How bird flu becomes human flu
Colorized transmission electron micrograph of Avian influenza A H5N1 viruses (seen in gold) grown in MDCK cells (seen in green).
Scientists are closely monitoring the H5N1 virus that causes avian flu.
So far, bird flu doesn’t pass easily from person to person. If it did ? by coughing or sneezing ? this virus might cause a global pandemic. Earth & Sky asked experts how an animal flu comes to pass between humans.
One possibility is someone becomes infected with human flu and bird flu at the same time. The two viruses share genes to create a new hybrid form of flu that easily passes from person to person. That’s apparently what happened in 1957 with Asian flu ? and in 1968 with Hong Kong flu ? both of which created pandemics that were considered “moderate” to “mild.”
But the H5N1 bird flu virus could become a pandemic strain – without interacting with an established human flu. In this case, bird flu adapts to humans through mutation of its own genes. Over time, mutated genes are passed between two bird flu viruses.
In October 2005, scientists announced that the extremely virulent Spanish flu virus of 1918 adapted to people in this second way. Some scientists think that flu strains formed in this way are more likely to cause a severe pandemic, such as Spanish flu, which killed millions.
So far, bird flu has evolved only in this second, more dangerous way. Scientists continue to monitor its progress.
The latest information on avian flu is available from the Centers for Disease Control and Prevention (CDC).
Our Earth & Sky radio report today refers to a Letter to Nature, in October 2005, in which Jeffery K. Taubenberger and others present findings that the 1918 flu virus became a pandemic strain in an unexpected way. It didn’t do so through “reassortment:” a process of swapping genes within two species, each of which carry a flu virus. Instead, the 1918 flu adapted to humans through “recombination:” that is, random mutations of the virus within a single species, in this case birds, before being passed to humans.
Pandemic strains created by reassortment are thought to be milder, causing fewer deaths among humans, than those created by recombination.
So far, the H5N1 bird flu virus has behaved in much the same way as the very deadly 1918 Spanish Flu virus, mutating by recombination alone. A January 2006 news item in Nature (available here) describes an example of a bird flu mutation that is giving scientists cause for concern. The concern is that the H5N1 virus might be mutating to a form that affects the throat and nose, in contrast to just the lungs. If so, this mutation of the virus might indicate that it is on a pathway leading to human–to–human transmission, via, for example, sneezing.
Avian flu might or might not be the cause of the next pandemic. Read about other possibilities in another Earth & Sky report.
Also see: Banknote tracking helps model spread of disease from newscientist.com
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