WASHINGTON (Reuters) - U.S. government health agencies are strengthening ties to airlines and aviation regulators to guard against the spread of infectious diseases or other deadly agents aboard commercial aircraft, federal officials said on Wednesday.

"With the potential of dramatic economic losses caused either by viruses or terrorists with viruses, a proactive posture rather than a reactive posture is an absolute necessity," said Rep. John Mica, a Florida Republican and chairman of the House aviation subcommittee.

"With over 1.6 billion passengers traveling worldwide each year on commercial air carriers, there is a real threat that these sometimes deadly diseases can be transmitted around the world in a matter of hours," Mica told a hearing attended by government health and aviation experts.

The Severe Acute Respiratory Syndrome (SARS) virus in 2002 and 2003 killed more than 700 people in two dozen countries. It limited airline service to Asia where it first occurred and frightened off many international travelers.

Of particular concern now is the outbreak of avian influenza or "bird flu" that has killed 50 people, also in Asia, since 2003. Experts fear the virus could mutate into a more contagious form and unleash a global pandemic.

Airlines have come under scrutiny for air quality in passenger cabins and complaints from flight attendants that recirculated air can become stale and unhealthful. But health experts say aircraft generally do a good job of filtering germs and the main concern is that sick travelers will spread disease, especially airborne agents, through direct contact with other travelers or after they reach their destination.

Anne Schuchat, acting director for the U.S. Center for Infectious Diseases, noted a case in 2004 when a traveler died from an acute viral illness contracted in Africa soon after arriving in New Jersey. An investigation identified a number of air and train passengers who may have been at risk for the virus but no one else turned up sick.

Schuchat credited cooperation among federal and state agencies, hospital and medical labs for a successful investigation.

But she told lawmakers that government health agencies are working harder to try to detect problems overseas before someone boards a flight to the United States.

"The best strategy for preventing disease introduction into the United States is through disease surveillance, early detection, and rapid response," Schuchat said.

U.S. airlines are working closely with the Centers for Disease Control to expedite information electronically about passengers and crew who may have been exposed to a contagious disease or who are sick.

To address the threat of a potential biological attack on the aviation system, the Homeland Security Department is concentrating on airports.

Among other strategies to mitigate exposure and spread of a biological agent, authorities are focusing on airport terminal airflow patterns and evacuation strategies. Also, the government is testing early warning technology that can detect aerosol and other deadly agents.

The Homeland Security Department is also in the early phase of studying biological detection systems to protect airliners.