A novel flu virus has struck hundreds of people in Mexico, and at least 18 have died. It has also infected eight people in the US, and appears able to spread readily from human to human. The World Health Organization is calling an emergency meeting to decide whether to declare the possible onset of a flu pandemic.
Ironically, after years of concern about H5N1 bird flu, the new flu causing concern is a pig virus, of a family known as H1N1.
Flu viruses are named after the two main proteins on their surfaces, abbreviated H and N. They are also differentiated by what animal they usually infect. The H in the new virus comes from pigs, but some of its other genes come from bird and human flu viruses, a mixture that the US Centers for Disease Control and Prevention calls "very unusual".
On Wednesday, the CDC announced that routine surveillance had uncovered mild flu cases during late March and April, caused by a novel swine flu virus. Those affected, aged 9 to 54, live in and around San Diego, California, and San Antonio, Texas, near the Mexican border. None was severe. Symptoms were normal for flu, with more nausea and diarrhoea than usual.
Mongrelised mix
On Thursday, Canadian public health officials warned Canadians travelling to Mexico of clusters of severe flu-like illness there. Then on Friday the WHO in Geneva said in a statement there have been around 900 suspected cases of swine flu in Mexico City and two other regions of Mexico, with around 60 suspected deaths. Of those, 18 have been confirmed as H1N1 swine flu, says the WHO, and tests so far have shown that 12 of those are "genetically identical" to the California virus.
On Friday, Richard Besser, head of the CDC, confirmed that Mexican samples tested at CDC were also "similar" to the US virus. "From everything we know to date, this virus appears to be the same," he said.
To be declared a pandemic, Besser said, the virus must be new, cause severe disease, and transmit easily enough to be sustained.
It is new. Anne Schuchat, head of science and public health at the CDC, said that the US virus is an unusually mongrelised mix of genetic sequences from North American pigs, Eurasian pigs, birds and humans. The H protein on its surface, having hitherto circulated only in pigs, is one most human immune systems have never seen, the crucial requirement for a pandemic flu.
Too late to contain
The virus's severity will depend on how many people who catch it die. While suspect deaths in Mexico are being tested for H1N1, is not yet known how many mild cases of virus there may have been in the affected region that have gone untested. Both numbers are needed to calculate how deadly a case might be. One ominous sign, however, is that the Mexican cases are said to be mainly young adults, a hallmark of pandemic flu.
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