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.

猪流感的起源
 
你现在可以正式把猪流感归咎于猪了。在四月份出现了一种具有大规模流行病爆发可能性的流感病毒，我们一般把它称之为“猪流感”，这是因为这种病毒和存在于猪体内的A/H1N1病毒类似。但是，当人们发现这种新的病毒是通过人传播，而不是通过猪传播的时候，生猪饲养行业的人抱怨说，他们受到了不公正的指责。 人们还清楚地发现，这种新的病毒除了含有猪类流感病毒的片段外，含有来自禽类和人类流感病毒的一些片段，这使得要弄清猪流感病毒的起源就更加不容易了。
 
但是，一项新的研究表明，猪流感确实要归咎于猪。美国疾病控制和预防中心（America’s Centres for Disease Control and Prevention (CDC)）的Rebecca Garten领导的几十名研究者测定了这种新病毒的76个样本全部或者部分的基因组序列。这种新的病毒到目前为止致使全球将近13000人染病。他们的研究结果发表于《科学》（Science）杂志上，他们确认这种新病毒最近的基因物质是来自北美和欧亚的猪流感病毒类型。这种病毒由8个来源已知的基因片段组成，但是以前没有发现过这种基因组合。这些病毒中的基因物质确实是一个大杂烩，它们来自禽类、人类和猪，但是所有8个片段的基因物质主要最近来自于猪。

有利的一面是，研究者们发现，在所有不同样品中发现的抗原（引发免疫反应的蛋白质）相互之间仍然类似。根据剑桥大学（Cambridge University）Derek Smith（他是这篇论文的作者之一）的观点，这可能使得设计针对这种新病毒的疫苗要容易一些。世界卫生组织（World Health Organisation）以及很多欧洲政府目前正在与生产商洽谈有关加速开发这种疫苗的事情，美国官员在5月22日也宣布了一项旨在开发这种疫苗的项目，投资达10亿美元。

令人担忧的消息是，上述研究发现，这种新流感病毒的基因物质与其相近的已知病毒物种的基因物质显著不同。研究者得出结论是，这意味着这种病毒在人类第一次感染前就已经在猪体内传播并且逐渐演变了相当长一段时间，而人们对这些并不知晓。换句话说，早期的预警系统不起作用。

另外一个让人担心的原因是，加拿大官员最近确认，在阿尔伯塔（Alberta）这种新的病毒已经从人类“跳回”到猪身上。因为对于来自不同物种的流感病毒来说，猪是一个天然的混合媒介（即Garten博士和她的同事发现的大杂烩），因此人们害怕一种更加致命的病毒形式可能会出现，并且这种病毒可能会“跳回”至人类身上。
 
这项新的研究没有回答一些重要的问题：这种病毒到底是如何传到人类身上的；为什么这种病毒使一些人致死，而另外一些人则幸免——确切地说，相对于老年人，为什么年轻人（因此可以认为更健康一些）受这种病毒的攻击更甚，CDC的另外一项研究发现，在美国，将近三分之二的猪流感患者的年龄在5至24岁之间，而年龄超过65岁的患者病例只有1%。这和季节性流感导致的情形正好相反，每年冬天，季节性流感导致几千老年人死亡。根据来自CDC的Anne Schuchat的观点，一个可能的解释是：“老年人可能在很早之前的时间里已经和某些病毒接触过，而这些病毒与我们目前所看到的病毒是类似的。”她推测到，这可能使他们对这种新的病毒产生一种免疫力，而年轻人则缺乏这种免疫。

Vocabulary:

Permit：允许
Strain：（疾病的）类型
Influenza：流感；简写为flu
Pandemic：大规模流行病的
Porker：食用猪
Malign：（公开地）毁谤，中伤
Avian：鸟的；鸟类的
Porcine：猪的
Muddy：（动词）使浑浊
Sequence：按顺序排列
Genome：基因组
Afflict：使痛苦；折磨
Hotchpotch：大杂烩
Antigen：抗原
Provoke：激起；引发
Vaccine：疫苗
Expedite: 加速
Billion：十亿
Infect：感染
Surmise：推测；猜测
Immunity：免疫
Menace：危险