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Transmission and Symptoms of Avian Flu in Humans

In 1997, confirmed cases of Avian Influenza in humans were first reported.  There are three types of Influenza Virus – A, B, and C.  Type A Influenza viruses are the most common type associated with the flu.  Type A viruses affects a wide variety of species including humans, birds, pigs, whales, and horses among many.  There are genetic differences within Type A Influenza viruses, which can differentiated by its surface proteins: hemagglutinin (H) and neuroadiminase (N). 

Subtypes of the Influenza Virus are usually quite specific to an animal.  For example, subtypes that have caused widespread illness in people either in the past or the current period are H3N2, H2N2, H1N1, and H1N2.  In pigs, H1N1 and H3N2 subtypes have caused outbreaks; and H7N7 and H3N8 viruses have caused outbreaks in horses.  There are fifteen subtypes of Avian Flu Virus that have been identified thus far, and subtypes of H5 and H7 have been transmitted to humans.

Avian Flu Subtype H5N1 has been the most devastating.  It is this subtype that has caused considerable damage to the poultry industries in Hong Kong, China, and Southeast Asia, resulting in the deaths of millions of chickens, turkeys, geese, and ducks via the disease or culling.  It is this subtype that was transmitted into humans, resulting in 15 deaths in Hong Kong, and 23 in Thailand and Vietnam.  

Although the most damage caused by Avian flu has been in Asia, North America reported the disease in 2003-2004.  Earlier this year in British Columbia, Canada, two humans were infected with Avian Flu virus subtype H7N3, with 10 others suspected of also being infected.  All individuals infected had worked closely with live infected animals, either in farms, processing plants, or live markets.  

This was not the first time an influenza virus normally seen in one species sometimes has crossed over and cause illness in another species.  In the United States, H1N1 was the only Flu virus subtype that was found in pigs.  However, in 1998, H3N2 viruses from humans were introduced into the pig population and caused widespread disease among pigs.

Recently, the Avian Flu Virus has been found in pigs in Asia.  The pigs, which showed no symptoms of disease, tested positive for H5N1 in nasal swabs.  The WHO says that the presence of the virus in nasal cavities means the pigs may simply be contaminated but not infected.  In any case, this has caused great concern in the health organizations around the world.  With this recent evidence, pigs can potentially serve as a host for avian and human influenza viruses.  This may allow these different flu viruses to interact together to produce a strain of Flu virus that is easily transmittable between humans.  If the flu strain is pathogenic, it could lead to a worldwide pandemic. 


The Avian Influenza virus can be transmitted to humans in two ways:

1)     Directly from infected birds (usually poultry) or virus-contaminated environments (equipment, feed, clothes,  etc.)                                                                      

2)     Through an intermediate host, such as a pig

To date, almost all cases of humans suffering from Avian Flu were reported in individuals who had worked closely with infected poultry, whether it was a farm, processing plant, or market.  Some infected humans had also worked with pigs, so there is some speculation that these people may have contracted the flu virus from pigs, which can also harbor the Avian Flu virus. 

It is known that infected birds shed large amounts of the virus through nasal (sneeze, saliva) or gastrointestinal tracts (feces).  Birds infected with the Highly Pathogenic Avian Flu subtypes may shed the virus Infected birds may shed viruses for up to 2 weeks before succumbing to their fate (usually mortality).  In contaminated feces, the fecal matter can dry up and become pulverized, allowing the virus to be airborne.  Thus, a route of transmission into humans can be inhalation of airborne influenza viruses. 

The avian flu virus can also be harbored in pigs.  Although pigs don’t show any signs of avian flu disease, it can secrete the virus in the spray from its airways.  A person in close proximity, for example during feeding, can then inhale the spray and become infected (just as human flu is passed on in the aerosols from sneezing).  Some speculate that some individuals, who have caught the avian flu in Hong Kong, Thailand, and Vietnam, did so through this scenario, as they purportedly working in close contact with pigs.

To date, human-to-human transmission of avian flu has not occurred.


Because symptoms of the avian flu are similar to the human flu, specific diagnosis of avian flu must be performed in a laboratory to determine the subtype of the Avian flu virus. 

The Avian Influenza virus attacks the cells in the nasal and gastrointestinal tracts.  Illnesses caused by the influenza virus range from symptomatic to fatal.  The avian influenza cases that arose in Hong Kong were severe.  Six of the eighteen people reported to have avian flu (infected by Influenza A H5N1) died of fatal illnesses caused by this avian-flu virus.  

The majority of human influenza infections result in acute febrile respiratory illness.  Some symptoms associated with Avian Flu is listed:   

  • Fever (temperatures of 100-103 degrees Fahrenheit in adults)
  • Malaise, lethargy, fatigue, extreme tiredness
  • Cough
  • Sore throat
  • Muscle aches
  • Eye infections (conjunctivitis)
  • Headaches
  • Diarrhea 
  • In severe cases of avian flu, it can cause severe breathing problems and pneumonia, and can lead to mortality