A research team in Hong Kong may have discovered a key reason why bird flu is so much more deadly than human flu viruses.

They report that the bird flu virus H5N1 promotes much more inflammation in human lung cells than the human flu virus H1N1.

In a human cell line, the bird flu virus triggered more than 10 times the level of inflammatory proteins as human flu virus.

A well-known influenza expert who worked on the study tells WebMD that this hyper-response may upset the delicate balance that drives the immune system, making it more vulnerable to the infection.

“It is like nuclear power,” says Malik Peiris, PhD. “If it is well controlled it can be very beneficial, but if it is out of control there can be huge problems.”

Early Bird Flu Cases

The H5N1 bird flu virus was first reported in humans in 1997, and as of November there had been 122 confirmed cases among humans and just more than 60 deaths.

Most of the deaths have occurred among people who contracted the virus through direct contact with infected poultry and other birds. A few cases of human-to-human transmission are suspected to have occurred in Hong Kong, China, and Vietnam, but these have not been confirmed.

Peiris, who played a critical role in the discovery of the SARS virus nearly three years ago, worked with Michael Chan, PhD, and colleagues at the University of Hong Kong to identify differences in immune response between bird flu and the virus that most often causes influenza in humans.

They did this by studying levels of proteins known as chemokines and cytokines in human cells infected with the two viruses.

Chemokines and cytokines are the protein messengers that trigger inflammation in response to infections and other assaults.

Using human lung cells, the researchers compared protein levels induced by an H5N1bird flu virus isolated in Hong Kong in 1997 and another more recent bird flu virus from Vietnam with protein levels induced by the human flu virus.

Twenty-four hours after infection, levels of inflammatory proteins were roughly 10 times greater in cells infected by the bird flu strains than in cells infected by the human strain. The more recent Vietnam bird flu strain even had a higher response than the 1997 Hong Kong bird flu virus.

More Knowledge, Better Flu Treatments

Gaining a better understanding of why bird flu is so dangerous to humans may lead to better treatments and strategies for managing this disease, Peiris says. This could save countless lives if, as many predict, the flu evolves and becomes easily spread from person to person.

Scientists now believe that the deadly 1918 Spanish flu pandemic, which killed 20 to 100 million people, was initially a bird flu that evolved to infect humans.

University of Virginia bird flu researcher Frederick G. Hayden, MD, and colleagues reported on the illnesses and deaths caused by the new threat in late September.

The researchers found that most cases have occurred in apparently healthy adults and children, and about half of those who have acquired the virus have died from it. Bird flu victims typically progress from moderate symptoms to life-threatening respiratory distress within a week to 10 days.

Hayden tells WebMD that he believes the treatment approach that has the most likelihood of success in the event of a H5N1 pandemic will combine antiviral drugs with treatment that targets the human host inflammatory response highlighted in the new study.

“This research is certainly a step in that direction,” he says.

By Salynn Boyles, reviewed by Louise Chang, MD

SOURCES: Chan, M.C.W. Respiratory Research, Nov. 11, 2005; online. Malik Peiris, PhD, virologist, University of Hong Kong. Frederick G. Hayden, MD, professor of clinical virology and internal medicine, University of Virginia, Charlottesville. WebMD Medical News: “Bird Flu: 10 Questions, 10 Answers.” WebMD Medical News: “Scientists: 1918 Killer Flu Was a Bird Flu.” WebMD Medical News: “Scientists in Desperate Race with Bird Flu.” WebMD Medical Reference:“Bird Flu Time Line.”