The deadly H1N1 influenza virus that’s fueling fears of a global pandemic is a hybrid of two common pig flu strains, scientists who have studied the disease told Wired.com Tuesday. Earlier reports called it a combination of pig, human and avian influenza strains.

The findings may resolve some uncertainty about the nature of the virus, but much is still unknown about its origins and effects.

“This is what we call a reassortment between two currently circulating pig flu viruses,” said Andrew Rambaut, a University of Edinborough viral geneticist. “Why it’s emerged in humans is anyone’s guess. It hasn’t been seen before in pigs as far as I know.”

Rambaut analyzed the gene sequences of viral samples taken from two infected California children. The samples were collected by the Centers for Disease Control and Prevention, and made available to researchers through an international database of flu genomes.

His conclusions were echoed by Eddie Holmes, a virus evolution specialist at the University of Pennsylvania, and Steven Salzberg, a University of Maryland bioinformaticist. Both have looked at the CDC-provided sequences. The CDC could not be reached for comment, but a document released to scientists and obtained by Wired.com affirms their analysis.

Researchers believe the samples from California represent the same viral strain as one that is believed to have killed as many as 150 of an estimated 1,600 hospitalized Mexicans, and caused hundreds more infections worldwide, including at least 64 in the United States. However, as samples from Mexico have not yet been sequenced, the similarity is not conclusive.

The two strains whose genes are found in the California samples belong to influenza families known generally as North American and Eurasian pig flu. The former was first described in the 1930s, and the latter in 1979. The Eurasian strain is generally found in Europe and Asia, rather than North America.

Neither of the strains have ever proven contagious in humans. One of the genes inherited from the Eurasian strain has reportedly never been seen in humans. It codes for the neuraminidase enzyme — the N1 in H1N1 — which controls the expansion of the virus from infected cells.

“The new neuraminidase gene that came in from Eurasian swine is one we’ve never before seen circulating in humans,” said Rambaut. “That’s one of the reasons it’s spreading rapidly. Very few people will have any immunity to this particular combination, which is what gives the concern that this will be a pandemic rather than just a normal seasonal flu outbreak. It remains to be seen how much and to what extent there is existing immunity.”

In medical terms, the genetic origins of the virus may not matter. Whether it come solely from pigs rather than a mix of pigs, birds and humans doesn’t change its immunological novelty.

However, understanding the origins could eventually help scientists determine how the virus evolved and where it originally emerged.

The earliest cases occurred in the town of La Gloria in the Mexican state of Veracruz, not far from a large and notoriously unsanitary hog farm operated by Granjas Carroll, a subsidiary of giant American food company Smithfield Foods.

Vercruz residents and some journalists have alleged that the virus could have evolved in the farm’s pigs, then passed into humans through water or insects tainted by infected waste. Many researchers, including the authors of a report issued last year by the Pew Commission on Industrial Farm Animal Production, have warned that unsanitary conditions at industrial hog farms could prove a breeding ground for new forms of influenza.

The World Health Organization has sent inspectors to the Granjas Carroll farm. The results of the investigation have not been announced. Smithfield issued a press release on Saturday stating that “it has found no clinical signs or symptoms of the presence of swine influenza in the company’s swine herd or its employees at its joint ventures in Mexico.” The company declined further comment, though CEO Larry Pope told USA Today that “(The term) swine flu is a misnomer.”

Rambaut, Holmes and Salzberg declined to speculate on whether the new H1N1 virus evolved on a hog farm or specifically in the Granjas Carroll facility.

However, it seems likely that pigs were the original host.

“That’s a logical conclusion,” said Salzberger. “It was probably two different pigs, or one who got co-infected from others. The two strains mixed, and now you have a brand-new strain.”

“Presumably somewhere there was a pig infected with both forms. We don’t know where or when. It could have been circulating in this form for a while,” said Rambaut.

What comes next is anyone’s guess.

“Influenza virus mutates remarkably rapidly so there is no doubt that the virus will mutate and evolve in humans,” said Holmes. “Quite what this evolution will result in is difficult to tell.”

致命的、引发全球担忧大流行的 H1N1 流感病毒是两种常见的猪流感病毒的杂交品种，研究过这种疾病的科学家在星期二告诉 Wired.com 说。早先的报道称其为猪、人和禽流感菌株的结合。

这一发现可能解决关于这种病毒性质的一些不确定性问题，但是有关它的起源和影响还有许多不明之处。

“这就是我们称之为在两种目前流行的猪流感病毒之间的重组物的东西 ”Edinborough 大学的病毒遗传学家安德鲁 拉姆鲍特说。 “任何人都在猜测为什么它在人类中出现。据我所知，以前还没有在猪中见过它。”

拉姆鲍特分析了来自加利福尼亚州两个感染儿童的病毒样本的基因序列。样本是由疾病控制和预防中心收集的，通过一个国际流感基因组数据库提供给研究人员。

他的结论得到了宾夕法尼亚大学病毒演变专家埃迪 霍尔姆斯和马里兰大学生物信息学家史蒂芬 萨尔茨贝克的附和。两者都查看过疾病预防和控制中心提供的序列。该中心还没有就此发表评论，但是向科学家们公布的并且由 Wired.com 获得的一份文件申明了他们的分析。

研究人员们认为，来自加利福尼亚州的样本代表了与造成多达 150 名墨西哥人死亡，估计 1600 名墨西哥人进医院治疗，并且引起世界范围内数百人感染，其中包括在美国至少 64 人感染的同样的病毒菌株。然而，来自墨西哥的样品尚未排序，类似性尚未确证。

在加利福尼亚州的样本中发现其基因的这两个菌株，属于一般称为北美猪流感和欧亚猪流感的流感系列。前者首次在 20 世纪 30 年代被描述过，后者在 1979 年被描述过。欧亚菌株一般在欧洲和亚洲发现，而不是北美。

这两类菌株都未曾被证明过在人类中是传染性的。其中一个从欧亚菌株遗传的的基因据报告从未在人类中见过。它编码为神经氨酸酶——H1N1 中的 N1，它控制病毒从感染细胞的扩展。

“这种来自欧亚猪的新的神经氨酸酶基因是一种我们从未见过在人类中流行的基因，”拉姆鲍特说。 “这是它迅速蔓延的原因之一。极少数人会对这种特定的组合有免疫力，它让我们关注的是，这会是一个大流行，而不仅仅是通常的季节性流感暴发。现有的免疫性有多少以及达到多大的程度，还有待观察。”

病毒的遗传起源在医疗方面可能并不重要。不论它仅仅是来自猪还是来自猪、鸟类和人类的混合，都不改变其免疫新颖性。

然而，了解起源可能最终帮助科学家们确定病毒如何演变以及它最初是在哪里出现的。

最早的病例发生在墨西哥韦拉克鲁斯州的拉格洛里亚镇，离一个大的、人所周知的不卫生的养猪场不远，这个养猪场由巨头美国食品公司史密斯菲德食品公司的一家子公司格兰加斯 卡罗尔公司所经营。

韦拉克鲁斯的居民和一些记者声称，病毒可能是在农场的猪中演变的，然后通过受感染的废物所污染的水或者昆虫传递到人类。许多研究人员，其中包括去年通过佩尤产业家畜生产委员会发表一份报告的作者，已经警告过，产业养猪场的不卫生条件，可能被证明是新流感形式的一个温床。

世界卫生组织已经派出检查人员到格兰加斯卡罗尔公司的农场。调查结果还没有对外公布。史密斯菲德公司上周六发出的新闻稿声称，“在它在墨西哥的合资企业中，公司的猪畜群或者其雇员，没有发现任何存在猪流感的临床体状或者症候。”该公司拒绝作进一步的评论，尽管首席执行官拉里 珀布告诉《今日美国》说“猪流感（这个词）是一个误称。”

拉姆鲍特、霍尔姆斯和萨尔茨贝克拒绝推测是否新的 H1N1 病毒是在养猪场或者具体是在格兰加斯 卡罗尔公司的设施内演变的。

然而，似乎可能猪是最初的宿主。

“这是合乎逻辑的结论，” 萨尔茨贝克说。“这可能是两种不同的猪，或者是一种从另一种得到并合感染的猪。两种菌株混合，现在就有了一种全新的菌株。”

“大概某处有过一头感染了两种形式菌株的猪。我们不知道是在哪里或者是在什么时候。它可能已经以这种形式传播了一段时间，” 拉姆鲍特说。

任何人都在猜测下一步出现什么。

“流感病毒变异非常迅速，所以毫无疑问，该病毒会在人类中发生变异和演化，”霍尔姆斯说。 “难以讲这种演变完全会导致什么。”