The new flu virus has been confirmed in Mexico, the US, Spain and Canada (Illustration: Google Maps)

See also: Swine flu – what you need to know

A novel flu virus has struck hundreds of people in Mexico, and at least 18 have died. It has also infected 20 people in five states in the US, and appears able to spread readily from human to human. The US has declared a public health emergency, and the World Health Organization is holding emergency meetings 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.

On Saturday and Sunday, the CDC confirmed eight cases of swine flu had been confirmed at a girl’s school in New York, two cases in Kansas and a case in Ohio.

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 “case mortality” of swine flu – how many people die after being infected – is not yet known. While suspect deaths in Mexico are being tested for H1N1, we don’t know 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 pandemic might be. One ominous sign, however, is that the Mexican cases are said to be mainly young adults, a hallmark of pandemic flu.

It can transmit among people. Those infected in the US had no known contact with pigs, and the three separate clusters of cases did not contact each other. This suggests, said Besser, that “this virus has already been transmitted from person to person, for several cycles”, making it too late for emergency antiviral drugs to contain its spread to a limited area.

‘High concern’

“Because there are human cases associated with an animal influenza virus, and because of the geographical spread of multiple community outbreaks, plus the somewhat unusual age groups affected, these events are of high concern,” the WHO said.

CDC scientists are now examining people with current and recent flu-like illnesses in the areas affected to see how many contacts of known cases have traces of the virus, or antibodies to it. That should show how many cases there may have been, how readily the virus spreads, and how likely it is to maintain transmission. On Sunday Besser said that contacts of people diagnosed with the virus “have significant rates of respiratory infection”, suggesting it may be easily communicable.

Another H1N1 flu jumped from pigs to people in 1976, and killed an army recruit in New Jersey. The US went on high alert and vaccinated thousands of people – but the virus did not spread readily enough to maintain an epidemic, and fizzled out.

See also: Swine flu – what you need to know