萬病之王癌症可能會傳染嗎
For all its peculiar horror, cancer comes with a saving grace. If nothing else can stop a tumor’s mad evolution, the cancer ultimately dies with its host. Everything the malignant cells have learned about outwitting the patient’s defenses — and those of the oncologists — is erased. The next case of cancer, in another victim, must start anew.
儘管談癌色變，但癌症也有“可取之處”。就算無論如何也無法阻止腫瘤的瘋狂生長，癌最後也會和宿主同歸於盡。惡性癌細胞從擊敗患者——和腫瘤科醫生——的抵抗過程中學到的一切，都會化爲烏有。癌症的下一次攻擊，必須從下一個受害者身上重新開始。
Imagine if instead, cancer cells had the ability to press on to another body. A cancer like that would have the power to metastasize not just from organ to organ, but from person to person, evolving deadly new skills along the way.
試想，如果癌細胞有能力傳染給另一個個體，癌症將不僅有能力從一個器官轉移到另一個器官，還可以從一個人轉移到另一個人，同時演化出新的致命技能。
While there is no sign of an imminent threat, several recent papers suggest that the eventual emergence of a contagious human cancer is in the realm of medical possibility. This would not be a disease, like cervical cancer, that is set off by the spread of viruses, but rather one in which cancer cells actually travel from one person to another and thrive in their new location.
雖然沒有明確跡象顯示這是一個迫在眉睫的威脅，但近期許多論文提出，最終出現具有傳染性的人類癌症，在醫學上是有可能的。它不會是一種疾病，比如由病毒傳播引起的子宮頸癌；而是癌細胞實際上從一個人轉移到另一個人身上，並在這個新地方肆意生長。
So far this is known to have happened only under the most unusual circumstances. A 19-year-old laboratory worker who pricked herself with a syringe of colon cancer cells developed a tumor in her hand. A surgeon acquired a cancer from his patient after accidentally cutting himself during an operation. There are also cases of malignant cells being transferred from one person to another through an organ transplant or from a woman to her fetus.
迄今爲止，只在極爲罕見的狀況下發現過這種情況。一名19歲的化驗室工作人員被帶有結腸癌細胞的注射器戳傷後，她的手上長出一個腫瘤。一位外科醫生在手術工作中被意外割傷後，從患者身上感染了癌症。癌細胞通過器官移植從一個人轉移到另一個身上的案例，或者從母親轉移到胎兒身上的案例也有很多。
On each of these occasions, the malignancy went no further. The only known cancers that continue to move from body to body, evading the immune system, have been found in other animals. In laboratory experiments, for instance, cancer cells have been transferred by mosquitoes from one hamster to another. And so far, three kinds of contagious cancers have been discovered in the wild — in dogs, Tasmanian devils and, most recently, in soft shell clams.
在每一樁這樣的案例中，癌細胞都存活不了太久。所有現在已知的能躲過免疫系統，在個體之間轉移的癌症，都是在其他動物身上發現的。比如，在實驗室裏，癌細胞通過蚊子從一隻倉鼠轉移到另一隻身上。目前，在野外環境中發現了三種傳染性癌症，分別發生在犬類、袋獾，以及砂海螂身上，後者是最近發現的。
The oldest known example is a cancer that spreads between dogs during sexual intercourse — not as a side effect of a viral or bacterial infection, but rather through direct conveyance of cancer cells. The state of the research is described in a review, “The Cancer Which Survived,” published last year by Andrea Strakova and Elizabeth P. Murchison of the University of Cambridge.
我們所知道的最早的例子是一種癌在犬類之間通過交媾轉移的——不是感染病毒或細菌後導致的副作用，而是癌細胞直接從一隻犬轉移到另一隻犬體內。劍橋大學的安德烈婭·斯特拉科娃(Andrea Strakova)和伊麗莎白·P·默奇森(Elizabeth P. Murchison)去年發表的一篇名爲《離開宿主依然存活的癌症》的綜述文章，對相關研究進行了梳理。
The condition, canine transmissible venereal tumor disease, is believed to have sprung into existence 11,000 years ago — as a single cell in a single dog — and has been circulating ever since. (Why did this happen in dogs and not, say, cats? Perhaps because of what the authors demurely call the dogs’ “long-lasting coital tie” — the half an hour or so that a male and female are locked in intercourse, tearing genital tissues and providing the cancer cells with a leisurely crossing.)
這種名爲犬類生殖器傳染性腫瘤的疾病，被認爲是在1.1萬年前突然出現在一隻犬身上，而且以一個單一癌細胞的形式一直傳播至今。（爲什麼這種病會出現在犬類身上，而不是在貓等其他動物身上？原因或許在於作者用文明措辭所說的犬類之間“持久的交媾聯繫”，即在一隻母犬和一隻公犬持續約半小時的交媾過程中，會撕裂生殖器官，使腫瘤細胞比較容易通過傷口發生轉移。）
Normally a cancer evolves in a single body over the course of years or decades, accumulating the mutations that drive it to power. But to have survived for millenniums, researchers have proposed, canine cancer cells may have developed mechanisms — like those in healthy cells — to repair and stabilize their own malignant genomes.
通常一種癌症會在幾年或幾十年的時間裏一直在一個生物體內發展，突變積累到一定程度，就會開始顯現威力。但有研究人員提出，這種已經存在上萬年之久的犬類癌症細胞，有可能發展出了一些機制——就像健康細胞一樣——可以修復和穩定自身的惡性基因。
Early on, cancer cells typically flourish by disabling DNA repair and ramping up the mutational frenzy. Somewhere along the way, the age-old canine cells may have reinvented the device to extend their own longevity. There is also speculation that this cancer may have learned to somehow modify canine sexual behavior in ways that promote the disease’s spread and survival.
早前，癌症細胞往往是通過破壞DNA修復功能而快速生長，引發瘋狂的突變。中間的某個時刻，古老的犬類生殖器傳染性腫瘤細胞可能改變了行爲方式，由此延長自身的壽命。還有一種猜測是，這種癌症可能不知怎麼改變了犬類的交媾方式，使之更利於這種疾病的傳播和腫瘤細胞的存活。
The second kind of contagious cancer was discovered in the mid-1990s in Tasmanian devils, which spread malignant cells as they try to tear off one another’s faces. Though it may be hard to sympathize, devil facial tumor disease threatens the creatures with extinction.
第二種傳染性癌症是在上世紀90年代中期發現於袋獾身上。它們在相互撕咬對方的臉時，會把惡性腫瘤細胞傳給其他同類。儘管可能比較難以引起人們的同情，但這種袋獾面部腫瘤疾病的確威脅到了這一物種的生存，使它們瀕臨滅絕。
With so few examples, transmissible cancer has been easy to dismiss as an aberration. But in December, scientists at the Universities of Tasmania and Cambridge reported in Proceedings of the National Academy of Sciences that Tasmanian devils are passing around another kind of cancer — genetically distinct from the first. It’s weird enough that one such cancer would arise in the species. What are the chances that there would be two?
因爲例子非常少，所以可轉移癌症很容易被當做一種反常的現象，不予重視。但是去年12月，塔斯馬尼亞大學和劍橋大學的科學家們聯合發表在《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)上的論文表示，袋獾還在傳播另一種癌症，與前一種癌症的基因組成截然不同。這種生物身上會出現一種這樣的癌症，就已經夠奇怪的了。現在居然說有兩種，這種機率有多大？
One theory is that the animals are unusually vulnerable. Driven so close to extinction — by climate change, perhaps, or human predators — the species is lacking in genetic diversity. The cells of another devil injected through a vicious wound may seem so familiar that they are ignored by the recipient’s immune system. If some of the cells carry the mutations for the facial cancer, they might be free to flourish and develop into a new tumor.
有一種理論認爲，這種動物非常脆弱。因爲氣候變化，或許還有人類獵捕的因素，它們已經幾乎要滅絕，因此缺乏基因多樣性。通過一個嚴重的傷口進入體內的另一隻袋獾的細胞，可能會看似非常熟悉，以至被接收者的免疫系統忽略。如果這些細胞中有一部分攜帶着面部腫瘤的基因突變，它們可能就會迅速裂變，發展成一種新的腫瘤。
But the scientists also proposed a more disturbing explanation: that the emergence of contagious cancer may not be so rare after all. “The possibility,” they wrote, “warrants further investigation of the risk that such diseases could arise in humans.”
但這些科學家也提出了一種更加讓人不安的解釋：傳染性癌症可能並不那麼罕見。“這種可能性，”他們寫道，“給了我們進行進一步研究的理由，即研究這類疾病在人類身上出現的風險有多大。”
Cancer has probably existed ever since our first multicellular ancestors appeared on Earth hundreds of millions of years ago. The life spans of even the longest-lived animals may be just too brief for cancers to easily evolve the ability to leap to another body. Otherwise, contagious cancer would be everywhere.
在我們最早的多細胞生物祖先於數億年前在地球上出現時，癌症可能就已經存在。就算是最長壽的動物，其生命週期對於癌症而言也太過短暫，讓它來不及演化出跳轉到另一種生物身上的能力。否則，現在就到處都是傳染性癌症了。
For now, at least, it remains a curiosity. Consider the case of a 41-year-old man in Medellin, Colombia, who was examined by doctors in 2013 because of fatigue, fever and weight loss. His lymph nodes were clogged with cancer cells that had also spread to his lungs and liver.
至少就目前來說，它還是一種很罕見的現象。比如發生在哥倫比亞麥德林一名41歲男子身上的事。2013年他因出現疲勞、發燒和體重下降等症狀而接受醫生的檢查，結果發現他的淋巴結里長滿了癌細胞，還轉移到了他的肺部和肝部。
Yet the cells looked far too small and simple to be human. “This case posed a diagnostic conundrum,” the doctors wrote in November in The New England Journal of Medicine.
但是，相對於人類而言，這些癌細胞顯得太小，結構也太過簡單。“這個病例成了一個難解的謎，”這些醫生在去年11月發表於《新英格蘭醫學雜誌》(The New England Journal of Medicine)上的文章中寫道。
The solution to the puzzle came when the man was also found to be harboring a tapeworm called Hymenolepis nana. Further analysis concluded that the cancer cells had originated in the parasite and then metastasized through the man’s body.
當這名男子被發現還攜帶有微膜殼絛蟲時，謎底解開了。進一步的分析推斷出，這名男子身上的癌細胞最初存在於這種寄生蟲內，然後通過他的身體進行了轉移。
There is no reason to think that tapeworm cancer is about to become a threat to public health. The patient’s immune system had been compromised by H.I.V., and he died several months later.
沒有理由認爲，絛蟲癌症即將對我們的公共衛生安全構成威脅。這名病人的免疫系統之前就已經因爲感染艾滋病毒而遭到了破壞，而且查出這種癌症幾個月後，他也去世了。
But nature is infinite in its surprises.
只不過，大自然總會有無限多的意想不到。
