Two Queenslanders feared to have picked up swine flu in Mexico have been given the all-clear, but Australia\’s top influenza expert says swine flu is probably already in Australia.

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“It\’s very hard to screen for influenza coming into the country because what we do know is that people can be infected and not show the signs of disease in the early stages, but can be spreading the disease now,” virologist Alan Hampson told News深圳桑拿,.

A spokesman for Queensland Premier Anna Bligh says tests on the two people one from the Gold Coast and one from Brisbane have come back negative for the virus.

Ms Bligh has reassured Queenslanders that the threat\’s being taken seriously and GPs and hospital emergency departments are on the lookout for symptoms.

Chief health officer Dr Jeannette Young will hold a media conference shortly.

US declares public emergency

The United States declared the swine flu outbreak a public health emergency as officials confirmed 20 flu cases in five US states and warned that they expected more infection in coming days.

The United States will screen visitors arriving in the US from infected areas, Homeland Security Secretary Janet Napolitano said, as 20 cases were confirmed in five states.

Suspected cases were also being investigated in Europe, the Middle East and Asia, and six new infections were confirmed in Canada.

Mexico death toll reaches 81

Authorities in Mexico, where the new multi-strain swine flu was first detected, confirmed 20 people have died and warned the death toll could be as high as 81.

Five people were said to have died in the capital in the past 24 hours.

“As we look for cases of swine flu, we are seeing more cases of swine flu.

We expect to see more cases of swine flu,” Richard Besser, the acting head of the Centers for Disease Control, told a White House press conference.

Besser said there were eight confirmed cases in New York City, seven in California, two in Texas, two in Kansas and one in Ohio.

US President Barack Obama is monitoring the swine flu outbreak closely and has ordered a “very active, aggressive, and coordinated response,” said White House homeland security advisor John Brennan.

Flu has a pandemic potential : WHO

World Health Organization (WHO) officials warned Sunday the new strain, apparently born when human and avian flu viruses infected pigs and became mixed, could further mutate.

“Yes, it\’s quite possible for this virus to evolve,” Keiji Fukuda, acting WHO assistant director-general for health, security and the environment, told journalists in Geneva.

“When viruses evolve, clearly they can become more dangerous to people.”

The WHO has already recommended that all nations “intensify surveillance for unusual outbreaks of influenza-like illness and severe pneumonia.”

As thousands of panicked Mexicans wore surgical masks on the streets and sought to avoid physical contact, President Felipe Calderon called for calm, urging citizens to work with authorities to contain the virus.

Calderon said if people can make timely medical visits, Mexico “can contain this outbreak.”

More than 1,300 treated for swine flu worldwide

Of the 1,364 people treated for suspected swine flu, 929 of them — or 67 percent — have been discharged, Calderon said, adding that the country had the necessary drug stock piles to fight the virus.

Two city zoos on Sunday joined schools, museums, theaters and music festivals which have closed down, and authorities sought to shut down bars and nightclubs.

Church officials took the unusual step of canceling Sunday services in this mainly Catholic country, after Calderon gave his government extraordinary powers to tackle the outbreak.

“This virus has clearly a pandemic potential,” World Health Organization (WHO) director general, Margaret Chan, warned on Saturday.

On CNN, senior White House adviser Valerie Jarrett was asked if Obama had tested negative for swine flu after he paid his first official visit to Mexico just over a week ago.

“He\’s fine. He is just fine,” she said without elaborating. Asian health officials also went on alert as the flu strain appeared to have spread to New Zealand.

Governments across the region, which has in recent years been at the forefront of the SARS and bird flu epidemics, stepped up checks at airports and urged the public to be on guard for symptoms of the new flu.

NZ investigates possible swine flu cases

Thirteen New Zealand students who recently traveled to Mexico are “likely” to have contracted swine fever, Health Minister Tony Ryall said — the first suspected cases in the region of more than three billion people.

Ten students from Rangitoto College, the country\’s largest school, tested positive for influenza A yesterday, and are thought to have swine flu.

Three students from Northcote College are also feared to have the disease.

Ryall said all 40 students and teachers from both groups had now been quarantined.

Officials are tracing hundreds of other passengers who were on the same flights from Los Angeles, to check their health status.

EU on alert for disease

The European Commission said it was on alert Sunday for swine flu, although there have not been any confirmed cases in Europe.

Three people out of four tested for swine flu in France were on Sunday given the all-clear by doctors, but one woman in the Paris region is still suspected of having contracted the virus, officials said.

Seven suspected cases were also reported in Spain. In the first suspected swine flu case in the Middle East, a 26-year-old Israeli man has been hospitalized in Netanya on returning from Mexico, hospital officials told AFP.

Russia Sunday banned meat imports from Mexico, several US states and nine Latin American nations, a spokesman for Prime Minister Vladimir Putin said. US officials have said it is impossible to catch the flu from eating meat.

According to the WHO, pigs have already been factors in the appearance of two previously unknown diseases that gave rise to pandemics in the last century