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標題: | 高病原性禽流感持續存在於禽類族群的條件分析與防治策略建議:數理模式研究 Persistence of Highly Pathogenic Avian Influenza (HPAI) Virus in Avian Population and the Implication on Control Strategy: A Dynamic Modeling Study |
作者: | Tai-Ting Chen 陳岱廷 |
指導教授: | 方啟泰 |
關鍵字: | 高病原性禽流感,交叉免疫,疫情,移動管制,數理模式, Highly pathogenic avian influenza (HPAI),cross-immunity,epidemic,movement restriction,mathematical modeling, |
出版年 : | 2017 |
學位: | 碩士 |
摘要: | 背景:高病原性禽流感 (Highly pathogenic avian influenza, HPAI) 是具有高度傳染性及致死性的全球性禽類傳染病。HPAI病毒的持續傳播,造成在地養禽業重大損失,亦是公共衛生潛在威脅。然而,何以一個迅速殺死禽類宿主的病毒能夠違反演化法則,於禽類族群中持續傳播?其機制尚未明瞭。先前數理模型研究指出交叉免疫力在某些條件下反而促使病毒持續傳播,但先前研究未考慮野鳥與家禽差異性、商業養殖模式、監測及防疫措施,與現實情境未盡符合。本研究擬透過建構符合獸醫防疫實務情境的數理模式,分析HPAI在禽類族群中持續傳播的條件,及對防治策略的影響。
方法:本研究分成兩部分,第一部分以確定性模型 (Deterministic model) 分析HPAI持續傳播條件;第二部分則以隨機性模型 (Stochastic model) 分析移動管制在預防禽場間HPAI傳播的成效。確定性模型將多病原 (Multi-pathogen) 傳播過程納入考量,探討野鳥和家禽、與交叉免疫力影響之下的疫情傳播動態變化。隨機性模型則模擬HPAI隨養禽業商業網路場間擴散的隨機過程,並考慮環境HPAI風險對移動管制效果的影響。 結果:若無交叉免疫力時,HPAI疫情在所有模擬情況下,均將於兩個月內自行滅絕,無法持續傳播;但當交叉免疫力存在,且達99.9 % 時,高、低病原性禽流感基礎再生數 (Basic reproduction number, R0) 相近的情境下 (同源突變),HPAI疫情則能在野鳥族群中持續傳播;若交叉免疫力50 % 時 (高低致病性病毒為異源),高R0 (大於4.5) HPAI病毒仍可因交叉免疫力存在而在野鳥族群中持續傳播。但對有常規監測及撲殺機制的養禽場,除了極低R0組合 (R0H = 1.1, R0L = 1.5) 之外,HPAI疫情皆無法維持。模擬移動管制的結果指出,立即實施預防性移動管制可有效遏止經由商業網路擴散之HPAI疫情,使98 % 場次免於感染。延誤兩周實施則須擴大範圍,納入近10倍場次才能預防80 % 場次免於感染。環境來源HPAI導致案例場散發性爆發,HPAI環境風險越大,移動管制成效愈低。 結論:交叉免疫力導致HPAI持續存在野鳥族群,成為環境中保毒宿主。迅速實施移動管制可有效防範經由商業網路擴散之HPAI疫情,但環境來源HPAI的威脅仍需以推廣養禽場之生物安全防護措施為根本之道。 Background: Highly pathogenic avian influenza (HPAI) is a global epizootic disease and potential threat to public health. A major unanswered question is how HPAI, which rapidly kill its host with very short latency, evolutionarily thrive? Mathematical simulations have shown that, under certain conditions, prior low pathogenic avian influenza (LPAI) infection with cross-immunity may allow HPAI persistence. However, previous work did not take veterinary surveillance and culling practice into account. We aim to analyze the conditions for HPAI persistence in both wild birds and domestic poultry under realistic scenarios, and the implication on control strategy. Methods: We constructed a deterministic multi-pathogen co-circulation model to analyze conditions of HPAI persistence, with emphasis on the role of LPAI cross-immunity. Sensitivity analyses include various combinations of R0 values between HPAI and LPAI, and the effect of veterinary surveillance and culling in domestic poultry. To assess the impact of environmental HPAI persistence, we built spatial stochastic model for farm-to-farm HPAI transmission. The efficacy of movement restriction policy on containing the spread of HPAI epidemic was evaluated in the scenarios of different levels of environmental source HPAI. Results: Without prior LPAI infection, HPAI outbreaks will take a natural course to burn-out within two months in all simulated scenarios. In contrast, with prior LPAI infection, HPAI can persist in a wide range of conditions. Such conditions are characterized by equal or similar R0 values between homologous HPAI and LPAI with a 99.9% cross-immunity in wild bird populations. For heterologous HPAI and LPAI, a 50% cross-immunity allows high R0 (> 4.5) HPAI persists in wild bird populations. Veterinary surveillance and culling effectively prevents HPAI persistence except the extremely low R0 combination (R0H = 1.1 and R0L = 1.5 for 99.9% cross-immunity; R0H < 2.0 for 50% cross-immunity). Stochastic modeling shows that immediately implement pre-emptive movement restriction will prevent HPAI epidemics from spreading via commercial network and 98% poultry farms can be preserved. A two-week delay, however, will decrease the efficacy to 80% and 10-fold more farms will need to be put under movement restriction. The presence of environmental source HPAI will cause a jumping pattern of outbreaks. The higher environmental risk, the lower efficacy of movement restriction. Conclusions: HPAI can persist in wild bird populations previously exposed to LPAI. This results indicate that wild birds, but not domestic poultry, serve as main HPAI reservoirs. The environmental nature of HPAI presence in wild birds makes HPAI elimination or containment difficult to achieve. Protective measures that prevents domestic poultry from being exposed to wild birds should be part of the overall avian influenza control strategies. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/67046 |
DOI: | 10.6342/NTU201702624 |
全文授權: | 有償授權 |
顯示於系所單位: | 流行病學與預防醫學研究所 |
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