Avian flu, or bird flu, is caused by an influenza virus that normally only infects birds. The strain currently circulating in the avian populations, particularly in Asian countries, is a Type A influenza virus known as H5N1. In rare instances, this strain is also found in pigs.
Where H5N1 Started
Influenza has been infecting humans and animals for centuries. The Oxford Dictionary states the origin of the word “influenza” to be Italy in 1743, the location of the first reported epidemic according to CBC (Canada Broadcasting Company). Specifically, the H5N1 strain has infected fowl for more than a century. The first historical cases of avian flu were identified in the 1870s in Italian chickens. At some point, the strain moved into an animal reservoir. The first cases of bird flu in recent years were documented in Hong Kong, and another outbreak occurred there in 2003, a few years before documented human cases.
How H5N1 Spread
The virus has spread outside of Asia, infecting ducks in Nigeria, and it has been documented as endemic to Europe as early as 2005. The World Health Organization (WHO) supported a theory that migratory birds were carrying the virus to other bird populations, but some researchers dispute that migratory pathways are to blame, choosing airplane travel as a method that human carriers spread the disease. However, the virus rarely infects humans, and when it does it has a high mortality rate, so asymptomatic human carriers are not currently known to exist. On the other hand, the migration theory is supported by the die-off of 6000 migratory birds in April 2005 at the Qinghai Lake nature reserve in central China. There is also accumulating evidence that the H5N1 virus follows trade routes, most likely in birds being transported for sale.
Human Bird Flu Infection
The first human cases of the recent highly pathogenic H5N1 outbreak were documented in the open farm markets of Vietnam, China, and Indonesia in 2005. By 2006, avian influenza infection was confirmed in the Middle East. The WHO followed a line of reasoning that the origin of human infection was via bird droppings due to the proximity of the people to the birds in their general living conditions. This new route of transmission for the virus may be ingestion or fluid exchange, which is in contrast to the familiar aerosol transmission of influenza.
Though the outbreak of avian influenza among birds was extensive in recent years and resulted in the culling or death of many thousands of birds, the number of human cases has been in the low hundreds, even in the Asian countries where birds and humans live in close proximity. The problem lies in the high mortality for those who are infected.
The Risk of Infection
The current avian H5N1 strain has shown a decreased ability to bind to the cells of the human respiratory tract, limiting its spread. Human-to-human transmission has been rare, with only a handful of family cases. One such case was a Chinese father who contracted bird flu from his son, but he did not pass the infection to 91 people with whom he had been in contact.
Genetic analysis has found that the bird flu strain has not undergone any major mutation that would allow it to spread more readily among humans. This genetic analysis has been ongoing since the 1990s, prior to the current outbreak, as researchers have monitored emerging strains that may pose a threat to the human population. This analysis, however, has not uncovered exactly how the virus spread from Europe to Asia and back again.
Alicia Mae Prater - Alicia received her doctorate in Experimental Pathology in 2007. She has been a freelance writer and scientific editor since 2008.
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