密度依賴效應對⽩線斑蚊⽣活史特徵及登⾰熱病媒 傳播能⼒之影響

Abstract

背景：登革熱是一種由埃及斑蚊（Aedes aegypti）及白線斑蚊（Aedes albopictus）等病媒蚊傳播的蟲媒傳染病，過去 30 年來其地理分布範圍與發生率快速擴張，已成為全球最重要的公共衛生威脅之一。已知幼蟲生長環境，包括幼蟲密度，會影響多項與疾病傳播相關的成蟲特徵，例如體型、存活率及病媒能力（vector competence）。然而，幼蟲密度對登革熱病媒傳播能⼒（vectorial capacity, VC）的整體累積效應仍未被充分了解。病媒傳播能⼒可衡量病媒族群整體的疾病傳播潛力。本研究測量臺灣白線斑蚊品系在不同幼蟲密度下的生活史特徵（發育時間、翅長及產卵力），並估算幼蟲密度對登革熱病媒傳播能⼒的影響。材料與方法：白線斑蚊幼蟲分別以三種密度飼養：0.2、0.4 與 0.6 隻幼蟲／毫升。每日記錄化蛹數與羽化成蟲數，以計算發育速率。雌蚊於羽化後測量翅長及產卵力。登革熱病媒傳播能⼒則根據實驗室所得的性狀數值，結合已發表文獻中的相關資料，在三種密度條件下進行估算。此外，研究亦進行敏感度分析，以評估幼蟲密度對叮咬率及病媒能力影響的不確定性，如何影響病媒傳播能⼒的計算結果。結果：較高的幼蟲密度顯著延長發育時間，並降低成蟲翅長。然而，不同密度處理間的產卵力並無顯著差異。在幾乎所有模擬情境中，病媒傳播能⼒皆隨幼蟲密度增加而下降，其主要原因為較高密度下成蟲羽化率（即病媒密度）及每日存活率降低所致。敏感度分析顯示，幼蟲密度與病媒傳播能⼒之間的負向關係，對於叮咬率及病媒能力受幼蟲密度影響的不確定性具有相當穩健性。只有在最極端的假設情境下（即叮咬率與病媒能力均隨密度增加，且同時達到模型設定的最大增幅時）最高密度組的病媒傳播能⼒才會超過最低密度組。此結果顯示，高幼蟲密度導致病媒傳播能⼒增加，必須同時滿足一組較不可能發生的條件。結論：本研究結果顯示，在白線斑蚊族群中，較高的幼蟲密度通常與較低的登革熱傳播潛力相關。此發現對病媒蚊防治策略具有重要意義。若防治措施僅降低而未完全消除幼蟲族群，可能會降低幼蟲密度，使存活個體發育成體型較大且壽命較長的成蟲，進而意外提高病媒傳播能⼒。因此，相較於單純抑制幼蟲數量，使用成蚊防治措施或清除孳生源可能更能有效降低登革熱傳播風險。更廣泛而言，本研究凸顯了幼蟲期生態條件在塑造成蟲病媒特徵及最終決定登革熱傳播潛力方面的重要性。

Introduction: Dengue, a vector-borne disease transmitted by Aedes mosquitoes, has emerged as one of the most pressing global health threats, with its geographic spread and incidence rapidly expanding over the past 30 years. The larval environment, including larval density, has been known to shape a number of adult mosquito traits relevant to disease transmission, including body size, survival, and vector competence. However, the cumulative effect of larval density on dengue vectorial capacity (VC), which measures a vector population’s overall transmission potential, is not well understood. In this study, we measured the effect of larval density on life history traits (development time, wing length, fecundity) of a Taiwanese strain of Aedes albopictus, and estimated the effect of larval density on dengue VC. Materials and Methods: Aedes albopictus larvae were raised at three 3 densities (0.2, 0.4, and 0.6 larvae/mL). The number of pupae and adults emerging each day was counted to determine the development rate. Female wing length and fecundity were measured upon adult emergence. Dengue VC was estimated at the three densities measured in the laboratory using a combination of empirically derived trait values from our laboratory experiment and data from published literature. Sensitivity analyses were conducted to assess how uncertainty in the effects of larval density on biting rate and vector competence affected calculated VC values. Results: Higher larval density significantly increased development time and reduced wing length. Fecundity did not differ significantly across density treatments. VC decreased with higher larval density across nearly all modeled scenarios, primarily driven by reductions in adult emergence rate (i.e., vector density) and daily survival at higher densities. Sensitivity analyses showed that this negative relationship between larval density and VC was robust to uncertainty about the effect of larval density on biting rate and vector competence. Only under the most extreme combined scenario, in which both biting rate and vector competence were simultaneously assumed to increase with density at their maximum modeled magnitude, did VC at the highest density exceed that at the lowest density, suggesting that high density leading to an increase in VC would require an unlikely combination of conditions. Discussion: These findings suggest that high larval densities are generally associated with reduced dengue transmission potential in Ae. albopictus populations. This finding has important implications for vector control. Control methods that reduce but do not eliminate larval populations may decrease larval density, producing larger, longer-lived adults, inadvertently leading to higher VC. Therefore, using adulticides or source reduction may be more effective at reducing dengue transmission than larval population suppression. More broadly, these results highlight the importance of larval ecological conditions in shaping adult vector traits and ultimately dengue transmission potential.