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

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.

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