World health authorities on Friday rushed to tackle flu outbreaks in the United States and Mexico that have killed at least 60 people and have pandemic potential.
Hundreds of people in Mexico have been infected and 60 have died from suspected swine flu, while seven human cases have been confirmed in the United States, a World Health Organisation spokeswoman said.
The WHO, which has identified swine influenza as a potential source of a human flu pandemic, activated its global epidemic operations centre while Mexican authorities closed schools in Mexico City and the centre of the country.
Mexico's Health Minister Jose Angel Cordova urged people to avoid large crowds, shaking hands, kissing people as a greeting, or using the subway.
"This afternoon the epidemic was confirmed by Canadian and US labs to be a new influenza virus," Cordova said in a televised statement late Thursday.
But it was unclear if the Mexican flu virus was the same one implicated in the confirmed swine flu outbreaks in the southwestern United States.
The European Centre for Disease Prevention and Control said the "novel" A/H1N1 influenza identified in at least two of the recent cases by US counterparts might have a risk of developing into a pandemic-type virus.
"The infection of humans with a novel influenza A virus infection of animal origins as has happened here is of concern because of the risk, albeit small, that this could represent the appearance of viruses with pandemic potential," the ECDC said on its website.
Referring to "swine flu cases that have broken out in the United States in Mexico," WHO spokeswoman Fadela Chaib said there are "now a total of seven known cases in the United States."
They included five in California and two in Texas, in three clusters, Chaib told journalists.
"Meanwhile in Mexico unusual end of season influenza activity began to be noticed at the end of March, peaking in April."
"To date there have been some 800 suspected cases with flu-like illness, with 57 deaths in the Mexico City area," the WHO spokeswoman said.
Twenty four suspected cases and three deaths were also recorded in San Luis Potosi in central Mexico, she said.
Mexico's health minister reported a total of 20 deaths late Thursday.
"The virus is being described in the United States as a new subtype of A/H1N1. Mexico so far hasn't said anything about the virus, what type of virus it is," said Chaib.
Most of the Mexican cases were found in healthy young adults with no known record of prior illness.
The WHO said it was in constant contact with health authorities in the United States and in Mexico.
Human outbreaks of H1N1 swine influenza virus were recorded in the United States in 1976 and 1988, when two deaths were recorded, and in 1986. In 1988 a pregnant woman died after contact with sick pigs, according to the WHO.
In recent years the global focus for a pandemic has shifted to the H5N1 bird flu virus, which has spread from poultry to humans and killed 257 of the 421 people infected by the virus since 2003.
WHO experts have pointed out that pigs have been implicated in the emergence of new influenza viruses responsible for two of the previous century's influenza pandemics.
If a pig is simultaneously infected with a human and an avian influenza virus, it can serve as a "mixing vessel" for the two viruses that could combine to create a new more virulent strain.
(c) 2009 AFP