Avian Flu Arose Through Gene Swapping

Researchers have found that the avian influenza virus H5N1, which has caused major outbreaks among poultry and killed several people over the past seven years, evolved through a series of genetic-reassortment, or gene-swapping, events with other viruses. Their findings were reported in the July 8, 2004, issue of Nature.

Gene swapping occurs when two or more viruses infect the same animal. The researchers studied the genetic makeup of H5N1 subtypes collected since 1997, tracing the evolution of the virus into a dangerous pathogen. They collected samples of the virus from poultry in Indonesia, Thailand, and Vietnam and from humans in Thailand and Vietnam. The eight genes carried by these viruses were then compared with the eight genes carried by samples of the 253 viruses collected from live poultry markets in Hong Kong and the Chinese provinces of Guangdong, Hunan, and Yunnan. The results showed that H5N1 viruses occurred only in ducks in 2000, but from 2001 on, the viruses spread to chickens, too.

Furthermore, the various H5N1 viruses isolated over the years all contained two genes (HA and NA) derived from the same, older virus called Goose/Guangdon/1/96. The other six genes came from various influenza viruses through reassortment. Another finding was that a specific type of H5N1 called the Z genotype, is still adapting to these birds, suggesting that these viruses will continue to evolve through mutation or reassortment to achieve greater fitness. Some genotype B, Y, and Z viruses carried a specific mutation known to cause resistance to antiviral drugs used to treat influenza.

"Although these deadly viruses were isolated from dead migrating birds, we don't know if the birds are actually spreading H5N1,” said Richard Webby, Ph.D., of the department of infectious diseases at St. Jude Children's Research Hospital (Memphis, TN, USA), which conducted the study along with researchers from a number of Asian countries.




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