The origin of swine flu
 
YOU are now officially permitted to blame the pigs. When a strain of influenza with pandemic potential struck in April, it was generally referred to as “swine flu” because it seemed similar to an existing group of strains, known as A/H1N1, which are commonly found in pigs. But when it became clear that the new bug was being spread by people, not porkers, the pig-breeding industry complained that it was being unfairly maligned. It also became apparent that the new virus contains bits and pieces derived from avian and human strains of influenza, as well as porcine ones, further muddying its origins.

A new study, however, suggests pigs really were to blame. Several dozen researchers, led by Rebecca Garten of America’s Centres for Disease Control and Prevention (CDC), sequenced full or partial genomes of 76 samples of the new virus, which has afflicted almost 13,000 people around the world so far. In a paper published in Science, they confirm that the closest genetic relatives of the new virus are swine-flu strains from both North America and Eurasia. The virus is made of eight gene segments of known provenance but which have not previously been seen in this combination. The genetic material in them is indeed a hotchpotch derived from avian, human and swine sources, but all eight segments come most recently from pigs.

On the positive side, the researchers discovered that the antigens (proteins that provoke an immune response) found in different samples have remained similar to each other. That, according to Derek Smith of Cambridge University, one of the paper’s authors, may make it easier to design a vaccine against the new virus. The World Health Organisation and a number of European governments are now talking to manufacturers about expediting the development of such a vaccine and, on May 22nd, American officials announced a $1 billion scheme with the same goal.

The worrying news is that the study found the genetic material in the new influenza is significantly different from that in its known close relatives. This, they conclude, means the bug was circulating and evolving undetected in swine for quite some time before the first people were infected. In other words, the early-warning systems did not work.

Another cause for concern is that Canadian officials recently confirmed that the new strain has hopped from humans back into pigs in Alberta. Because swine are natural mixing vessels for influenza strains from different species (hence the hotchpotch found by Dr Garten and her colleagues), the fear is that a deadlier form could emerge and jump back into humans.

This new study does not answer the big questions of how, exactly, the virus crossed over to humans and why it kills some people and not others—in particular, why it hits the young (and thus, presumably, healthy) harder than the elderly. A different study by the CDC has found that nearly two-thirds of swine-flu infections in America have been in people aged between five and 24, whereas only 1% of cases affected those over 65. This is the reverse of the pattern seen in seasonal flu, which kills thousands of old people every winter. One possible explanation, according to Anne Schuchat of the CDC, is that “older adults might have been in contact a long time ago with a virus similar to the one we see now.” That, she surmises, might give them an immunity to this new menace that young people lack.

豬流感的起源
 
你現在可以正式把豬流感歸咎于豬了。在四月份出現了一種具有大規模流行病爆發可能性的流感病毒，我們一般把它稱之為“豬流感”，這是因為這種病毒和存在于豬體內的A/H1N1病毒類似。但是，當人們發現這種新的病毒是通過人傳播，而不是通過豬傳播的時候，生豬飼養行業的人抱怨說，他們受到了不公正的指責。 人們還清楚地發現，這種新的病毒除了含有豬類流感病毒的片段外，含有來自禽類和人類流感病毒的一些片段，這使得要弄清豬流感病毒的起源就更加不容易了。
 
但是，一項新的研究表明，豬流感確實要歸咎于豬。美國疾病控制和預防中心（America’s Centres for Disease Control and Prevention (CDC)）的Rebecca Garten領導的幾十名研究者測定了這種新病毒的76個樣本全部或者部分的基因組序列。這種新的病毒到目前為止致使全球將近13000人染病。他們的研究結果發表于《科學》（Science）雜志上，他們確認這種新病毒最近的基因物質是來自北美和歐亞的豬流感病毒類型。這種病毒由8個來源已知的基因片段組成，但是以前沒有發現過這種基因組合。這些病毒中的基因物質確實是一個大雜燴，它們來自禽類、人類和豬，但是所有8個片段的基因物質主要最近來自于豬。

有利的一面是，研究者們發現，在所有不同樣品中發現的抗原（引發免疫反應的蛋白質）相互之間仍然類似。根據劍橋大學（Cambridge University）Derek Smith（他是這篇論文的作者之一）的觀點，這可能使得設計針對這種新病毒的疫苗要容易一些。世界衛生組織（World Health Organisation）以及很多歐洲政府目前正在與生產商洽談有關加速開發這種疫苗的事情，美國官員在5月22日也宣布了一項旨在開發這種疫苗的項目，投資達10億美元。

令人擔憂的消息是，上述研究發現，這種新流感病毒的基因物質與其相近的已知病毒物種的基因物質顯著不同。研究者得出結論是，這意味著這種病毒在人類第一次感染前就已經在豬體內傳播并且逐漸演變了相當長一段時間，而人們對這些并不知曉。換句話說，早期的預警系統不起作用。

另外一個讓人擔心的原因是，加拿大官員最近確認，在阿爾伯塔（Alberta）這種新的病毒已經從人類“跳回”到豬身上。因為對于來自不同物種的流感病毒來說，豬是一個天然的混合媒介（即Garten博士和她的同事發現的大雜燴），因此人們害怕一種更加致命的病毒形式可能會出現，并且這種病毒可能會“跳回”至人類身上。
 
這項新的研究沒有回答一些重要的問題：這種病毒到底是如何傳到人類身上的；為什么這種病毒使一些人致死，而另外一些人則幸免——確切地說，相對于老年人，為什么年輕人（因此可以認為更健康一些）受這種病毒的攻擊更甚，CDC的另外一項研究發現，在美國，將近三分之二的豬流感患者的年齡在5至24歲之間，而年齡超過65歲的患者病例只有1%。這和季節性流感導致的情形正好相反，每年冬天，季節性流感導致幾千老年人死亡。根據來自CDC的Anne Schuchat的觀點，一個可能的解釋是：“老年人可能在很早之前的時間里已經和某些病毒接觸過，而這些病毒與我們目前所看到的病毒是類似的。”她推測到，這可能使他們對這種新的病毒產生一種免疫力，而年輕人則缺乏這種免疫。

Vocabulary:

Permit：允許
Strain：（疾病的）類型
Influenza：流感；簡寫為flu
Pandemic：大規模流行病的
Porker：食用豬
Malign：（公開地）毀謗，中傷
Avian：鳥的；鳥類的
Porcine：豬的
Muddy：（動詞）使渾濁
Sequence：按順序排列
Genome：基因組
Afflict：使痛苦；折磨
Hotchpotch：大雜燴
Antigen：抗原
Provoke：激起；引發
Vaccine：疫苗
Expedite: 加速
Billion：十億
Infect：感染
Surmise：推測；猜測
Immunity：免疫
Menace：危險