登革熱在美洲打破了記錄——激增背後的原因是什麼？
氣溫升高導致登革熱季節延長，並可能推動該疾病的地理擴張。
Mariana Lenharo / 2023 年 7 月 27 日 / 新聞 / 自然
登革熱患者可能會出現發燒、關節疼痛和頭痛的症狀。 嚴重的病例可能致命。
圖片來源：Ernesto Benavides/AFP via Getty
今年迄今為止，美洲已報告超過三百萬登革熱病例。 這意味著 2023 年該病的年發生率已經是自 1980 年以來第二高的年發生率，當時泛美衛生組織開始收集病例數數據（參見「登革熱呈上升趨勢」）。
巴西里約熱內盧生物科學和公共衛生機構奧斯瓦爾多·克魯茲基金會的流行病學家 Cláudia Codeço 表示：「我們確實觀察到病例數量的增加超出了這一時期的預期」。 2023年是否會打破2019年報告的320萬例病例記錄，取決於該疾病在中美洲和北美的傳播方式，因為大多數研究人員認為南美洲登革熱季節的高峰已經過去。
登革熱呈上升趨勢
美洲之溫度和降雨型態的改變有助於解釋近年來登革熱的高發生率
資料來源：泛美衛生組織。
登革熱是由四種密切相關血清型的病毒所引起的，這使得確定病例激增的確切原因具有挑戰性。「這些血清型之間存在相互作用，對一種血清型的免疫力會干擾另一種血清型。 當我們把這些放在一起時，可能會導致不可預測的動態」，Codeço 說。
但研究人員表示，氣溫上升和降雨模式的變化可能有助於解釋這一趨勢。 登革熱的主要媒介埃及斑蚊在 30°C 左右的溫度和潮濕的條件下繁殖，由於創紀錄的高溫和極端天氣事件，這種情況在過去幾年變得更加頻繁。
這種疾病沒有具體的治療方法，它會導致發燒、頭痛和疲勞。 嚴重病例可能致命：今年迄今為止，美洲已有 1,300 多人死於登革熱。
範圍擴展
登革熱正在蔓延到曾經是埃及斑蚊禁區的地區。 在巴西，今年報告了近 240 萬例病例，這種疾病正在蔓延到南部各州，這些州以前對蚊子來說太冷了。 根據 Codeço 及其同事的分析，過去 5 年裡，481 個巴西城市首次出現登革熱持續本地傳播。 海拔 2,240 米的墨西哥城於 2015 年記錄了首次埃及斑蚊入侵。「如果你讀過有關埃及斑蚊生物學的書籍，他們會說這種蚊子在海拔 1,200 米以上無法繁殖」，說 何塞·拉莫斯–卡斯塔涅達 (José Ramos-Castañeda)，庫埃納瓦卡墨西哥國家公共衛生研究所的病毒學家。「在這方面，全球變暖正在影響虫媒的分佈，從而影響病例的可能分佈」。
巴西大型蚊子工廠旨在阻止登革熱
安娜堡密歇根大學的研究人員調查了巴西部分地區氣溫上升可能如何影響 2040 年代末登革熱的流行潛力（即登革熱在人群中傳播的可能性）。 該研究的作者之一、計算流行病學家 Andrew Brouwer 表示：「我們發現，無論具體的氣候變化情況如何，流行的潛在可能性都比現在更高」。「在大多數地方，我們發現流流行的潛在可能性增加了 10-20%」，他說。
這種現像不僅限於南美洲。「在南半球和北半球，虫媒和病原體能夠持續生存的區域將會增加」，布勞威爾說。 在美國大陸，佛羅里達州、德克薩斯州和亞利桑那州已經出現登革熱本地傳播情況。
更長的季節
登革熱通常是季節性的——病例數往往在夏季或雨季上升，在冬季或旱季下降。 但全球氣溫上升意味著登革熱季節可能會更長。 在布勞威爾和他的同事的預測中，「我們發現兩端的傳播季節通常增加了大約一個月」，布勞威爾說。
短期內，持續的聖嬰現象 (El Niño) 天氣事件預計會帶來洪水、乾旱和創紀錄的氣溫，可能會對登革熱產生影響。 6月下旬，世界衛生組織秘書長譚德塞警告說，這種現象「可能會增加登革熱和其他所謂蟲媒病毒（如玆卡病毒和屈公熱）的傳播」。
聖嬰現象現象可能對中美洲和北美部分地區的登革熱發病率產生最大影響，這些地區目前正在經歷雨季。
已採用多種策略來控制登革熱傳播。 其中包括使用誘捕器或殺蟲劑殺死蚊子宿主，以及清除裝有昆蟲繁殖的開放容器的積水。人們還努力開發不能傳播這種疾病的改良蚊子。
拉莫斯–卡斯塔涅達說，所有這些方法都有幫助，但「真正影響傳播的是人群的免疫力」。 自2015年以來，兩種登革熱疫苗已在一些地方獲得當局批准，但由於功效、安全問題和價格高昂等問題，尚未得到廣泛採用。doi：https://doi.org/10.1038/d41586-023-02423-w
參考文獻：
- Codeco, C. T. et al. Lancet Reg. Health Am. 12, 100274 (2022).
- Van Wyk, H., Eisenberg, J. N. S. & Brouwer, A. F. PLoS Negl. Trop. Dis. 17, e0010839 (2023).
Dengue is breaking records in the Americas — what’s behind the surge?
Increasing temperatures contribute to longer dengue seasons, and could drive the geographical expansion of the disease.
Mariana Lenharo / 27 July 2023 / NEWS / Nature
People with dengue can experience fever, joint pain and headaches. Severe cases can be fatal.Credit: Ernesto Benavides/AFP via Getty
More than three million cases of dengue have been reported in the Americas so far this year. That means 2023 already has the second-highest annual incidence of the disease since 1980, when the Pan American Health Organization began collecting data on the number of cases (see ‘Dengue on the rise’).
“We do observe an increase in cases beyond what was expected for this period,” says Cláudia Codeço, an epidemiologist at the Oswaldo Cruz Foundation, a biosciences and public-health institution in Rio de Janeiro, Brazil. Whether the record of 3.2 million cases reported in 2019 will be broken in 2023 depends on how the disease spreads in Central and North America, because most researchers think the peak of dengue season in South America has passed.
Source: Pan American Health Organization.
Dengue is caused by four closely related viruses, or serotypes, which makes identifying the exact cause of the surge challenging. “There is an interaction between these serotypes, with the immunity against one interfering with the others. When we put this together, it can lead to unpredictable dynamics,” Codeço says.
But rising temperatures and changes in rainfall patterns might help to explain the trend, researchers say. Dengue’s main vector, the Aedes aegypti mosquito, thrives at temperatures around 30 °C and in humid conditions, which have become more frequent in the past few years as a result of record heat and extreme weather events.
There is no specific treatment for the disease, which can cause fever, headache and fatigue. Severe cases can be fatal: more than 1,300 people have died from dengue in the Americas so far this year.
Range expansion
Dengue is spreading to regions that were once off-limits to A. aegypti. In Brazil — which has reported nearly 2.4 million cases this year — the disease is expanding into southern states, which were previously too cold for the mosquito. Over the past 5 years, 481 Brazilian municipalities have registered sustained local transmission of dengue for the first time, according to an analysis by Codeço and her colleagues. And Mexico City, at an altitude of 2,240 metres, recorded its first A. aegypti invasion in 2015. “If you read books about the biology of the Aedes aegypti, they say the mosquito doesn’t reproduce at altitudes above 1,200 metres,” says José Ramos-Castañeda, a virologist at the Mexican National Institute of Public Health in Cuernavaca. “In that aspect, global warming is affecting the distribution of the vector and therefore the possible distribution of cases.”
Massive mosquito factory in Brazil aims to halt dengue
Researchers at the University of Michigan in Ann Arbor have investigated how rising temperatures in parts of Brazil might affect dengue’s epidemic potential — the chance of it spreading among people — in the late 2040s. “What we found was that, regardless of the specific climate-change scenario, the epidemic potential was higher than today,” says computational epidemiologist Andrew Brouwer, one of the authors of the study. “In most locations, we’ve seen a 10–20% increase in the epidemic potential,” he says.
The phenomenon is not restricted to South America. “Both in the Southern and Northern Hemisphere, the regions where vector and pathogen can be sustained are going to be increasing,” says Brouwer. In the continental United States, local transmission of dengue has already been registered in Florida, Texas and Arizona.
Longer seasons
Dengue is typically seasonal — case numbers tend to go up in summer or the rainy season and down in winter or the dry season. But global temperature increases mean that dengue seasons might get longer. In Brouwer and his colleagues’ projection, “we found that the transmission seasons generally increased by about a month on either end”, Brouwer says.
In the short term, the ongoing El Niño weather event — which is expected to bring floods, droughts and record temperatures — could have consequences for dengue. In late June, the director-general of the World Health Organization, Tedros Adhanom Ghebreyesus, warned that the phenomenon “could increase transmission of dengue and other so-called arboviruses such as Zika and chikungunya”.
El Niño will probably have the greatest impact on dengue incidence in Central America and parts of North America, regions that are now going through the rainy season.
Several strategies have been used to control dengue transmission. T