猴痘走向全球：科學家為何保持警戒
科學家們正試圖了解為什麼這種病毒，天花的一種較不致命的親戚，會在世界各地如此多的人群中突然出現。
資料來源： Max Kozlov / NEWS 2022 年 5 月 20 日 / Nature / 財團法人台灣紅絲帶基金會編譯
猴痘病毒（此處顯示為彩色透射電子顯微照片）與天花病毒密切相關。圖片來源：UK Health Security Agency/Science Photo Library
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過去一周，至少有 11 個非屬非洲的國家報告了 120 多例確診或疑似猴痘病例，這是一種在非洲以外很少發現的罕見病毒性疾病。該病毒在世界各地通常不會在不同的人群中出現，這讓科學家們感到震驚——並讓他們競相尋找答案。
「看到這種傳播方式令人大開眼界」，加州大學洛杉磯分校的流行病學家 Anne Rimoin 說，她在剛果民主共和國研究了猴痘十多年。
天花和其他的病毒比懷疑時間更早地困擾著人類
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之所以稱為猴痘，是因為研究人員於 1958 年首次在實驗室猴子身上發現了這種病毒，這種病毒被認為是從囓齒動物等野生動物或感染者身上傳播的。非洲平均每年發生幾千例病例，通常發生在非洲大陸的西部和中部。但非洲以外的病例僅限於少數與前往非洲旅行或進口受感染動物有關的病例。僅在過去一周，非洲以外地區發現的病例數量——幾乎肯定會增加——已經超過了自 1970 年首次發現該病毒導致人類疾病以來在非洲大陸以外發現的病例數量。這種快速傳播使科學家們高度的警戒。
但馬里蘭州德特里克堡美國陸軍傳染病醫學研究所的病毒學家傑伊·胡珀說，猴痘不是導致 COVID-19 大流行的冠狀病毒 SARS-CoV-2。它不容易在人與人之間傳播，而且由於它與天花病毒有關，因此手頭上已經有治療方法和疫苗來控制它的傳播。因此，儘管科學家們擔心，因為任何新的病毒行為都令人擔憂——但他們並不恐慌。
與透過稱為氣溶膠的微小飛沫傳播的 SARS-CoV-2 不同，猴痘被認為是透過與體液（例如咳嗽產生的唾液）的密切接觸而傳播的。胡珀說，這意味著患有猴痘的人感染的密切接觸者可能比感染 SARS-CoV-2 的人要少得多。這兩種病毒都會引起流感樣症狀，但猴痘也會引發淋巴結腫大，並最終在面部、手和腳上引發獨特的充滿液體的病變。大多數人在幾週內從猴痘中恢復，無需治療。
5 月 19 日，葡萄牙的研究人員上傳了在那裡檢測到的猴痘病毒基因組的第一份草圖，但紐約市西奈山伊坎醫學院的病毒學家 Gustavo Palacios 強調，這仍然是一個非常早期的草圖，在得出任何明確的結論之前，還需要做更多的工作。
世界上第二致命的伊波拉疫情在剛果民主共和國結束
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研究人員從這些初步遺傳數據中可以看出，猴痘病毒與主要在西非發現的病毒株有關。與在中非流行的病毒相比，這種病毒引起的疾病較輕，死亡率較低——在貧困的農村人口中約為 1%。但是，導致當前疫情爆發的毒株與西非的毒株究竟有多大不同——以及在不同國家出現的病毒是否相互關聯——仍然未知。
對這些問題的回答可以幫助確定病例的突然增加是否源於一種突變，這種突變使這種猴痘病毒比過去更容易傳播，以及每次爆發是否都可以追溯到一個單一的起源，澳大利亞雪梨新南威爾斯大學的傳染病流行病學家Raina MacIntyre 說道。與 SARS-CoV-2（一種快速進化的 RNA 病毒，其變異株經常逃避疫苗和先前感染所獲得的免疫）不同，猴痘病毒是一種相對較大的 DNA 病毒。 MacIntyre 說，DNA 病毒比 RNA 病毒更擅長檢測和修復突變，這意味著猴痘病毒不太可能突然突變為擅長人類傳播。
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「深感擔憂」
儘管如此，在彼此沒有明顯關聯的人群中檢測到猴痘顯示該病毒可能一直在無聲傳播——美國疾病控制和預防中心痘病毒小組負責人、流行病學家安德里亞·麥科勒姆（Andrea McCollum）稱這一事實「令人深感擔憂」。
與可以無症狀傳播的 SARS-CoV-2 不同，猴痘在感染人時通常不會被忽視，部分原因是它會引起皮膚損傷。麥科勒姆說，如果猴痘可以無症狀地傳播，那將特別令人不安，因為這會使病毒更難追踪。
另一個謎團是為什麼幾乎所有病例群都包括 20-50 歲的男性，其中許多是同性戀、雙性戀和男男性行為者 (GBMSM)。 Rimoin 說，雖然猴痘並不被認知為透過性傳播，但性活動肯定構成密切接觸。 MacIntyre 說，對於這種意想不到的傳播模式，最可能的解釋是該病毒偶然被引入 GBMSM 社區，並且該病毒繼續在那裡傳播。一旦完成流行病學調查（這可能需要數週時間並涉及嚴格的接觸者追踪），科學家們將可更好地了解疫情的起源和感染的風險因素。
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遏制策略
自從 1970 年代根除天花（其表親病毒）的運動結束以來，科學家們一直在密切關注猴痘。由於全球範圍內的疫苗接種，天花不再是威脅，公共衛生官員就停止推薦天花疫苗接種——而這項接種也將猴痘阻絕於外。 MacIntyre 說，自從天花被根除後，隨著每一年過去，對這病毒的免疫力減弱或沒有免疫力的人數都在增加。
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草原犬鼠模型為解決猴痘病毒提供了希望
自從那時之後已經爆發了幾次。例如，剛果民主共和國幾十年來一直在與猴痘作鬥爭，而奈及利亞自 2017 年報告了第一例超過 39 歲的病例以來，一直在經歷大規模爆發，約有 500 例疑似病例和 200 多例確診病例。美國於2003 年也報告了一波突發的疫情，當時來自加納的一批囓齒動物將病毒傳播給伊利諾伊州的寵物草原土撥鼠，並感染了 70 多人。
然而，公共衛生當局對猴痘並非無能為力。作為對生物恐怖主義的預防措施，美國等國家維持著天花疫苗的供應，以及一種被認為對病毒非常有效的抗病毒治療。不過，麥科勒姆說，這些療法可能不會大規模部署。醫護人員可能會改用一種稱為「環形疫苗接種」的方法來控制猴痘的傳播：這將會為感染猴痘的人之密切接觸者接種疫苗，以切斷任何傳播途徑。
根據她迄今為止所看到的數據，麥科勒姆認為，目前暴發的疫情除了環形接種之外可能不需要其他的遏制策略。「即使在每天都會發生猴痘的地區」，她說，「它仍然是一種相對罕見的感染」。
Monkeypox goes global: why scientists are on alert
Scientists are trying to understand why the virus, a less lethal relative of smallpox, has cropped up in so many populations around the world.
•Max Kozlov / NEWS 20 May 2022 / Nature
The monkeypox virus (shown here in a coloured transmission electron micrograph) is closely related to the smallpox virus.Credit: UK Health Security Agency/Science Photo Library
More than 120 confirmed or suspected cases of monkeypox, a rare viral disease seldom detected outside of Africa, have been reported in at least 11 non-African countries in the past week. The emergence of the virus in separate populations across the world where it doesn’t usually appear has alarmed scientists — and sent them racing for answers.
“It’s eye-opening to see this kind of spread,” says Anne Rimoin, an epidemiologist at the University of California Los Angeles, who has studied monkeypox in the Democratic Republic of the Congo for more than a decade.
Smallpox and other viruses plagued humans much earlier than suspected
Called monkeypox because researchers first detected it in laboratory monkeys in 1958, the virus is thought instead to transmit from wild animals such as rodents to people — or from infected people. In an average year, a few thousand cases occur in Africa, typically in the western and central parts of the continent. But cases outside Africa have been limited to a handful that are associated with travel to Africa or with the importation of infected animals. The number of cases detected outside of Africa in the past week alone — which is all but certain to increase — has already surpassed the number detected outside the continent since 1970, when the virus was first identified as causing disease in humans. This rapid spread is what has scientists on high alert.
But monkeypox is no SARS-CoV-2, the coronavirus responsible for the COVID-19 pandemic, says Jay Hooper, a virologist at the US Army Medical Research Institute of Infectious Diseases in Fort Detrick, Maryland. It doesn’t transmit from person to person as readily, and because it is related to the smallpox virus, there are already treatments and vaccines on hand for curbing its spread. So while scientists are concerned, because any new viral behaviour is worrying — they are not panicked.
Unlike SARS-CoV-2, which spreads through tiny air-borne droplets called aerosols, monkeypox is thought to spread from close contact with bodily fluids, such as saliva from coughing. That means a person with monkeypox is likely to infect far fewer close contacts than someone with SARS-CoV-2, Hooper says. Both viruses can cause flu-like symptoms, but monkeypox also triggers enlarged lymph nodes and, eventually, distinctive fluid-filled lesions on the face, hands and feet. Most people recover from monkeypox in a few weeks without treatment.
On 19 May, researchers in Portugal uploaded the first draft genome of the monkeypox virus that was detected there, but Gustavo Palacios, a virologist at the Icahn School of Medicine at Mount Sinai in New York City, emphasizes that it’s still a very early draft, and more work needs to be done before drawing any definitive conclusions.
World’s second-deadliest Ebola outbreak ends in Democratic Republic of the Congo
What researchers can tell from this preliminary genetic data is that the monkeypox virus is related to a viral strain predominantly found in western Africa. This strain causes milder disease and has a lower death rate — about 1% in poor, rural populations — compared with the one that circulates in central Africa. But exactly how much the strain causing the current outbreaks differs from the one in western Africa — and whether the viruses popping up in various countries are linked to one another — remains unknown.
Answers to those questions could help determine if the sudden uptick in cases stems from a mutation that allows this monkeypox virus to transmit more readily than those of the past, and if each of the outbreaks traces back to a single origin, says Raina MacIntyre, an infectious disease epidemiologist at the University of New South Wales in Sydney, Australia. Unlike SARS-CoV-2, a rapidly-evolving RNA virus whose variants have regularly eluded immunity from vaccines and prior infection, monkeypox virus is a relatively large DNA virus. DNA viruses are better at detecting and repairing mutations than RNA viruses, which means it’s unlikely that the monkeypox virus has suddenly mutated to become adept at human transmission, MacIntyre says.
‘Deeply concerning’
Still, for monkeypox to be detected in people with no apparent connection to one another suggests that the virus might have been spreading silently — a fact that Andrea McCollum, an epidemiologist who heads the US Centers for Disease Control and Prevention poxvirus team calls “deeply concerning”.
Unlike SARS-CoV-2, which can spread asymptomatically, monkeypox does not usually go unnoticed when it infects a person, in part because of the skin lesions it causes. If monkeypox could spread asymptomatically, it would be especially troubling because it would make the virus harder to track, McCollum says.
Another puzzle is why almost all of the case clusters include men aged 20–50, many of whom are gay, bisexual and have sex with men (GBMSM). Although monkeypox isn’t known to be sexually transmitted, sexual activity certainly constitutes close contact, Rimoin says. The most likely explanation for this unexpected pattern of transmission, MacIntyre says, is that the virus was coincidentally introduced into a GBMSM community, and the virus has continued circulating there. Scientists will have a better idea of the origin of the outbreaks and the risk factors for infection once an epidemiological investigation is complete, which can take weeks and involves rigorous contact tracing.
Containment strategies
Scientists have been keeping an eye on monkeypox ever since an eradication campaign for smallpox, its cousin virus, wound down in the 1970s. Once smallpox was no longer a threat thanks to worldwide vaccinations, public-health officials stopped recommending smallpox inoculation — which also kept monkeypox at bay. With each year that has passed since smallpox’s eradication, the population with weakened or no immunity to these viruses has grown, MacIntyre says.
Prairie-dog model offers hope of tackling monkeypox virus
There have been a few outbreaks since then. The Democratic Republic of the Congo, for example, has been grappling with monkeypox for decades, and Nigeria has been experiencing a large outbreak, with about 500 suspected and more than 200 confirmed cases, since 2017, when the country reported its first case in more than 39 years. The United States also reported an outbreak in 2003, when a shipment of rodents from Ghana spread the virus to pet prairie dogs in Illinois and infected more than 70 people.
Yet public-health authorities are not powerless against monkeypox. As a precaution against bioterrorism, countries such as the United States maintain a supply of smallpox vaccines, as well as an antiviral treatment thought to be highly effective against the virus. The therapies probably wouldn’t be deployed on a large scale, though, McCollum says. Health-care workers would probably instead use a method called ‘ring vaccination’ to contain the spread of monkeypox: this would vaccinate the close contacts of people who have been infected with monkeypox to cut off any routes of transmission.
On the basis of the data that she has seen so far, McCollum thinks the current outbreaks probably won’t necessitate containment strategies beyond ring vaccination. “Even in areas where monkeypox occurs every day,” she says, “it’s still a relatively rare infection.”
doi: https://doi.org/10.1038/d41586-022-01421-8