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The next flu pandemic may be hibernating in an Arctic glacier or frozen Siberian lake, waiting for rising temperatures to set it free. Then birds can deliver it back to civilization.
New research suggests an influenza virus could go into hiding in the ice when earlier generations of humans, birds or other hosts developed immunity strong enough to drive the virus to extinction. It’s a sort of evolutionary loophole.
“It can bring a set of viral genes back to life that have been frozen for centuries or thousands of years,” said environmental biologist Scott Rogers of Bowling Green State University in Ohio. “If hosts haven’t seen the virus in a while, then there may be no active immunity.”
Rogers and Zeynep Ko?er, of Bowling Green State University in Ohio, found that influenza viruses can easily survive freezing in pond water, and emerge from the melting ice strong enough to infect bird eggs. They presented their latest evidence today at the American Society for Microbiology meeting in Philadelphia.
Rogers calls this evolutionary strategy “genome recycling.” He thinks migrating waterfowl regularly deliver influenza viruses to Arctic glaciers and lakes, where it becomes frozen in ice. When the ice melts, birds pick the virus up and transport it back south where it can infect humans.
The research comes amid a global alert over a new swine flu strain, H1N1, that has so far killed at least 80 people and could be headed toward full-blown pandemic. Influenza pandemics have struck periodically in historic times. The worst in recent memory were the Spanish flu in 1918, the Asian flu in 1957 and the Hong Kong flu in 1968.
These pandemics are hard to predict or trace back to their origins. Some researchers have proposed Siberia as a hub for the evolution of flu pandemics that eventually emerge in other locations — carried there by birds.
Scientists have in fact detected influenza viruses frozen in the ice and mud of lakes in Alaska, Siberia and elsewhere. These Arctic lakes are the summer grounds for ducks that migrate to China, Southern Asia, Europe and North America.
Dany Shoham, who studies biological warfare at the Begin-Sadat Center for Strategic Studies in Bar-Ilan University in Israel, first sidled up to the idea that influenza viruses may hide in ice during the 1990s. As influenza viruses pass from one person, or bird, to another, they normally pick up random changes in their genes because of errors in viral replication. This “genetic drift” happens at a constant rate.
But Shoham noticed something strange: Influenza viruses isolated decades apart sometimes showed little sign of genetic drift. One strain that came from Russia in 1977, was nearly identical to a strain of the virus last seen in 1950.
“In some cases,” he said, “they are absolutely identical.”
To Shoham, it seemed as though these viruses spent the intervening decades not infecting birds or people, but rather frozen in suspended animation — something like Buck Rogers spending 500 years drifting in space.
Shoham and Rogers believe that ice provides a perfect explanation. When they tested their theory with Siberian lake ice in 2006, they found an influenza virus almost identical to one that had infected people in the 1930s, and again in the 1960s.
“This phenomenon may take place regularly,” Shoham said, “far beyond what we witness.”
They are now trying to prove the viruses found in lake ice can actually survive well enough to re-infect birds when the ice melts. So far it has been shown only in lab experiments, but there’s already some indication that influenza has evolved a special capacity for surviving cold.
When cells and viruses are cooled, their membranes often change suddenly — similar to the way water molecules reorganize during freezing — and this can rupture the membrane and kill the cell. So, biophysicist Joshua Zimmerberg of the National Institutes of Health cooled an influenza virus below freezing while monitoring the properties of its membrane coating using a new technique called “magic angle spinning nuclear magnetic resonance.”
But the membrane coating of influenza was “like none we had ever looked at before,” said Zimmerberg, who published his results last year in Nature Chemical Biology. Influenza’s membrane capsule gradually hardened from an oily fluid into a hardened gel, without sudden changes. “It’s remarkably stable with freezing and thawing,” he said. “That’s the unique thing about influenza.”
The idea that influenza may hide out in ice has struck a chord among some experts. “One of the challenges is where does this virus persist between pandemics?” said virologist Richard Slemons of Ohio State University, who has studied bird flu for 35 years. “The idea needs to be considered and explored.”
Rogers believes global surveillance for influenza outbreaks should keep an eye on Arctic ice. Many other viruses may have evolved to lay dormant in ice when their host populations develop resistance, says Shoham. He suspects waterborne viruses such as polio, hepatitis A, and rotavirus could all potentially survive in ice. Even smallpox — a virus against which Americans are no longer routinely vaccinated — might survive in the bodies of victims buried in Arctic permafrost.
Meanwhile, Rogers and John Castello of State University of New York in Syracuse have isolated a plant virus called tomato mosaic virus from Greenland glacial ice up to 140,000 years old. “It’s our opinion that they are probably still viable,” says Rogers, “but we weren’t able to show that.”
And Ko?er is screening ice from Antarctic lakes that have remained frozen for at least hundreds of years, using a technique which can detect any type of virus — whether they infect, plants, animals, or bacteria.
“I want to see everything,” she says. One preliminary run turned up genetic sequences for what could be over 100 viruses.
未来的流感大流行可能蛰伏在北极冰川或冻结的西伯利亚湖泊之中，等到气温升高就会复活，然后由鸟类重新传播到文明世界。
新的研究指出，因为早期的人类祖先、鸟类或其它宿主体内产生了很强的免疫力，足以剿灭病毒，所以流感病毒可能藏匿到冻冰之中。这是进化的一个漏洞。
“ 它可能使已经冰冻几个世纪或数千年的一组病毒基因恢复生机，”俄亥俄州立鲍灵格林大学环境生物学家斯科特·罗杰斯说，“如果宿主在一段时间内意识不到病毒的存在，就不可能产生主动免疫力。”
俄亥俄州立鲍灵格林大学的罗杰斯和Zeynep 发现，冻结在池塘水中的流感病毒很容易复活，摆脱融冰的病毒也很强壮，足以感染禽蛋。他们在费城召开的美国微生物学会会议上，出示了最新获得的证据。
罗杰斯称这种进化策略为“基因组的回收利用”。他认为，迁徙水鸟定期将病毒送到北极冰河和冰湖中，病毒在那里冻入冰中。冰融化之后，水鸟又带上病毒，把它运回南方，感染人类。
此项研究开始于全球警惕新型猪流感病毒H1N1期间。H1N1至今已经至少杀死八十人，可能会导致流感大流行的全面爆发。历史上的流感大流行曾经给我们留下周期性的印象。我们印象中最严重的是1918年西班牙流感、1957年亚洲流感和1968年的香港流感。
科学家们已经实际检测了阿拉斯加、西伯利来和其他地方湖中冰块及淤泥中的流感病毒。那里是迁徙至中国、南亚、欧洲和北美的野鸭的避暑胜地。
丹尼·肖海姆在以色列巴伊兰大学比金─沙达特战略研究中心研究生物战。他最先接近了这个观点，认为流感病毒可能在上世纪九十年代期间隐藏在冰中。当流感病毒由一个人或一只鸟传给另一个人或鸟时，由于病毒复制错误，它们通常会带上基因中的随机变化。这种“遗传漂变”以恒定的速率产生结果。
肖海姆注意到一些奇怪的现象：隔离了几十年的流感病毒有时表现出很少的遗传漂变迹象。来自1977年的一个病毒株几乎和1950年最后看到的病毒株一样。
他说，“在某种情况下，它们完全相同。”
肖海姆认为，似乎这几十年来病毒没有感染禽类或人类，而是冻起来假死——有点像兔子罗杰花了五百年时间漂游太空。
肖海姆和罗杰斯坚信冰冻是最完美的理由。2006年，以西伯利亚湖冰来验证自己的理论时，他们发现，有一种流感病毒和曾经在二十世纪三十年代感染人类，又在六十年代再次出现的一种病毒几乎一样。
肖海姆说，“这种现象可能会定期发生，远非我们所看到的那样。”
他们现在正在努力证明，在湖冰中找到的病毒确实能很好地活下来，在冰融化之时再次感染鸟类。到目前为止，实验还仅限于实验室，不过，已经有迹象表明病毒已经进化出了一种能在寒冷中生存的能力。
当细胞或病毒被冷却时，它们的薄膜往往会突然变化——近似于水分子在结冰过程中的重组方式——这样会挣破薄膜杀死细胞。因此，美国国立卫生研究院生物物理学家约书亚·齐默伯格将一种流感病毒冷却至冰点以下，同时运用一种“魔角自旋核磁共振”的新技术监测其薄膜层的性能变化。
齐默伯格去年在《自然化学生物学》上发表了研究结果，他说，病毒薄膜层“不像我们以前见过的任何一种”。流感病毒的膜囊没有突然变化，而是逐渐硬化，由油状液态硬化为凝胶。他说，“相对于冻结和融化来说，它非常稳定。那是病毒的独特之处。”
病毒可能隐藏在冰中的观点已经引起一些专家的共鸣。俄亥俄州立大学滤过性病原体学者理查德·西里蒙研究了三十五年禽流感。他说，“有必要考虑和探索这个观点。”
罗杰斯认为，全球对流感爆发的监测应该留意北极冰河。肖海姆说，一些病毒在其宿主产生了抵抗力时，也许已经进化出许多另类病毒蛰伏于冰中。他怀疑水传播的病毒，比如小儿麻痹症、甲型肝炎和轮状病毒（一种致婴儿和新生畜胃肠炎的病毒）都可能生存于冰中。甚至天花——美国人已经不再经常接种疫苗的病毒 ——也许还存活于深埋在北极永冻层的受害者体内。
与此同时，罗杰斯和锡拉丘兹纽约州立大学的约翰·卡斯特罗已经从高达十四年的格陵兰冰川冰中分离出一种植物病毒——花叶病毒。“我们的看法是，它们可能还有繁殖能力。”罗杰斯说，“不过，我们还不能证明。”
Zeynep 正在从至少冰冻了几百年的南极湖泊中筛选冰块。他使用了一种技术，能够检测出所有类型的病毒——无论是传染性病毒，还是植物、动物或细菌病毒。
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发表于 2016-8-12 16:29:55