Discovery Guides Areas


Avian Influenza (Bird Flu)
(Released July 2007)

  by Sujata Suri  


Key Citations



Pathology and epidemiology


Typically, the Avian Influenza (AI) virus refers to Influenza A, found chiefly in birds. Infected birds show clinical symptoms like a sudden drop in egg production, brittle or soft-shelled and even shell-less eggs, swollen wattles and combs, congestion, and swollen skin under the eyes.

Usually the risk of human infection from birds is through coming in close contact with bodily fluids or with contaminated surfaces. Infection can be transmitted from infected bird droppings, saliva, nasal secretions, feces, or blood. These viruses can remain infectious for about 1 week at human body temperature or a month at 32 degrees F, and can survive at very low temperatures indefinitely. Symptoms of AI in infected humans are mild fever, myalgia, sore throat, cough, conjunctivitis, myositis, and myoglobinuria. However, some people develop life-threatening complications like respiratory distress syndrome, pneumonia, and multiorgan failure.

In 1997, the first documented infection of humans with an avian influenza virus occurred in Hong Kong. At the same time, the poultry population in Hong Kong was also found to be infected with avian influenza caused by the same pathogenic strain. Studies determined that the infection occurred when the virus jumped directly from birds to humans due to close contact with infected poultry. A pandemic was averted by rapid mass killing/burning of over a million birds - the entire poultry population of Hong Kong (Chan, 2002; Yuen et al., 1998).

In December 2003, a highly pathogenic form of H5N1 caused another outbreak in poultry in The Republic of Korea (Lee et al., 2005). Another human infection was confirmed in February 2004 when two fatal cases were reported in Hong Kong due to H5N1 (Peiris et al., 2004), followed by 112 cases (57 fatal) from Thailand, Cambodia, Indonesia, and Viet Nam. Until now the cumulative number of confirmed human cases is 317 (191 fatal) (WHO, 2007). RnH5N1 viruses also have been isolated from ducks in Southern China (Chen et al., 2004) and antiviral antibodies have been found in pigs in Viet Nam (Choi et al., 2005). These cases could be the result of new strains due to reassortant viruses, antigenic shift or antigenic drift, as explained earlier. People are not immune to these different strains. Generally speaking, an individual has immunity to only those microbes or viruses to which they are earlier exposed. The possibility of dreadful new strains is thus worrying, as people either have no immunity or extremely delayed immunity depending upon the individual's health and age. However, many new harmless strains causing symptomless infections go unnoticed. It was noticed that Spanish flu was most lethal in young adults, who generally are most able to fight off severe infections. One theory for why Spanish flu preferentially killed young people is because they are the one with robust and reactive immune systems and therefore were most likely to mount a self destructive response (Brown, 2007).

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