By Alistair Bell and Noel Randewich
MEXICO CITY (Reuters) - A strain of flu never seen before has killed as many as 61 people in Mexico and has also been seen in the United States, where eight people have been infected but recovered, health officials said on Friday.
Mexico's government said at least 16 people have died of the disease in central Mexico and that it may also have been responsible for 45 other deaths.
Four possible cases were also seen in Mexicali, near the border with California and Mexican officials said there were close to 1,000 suspected cases.
The World Health Organisation said tests showed the virus from 12 of the Mexican patients was the same genetically as a new strain of swine flu, designated H1N1, seen in eight people in California and Texas.
"Our concern has grown as of yesterday," U.S. Centers for Disease Control and Prevention acting director Dr. Richard Besser told reporters in a telephone briefing.
But global health officials were not ready to declare a pandemic -- a global epidemic of a new and deadly disease such as flu. "So far there has not been any change in the pandemic threat level," Besser said.
But because there is clearly human-to-human spread of the new virus, raising fears of a major outbreak, Mexico's government cancelled classes for millions of children in its sprawling capital city and surrounding areas.
Close analysis showed it is a never-before-seen mixture of swine, human and avian viruses, according to the CDC.
"We do not have enough information to fully assess the health threat posed by this new swine flu virus," Besser said.
Humans can occasionally catch swine flu from pigs but rarely have they been known to pass it on to other people.
NO CONTAINMENT
Besser said it was probably too late to contain this outbreak. "There are things that we see that suggest that containment is not very likely," he said. Once it has spread beyond a limited geographical area it would be difficult to control.
But there is no reason to avoid Mexico, CDC and the WHO said. "CDC is not recommending any additional recommendations for travellers to California, Texas and Mexico," Besser said.
The WHO said it was ready to help with antivirals if needed, and that both the United States and Mexico are well equipped to handle the outbreak. The CDC is already working on a vaccine.
Besser said scientists were working to understand why there are so many deaths in Mexico when the infections in the United States seem mild.
Worldwide, seasonal flu kills between 250,000 and 500,000 people in an average year, but the flu season for North America should have been winding down.
The U.S. government said it was closely following the new cases. "The White House is taking the situation seriously and monitoring for any new developments. The president has been fully briefed," an administration official said.
Mexico's government cautioned people not to shake hands or kiss when greeting or to share food, glasses or cutlery for fear of infection. Flu virus can be spread on the hands, and handwashing is one of the most important ways to prevent its spread.
The outbreak jolted residents of the Mexican capital, one of the world's biggest cities with 20 million residents.
One pharmacy ran out of surgical face masks after selling 300 in a day.
"We're frightened because they say it's not exactly flu, it's another kind of virus and we're not vaccinated," said Angeles Rivera, 34, a federal government worker who fetched her son from a public kindergarten that was closing.
The virus is an influenza A virus, carrying the designation H1N1. It contains DNA from avian, swine and human viruses, including elements from European and Asian swine viruses, the CDC has said.
The Geneva-based U.N. agency WHO said it was in daily contact with U.S., Canadian and Mexican authorities and had activated its Strategic Health Operations Centre (SHOC) -- its command and control centre for acute public health events.
The CDC said it will issue daily updates at http://www.cdc.gov/flu/swine/investigation.htm.
Surveillance for and scrutiny of influenza has been stepped up since 2003, when H5N1 bird flu reappeared in Asia. Experts fear that or another strain could spark a pandemic that could kill millions.
(Additional reporting by Stephanie Nebehay in Geneva and Maggie Fox in Washington; Writing by Kieran Murray; Editing by Eric Walsh)
