April 26 (Bloomberg) -- International health officials are wrestling with how to respond to a swine flu from Mexico that’s infecting people, causing a range of illnesses, and even death.

The World Health Organization called the outbreak a “public health emergency of international concern” yesterday, and as many as 81 deaths in Mexico were linked to the virus, normally transmitted among pigs. Twenty cases in California, Kansas, New York, Ohio and Texas, all of them mild, have been connected as well. In New Zealand, 10 students are “highly likely” to have swine flu, officials said.

Fears of a lethal pandemic lie in the nature of flu germs, which mutate readily and can become virulent by exchanging genes with related influenza viruses. While the H5N1 bird virus that spread across Asia in the last few years, killing millions of fowl and several hundred people, never gained genes to spread easily among humans, the Mexican swine flu already has, said Malik Peiris, a microbiologist from the University of Hong Kong.

“The concern is that this virus has the ability to transmit from humans to humans because a number of the cases who got infection have had no direct exposure to swine,” said Peiris, who has studied the SARS and avian flu viruses. “That is certainly a cause for concern.”

Health officials said they are trying to determine how the virus gained its ability to infect and spread among humans.

Swine-Flu Emergency

Mexican President Felipe Calderon declared a swine-flu emergency, giving him powers to order quarantines and suspend public events in the nation, where 1,324 patients are hospitalized with flu-like symptoms.

Authorities closed schools until May 6 in Mexico City and the states of Mexico and San Luis Potosi, where infections have been concentrated, and canceled most public and official activities.

In the U.S., Janet Napolitano, secretary of the Department of Homeland Security, also declared a public health emergency to free up resources to deal with the outbreak. The U.S. Centers for Disease Control and Prevention, an Atlanta-based agency, is leading the search for more cases.

The latest U.S. tally includes eight students at the St. Francis Preparatory School in Queens. New Yorkers should stay home from work if they are feeling ill and not go to the hospital unless they are seriously ill, said New York City Mayor Michael Bloomberg at a press conference today. City schools will remain open tomorrow, although St. Francis, which is private, will be closed.

Bloomberg is the founder and majority owner of Bloomberg LP, the parent company of Bloomberg News.

New Zealand Students

Ten New Zealand high school students who returned from Mexico are “highly likely” to have swine flu, Health Minister Tony Ryall said today in a phone interview.

The students don’t have “severe” symptoms and most already appear to be recovering, Ryall said. The students and their families are being isolated in their homes, and the families are being treated with Roche Holding AG’s Tamiflu, Ryall said.

France is investigating four possible cases of swine flu, including three in one family in the north of the country and one woman in the Paris region, officials said. Test results should be available within 24 hours, Francoise Weber, director general of the Sanitary Surveillance Institute, said at a news conference in Paris today.

Russian Imports

Russia suspended imports of all meat from Mexico and the U.S. states of Texas, California and Kansas shipped after April 21 on concern about the spread of swine flu, the country’s veterinary watchdog said.

“We have no evidence to suggest that people are getting infected from exposure to pork or to pigs,” Keiji Fukuda, the Geneva-based agency’s assistant director-general for health security and environment, said on a conference call with reporters.

Outbreaks in Mexico and the U.S. warrant an urgent assessment of its potential to spark the first influenza pandemic in 41 years, the WHO said yesterday. The Geneva-based United Nations agency held an emergency meeting and found that more evidence is needed to determine whether the level of pandemic alert should be increased, it said.

The emergency committee will meet again April 28 to consider raising the alert, Fukuda said. “We want to make sure that we are on pretty good solid ground,” he said.

The WHO’s pandemic threat level, a six-stage measure, is currently at 3. Evidence of increased human-to-human spread of a new virus would move it to level 4, according to the WHO Web site.

On Lookout

Health officials in the U.S. are asking both doctors and patients to be on the lookout for suspicious cases of flu. The lung virus normally causes symptoms such as coughing and sneezing, and can also bring on muscle and joint aches, headaches, and even diarrhea and vomiting, according to the CDC.

President Barack Obama is seeking an “active, aggressive, coordinated” response to the swine flu outbreak, John Brennan, assistant to the president for homeland security, said.

At a time when scientists can tailor drugs to match a patient’s genetic profile and people live longer than ever, the flu, first described by Hippocrates 2,400 years ago, still has the power to make millions bed-bound for a week and kill the very young, the elderly and those weakened by chronic disease.

The CDC estimates the germ is linked to more than 30,000 U.S. deaths annually.

New Viruses

In most cases, adults can resist succumbing to flu viruses that are identical or very similar to those they’ve been exposed to before. “New” viruses that the human immune system hasn’t seen earlier are the most dangerous, because they can overwhelm the body’s defenses.

Flu germs are classified by two proteins, one known by the letter H, for hemagglutinin, and the other N, for neuraminidase. The Mexican swine flu is an H1N1 flu, the same subtype that caused the pandemic of 1918. Many less-dangerous descendants of that virus are seasonal H1N1 viruses circulating worldwide today, scientists said.

The dominant form of flu circulating in the U.S. in the most recent flu season was an H1N1, said Frederick Hayden, professor of clinical virology at the University of Virginia Health Sciences Center in Charlottesville. That suggests that people who got this year’s flu vaccine, which gave protection against the H1N1 virus, might also have some protection against the swine flu, he said.

1976 Swine Flu

“We need to take the (blood) of individuals who got last season’s vaccine and see whether there’s any evidence of cross- reactivity for this new strain,” he said in a telephone interview. “We need to do the same thing with patients who have had recent infection with the human H1N1 strain.”

The CDC is conducting studies now that might show whether seasonal vaccination might protect people against swine flu. It’s also possible that people who were vaccinated in the 1976 swine flu outbreak, which many flu experts believed was the beginning of a pandemic at the time, are protected, he said.

Vaccine makers are taking the initial steps toward making shots against swine flu. Baxter International Inc., a maker of both seasonal and pandemic vaccines, has requested samples of the swine virus for laboratory testing, said Christopher Bona, a spokesman for Deerfield, Illinois-based Baxter. GlaxoSmithKline Plc, based in London, has had conversations with WHO’s flu division that’s responsible for distributing flu virus samples to drugmakers, said Deborah Alspach, a spokeswoman.

Other companies that make flu vaccine include Novartis AG, of Basel, Switzerland, and Sanofi-Aventis SA, of Paris.

Roche’s Tamiflu

Roche, of Basel, has an ample supply of Tamiflu, which can reduce the symptoms of swine flu. Roche has donated a “Rapid Response Stockpile” of 5 million treatment courses to the WHO that’s on 24-hour stand-by to be sent around the world, said Terence Hurley, a spokesman for the company. No request has been made to deploy the stockpile, he said in an e-mail.

Glaxo also has ample supplies of its inhaled Relenza antiviral, which also appears to be effective against the swine flu in CDC tests, Alspach said.

The virus has already evaded the first line of defense that health officials had hoped to use against a pandemic. International flu experts preparing for a pandemic had planned to contain the initial outbreak of a new, lethal strain of flu. The swine flu virus has already spread so far in Mexico and the U.S. that the containment strategy is out of the question, said Anne Schuchat, interim deputy director for science and public health programs at the Centers for Disease Control and Prevention, the Atlanta-based U.S. agency.

“We don’t think we can contain the spread of this virus,” she said yesterday in a conference call with reporters.

Fukuda, of the WHO, said there was no evidence that the spread of the virus was connected to terrorism.

To contact the reporter on this story: John Lauerman in Boston at jlauerman@bloomberg.net; Jason Gale in Singapore at j.gale@bloomberg.net

Last Updated: April 26, 2009 13:28 EDT