Swine Flu vs. Bird Flu: Which Is the Greater Pandemic Threat?

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As health officials around the globe continue to monitor the swine flu outbreaks that have occurred within the past month, comparisons with the periodic bird flu outbreaks over the past decade are difficult to avoid.

Specifically, some may find themselves asking the question: Since the bird flu failed to spark a full-blown pandemic, should we really worry about swine flu?

Infectious disease experts say there are numerous key differences between the two viruses and their pandemic potential, and a new study published on Friday in the journal PLoS Pathogens hints at just one of these differences. In the study, British and U.S. researchers revealed that the bird flu virus becomes impaired at 89.6 degrees Fahrenheit -- the temperature of the human nose -- therefore making it less likely that it can spread from person to person.

The human nose, in other words, may simply offer too cold a climate to encourage the spread of the avian flu virus. Whether this will always be the case is not yet known, but the researchers said that a substantial mutation would have to occur in the bird flu virus before it would become a pandemic threat for humans.

"Bird viruses are out there all the time, but they can only cause pandemics when they undergo certain changes," said Wendy Barclay of the Imperial College in London, one of the study's authors. "Our study gives vital clues about what kinds of changes would be needed in order for them to mutate and infect humans, potentially helping us to identify which viruses could lead to a pandemic."

The study may also provide scientists with the mechanisms of virus mutation in general, allowing them to predict which viruses are more likely to spread to humans.

Dr. William Schaffner, chair of the Department of Preventive Medicine at the Vanderbilt University School of Medicine in Nashville, Tenn., called the study "very nice work that adds another explanation why the bird flu is not being transmitted readily in people."

Swine Flu Beats Bird Flu in Human-to-Human Transmission

"Previously it has been shown that the avian influenza viruses did not attach to the cells of the upper respiratory tract, but could attach lower down," Schaffner said. "The current study provides an additional potential explanation."

Ed Hsu, associate professor of public health informatics at the University of Texas School of Health Information Sciences and School of Public Health, agreed. "This study renders potential explanations for why Avian Flu virus does not effectively transmit from human to human -- an important indicator for raising pandemic level."

But Dr. Pascal Imperato, dean of the Graduate Program in Public Health at SUNY Downstate Medical Center in Brooklyn, N.Y., warned that the fact that this was an in vitro study -- in other words, one that took place in a Petri dish -- meant that more research would be needed to determine whether the findings are an accurate reflection of what happens in humans.

"The findings would need to be corroborated in order to have credibility," Imperato said. "That said, there are many other variables that determine the infectivity of a pathogen. While these findings are of interest, they really do not alter our current understanding of the pathogenicity of influenza viruses."

Bird Flu vs. Swine Flu: Differences in Pandemic Potential?

As the human nose is a crucial site of infection for any flu bug, the new research might help explain why, even at the height of the bird flu scare, there was little evidence of human-to-human spread.

According to the World Health Organization website, since the bird flu virus re-emerged in 2003, there have been only 423 reported cases -- on average, about 60 cases per year worldwide.

Most avian flu cases occur after the victims had had close contact with live poultry. Few cases are currently suspected to have occurred through human-to-human contact, and in most cases the spread was successfully contained through the systematic destruction of bird populations.

The avian virus seems instead more suited for the balmy 104-degree conditions found in the gut of a bird. Exclusively human influenza viruses also do well under those conditions, but unlike the bird flu virus they are not as adversely affected when placed in colder temperatures.

Health Officials Still Warning of Swine Flu Threat

Viruses that are not affected by the colder temperatures are more likely to become a problem, the researchers said. The swine flu virus, for example, may be able to thrive in a wide range of temperatures, and that may explain its quicker spread. There have been 4,694 cases so far across 26 countries, including the U.S., Canada, Malaysia, and the U.K. Although the flu strain is mild, a more virulent mutation can prove to be devastating.

Take, for example, the virulence of bird flu. Despite the relative paucity of human cases of bird flu among humans, Hsu said, 258 people have died since 2003 as a result of the bird flu virus -- a case fatality rate of more than 60 percent.

"Once [bird flu] viruses get passed the proximal pathway, low temperature 'barrier,' they replicate so quickly that they often overpower the host and thereby cause high fatality," Hsu said.

And if the viruses were to somehow be able to bridge this barrier, there is a chance that they could become more problematic.

"As more people are exposed to the [bird flu] strains, it is possible that mutations may occur that lead to more efficient bird-to-human transmission," said Ella Nkhoma, a researcher and epidemiologist at the University of North Carolina at Chapel Hill. "However, whether such a mutated virus would maintain sufficient fitness to efficiently sustain human-to-human transmission is unclear. So far, human-to-human transmission has been very rare."

Indeed, infectious disease experts largely agree that the outbreaks at hand may warrant more worry than the bird flu.

"Many experts now think that, after 4-5 years, because the current H5N2 bird virus has not mutated into a human-transmissible strain, that it is unlikely to do so," Schaffner said. "At the moment, the H1N1 [swine flu] virus has a much greater potential for producing a pandemic."

This threat has not been lost on international health officials. On Friday, WHO cautioned against a false sense of security from the dwindling and seemingly mild outbreaks of swine flu, saying the worst may not be over for the newly-discovered virus.

WHO Director-General Margaret Chan said there remained "great uncertainty" about the strain that could pose particular threats in Southeast Asia.

The Associated Press contributed to this report.