A Clue Is Found on Why Bird Flu Is Slow to Spread Among Humans
Scientists have a new clue why the feared H5N1 strain of the avian-influenza virus hasn't spread easily among people.
According to research published this week by teams in the U.S. and the Netherlands, the bird virus primarily infects cells deep in the human lung, possibly making it difficult for the germ to spread.
The H5N1 avian-flu virus was first detected in Hong Kong in 1997, and has since claimed millions of birds and more than 100 human lives.
The virus has continued to spread across the globe and has been detected in birds in both Europe and Africa, although not yet in the U.S.
Humans have very limited immunity to such avian viruses, and health authorities say that if the bird-flu virus begins to spread between people, it could spark a global outbreak of deadly respiratory disease.
That hasn't happened, and the new findings may help explain why.
Using tissues from human cadavers, researchers at the Erasmus Medical Center in Rotterdam studied which cells the bird virus would become attached to. According to Thijs Kuiken, a veterinary pathologist who directed the research, H5N1 attached to cells deep in the lung, but not cells in the throat, where human-flu viruses multiply. The study appears in the journal Science.
Dr. Kuiken called the finding a possible explanation for why the avian virus doesn't jump easily between people. "It must reach the lower respiratory tract to replicate, and it's harder to spread by coughing and sneezing," he said.
Other doctors questioned that conclusion, saying that patients with such infections were likely to cough more heavily. "I don't think it directly affects the transmissibility just because it's in the lower respiratory tract," said Nikki Shindo, an influenza expert at the World Health Organization who has studied human cases, most recently in Turkey.
Dr. Shindo said that if the virus has to reach deep into the lung before causing an infection, that could make it harder to catch.
In a separate study released yesterday by the journal Nature, a research group headed by Yoshihiro Kawaoka of the University of Wisconsin-Madison reached similar conclusions after analyzing the specific molecules on human cells to which influenza viruses attach.
They found that molecules preferred by H5N1 are expressed mostly by cells deep in the lung. To spread between humans, the virus would need to adapt in a way that enabled it to attach to a different type of molecule present in cells higher in the respiratory tract.
"This restriction may contribute to the inefficient human-to-human transmission of H5N1 viruses seen to date," Dr. Kawaoka wrote in his report.
最新研究发现人类不易感染禽流感的原因
科学家对令人谈之色变的H5N1型高致病禽流感病毒很难在人类之间传播的原因有了新发现。
根据美国和荷兰的科研团队本周发表的研究报告，禽流感病毒主要感染人类肺部深处的细胞，这一点可能使病毒很难在人类中传播。
更多信息
? 禽流感专题H5N1型禽流感病毒于1997年在香港首次被发现，至今已导致数百万禽类死亡，并夺走了100多人的生命。目前禽流感病毒继续在全球扩散，尽管尚未登陆美国，但欧洲和非洲都已在鸟类身上发现禽流感病毒。
人类对禽流感的免疫力极其有限。卫生监管机构说，一旦禽流感开始在人类间传播，它将引发一场全球性的致命性呼吸疾病疫情。但这一场景尚未发生，科学家的最新发现或许可以给出解释。
鹿特丹Erasmus Medical Center的研究人员利用人类尸体上的细胞组织来研究禽流感病毒会附著在哪里。主持这次研究的负责人、兽类病理学家Thijs Kuiken说，H5N1型禽流感病毒不像普通流感病毒那样在喉咙细胞上繁殖，它附著在人类肺部深处的细胞上。这篇研究报告在《科学》(Science)期刊上发表。
Kuiken博士称这个新发现可能是禽流感病毒不容易在人类之间传播的原因。他说：“病毒必须到达下呼吸道才能繁殖，较难通过咳嗽和打喷嚏传播。”
其他研究人员对这一结论提出质疑，他们反驳说，禽流感病人会咳嗽得更厉害。世界卫生组织(World Health Organization)流感专家Nikki Shindo说：“我认为禽流感病毒附著在下呼吸道的特点不会直接影响它的传染性。”Shindo博士研究人感染禽流感的病例，最近在土耳其进行研究。他说，如果禽流感病毒必须到达肺部深处才能感染人体，那么人类才会较难感染禽流感。
本周三发表在《自然》(Nature)期刊上的另一篇研究报告中，麦迪逊威斯康辛大学(University of Wisconsin-Madison)的Yoshihiro Kawaoka带领的研究小组在分析了禽流感病毒附著的人类细胞之后，得出了类似的结论。
他们发现H5N1容易附著的分子主要集中在肺部深处的细胞中。要想在人类之间传播，病毒需要附著在上呼吸道的分子上。
Kawaoka博士在他的报告中写道：“这种局限性可能是H5N1型病毒迄今为止难以在人类之间传播的原因。”
