因子所建立的羅吉斯迴歸模式，可以進一步建立登革危險性指標的預警數值。經初步運算，此法優於僅以前期登革危險性指標(dengue risk index, DRI)值預測本期DRI值的效果，在加入氣候因子後，模式預測值與觀察值的相關係數(Spearman correlation)可達0.71(屏東地區)、0.84(高雄地區)以及0.86(台南地區)；至於泰國地區也有0.66-0.77的水準。由此可知此法不但適用於台灣登革疫情層級之預警，也適用於泰國海岸五省的疫情預測。 這些發現顯示，惟有氣象條件適宜時，境外移入登革病例才有可能引發本地的疫情。據此，經由境外移入病例的快速實驗診斷、早期發現以及管理，可以遏止其後大規模登革/登革出血熱流行的發生。因此整合氣象資訊的早期警示監測系統，將是登革疫情尚未成為地方性流行的地區用以成功防治疫情的無價利器。Travelers who acquire dengue infection are often routes for virus transmission to other regions. Nevertheless, the interplay between infected travelers, climate, vectors, and indigenous dengue incidence remains unclear. The role of foreign-origin cases on local dengue epidemics has thus been largely neglected by research. This study investigated the effect of both imported dengue and local meteorological factors on the occurrence of indigenous dengue in Taiwan. Using logistic and Poisson regression models, we analyzed bi-weekly, laboratory-confirmed dengue cases at their onset dates of illness from 1998 to 2007 to identify correlations between indigenous dengue and imported dengue cases (in the context of local meteorological factors) across different time lags. Our results revealed that the occurrence of indigenous dengue was significantly correlated with temporally-lagged cases of imported dengue (2-14 weeks), higher temperatures (6-14 weeks), and lower relative humidity (6-20 weeks). In addition, imported and indigenous dengue cases had a significant quantitative relationship in the onset of local epidemics. However, this relationship became less significant once indigenous epidemics progressed past the initial stage. Polytomous Logistic regression model with relevant meteorological variables stepwise selected were able to fitting the value of Dengue Risk Index (DRI) in both Taiwan and Thailand. The Spearman correlation between observed DRI and model-expected DRI ranged from 0.71 to 0.86 in Taiwan and from 0.66 to 0.77 in Thailand, respectively. These findings imply that imported dengue cases are able to initiate indigenous epidemics when appropriate weather conditions are present. Early detection and case management of imported cases through rapid diagnosis may avert large-scale epidemics of dengue/dengue hemorrhagic fever. The potential application of DRI with meteorological modeling in both Taiwan and Thailand demonstrated that its feasibility to be extended to other countries for the current important issue on global warming and dengue. The deployment of an early-warning surveillance system, with the capacity to integrate meteorological data, will be an invaluable tool for successful prevention and control of dengue, particularly in non-endemic countries