Infection with bird flu viruses in domestic poultry causes two main forms of disease that are distinguished by low and high extremes of virulence. The "low pathogenic" form of bird flu may go undetected and usually causes only mild symptoms (such as ruffled feathers and a drop in egg production).
However, the highly pathogenic form of bird flu spreads more rapidly through flocks of poultry. This form may cause disease that affects multiple internal organs and has a mortality rate that can reach 90 percent to 100 percent, often within 48 hours.
One strain of bird flu, the H5N1 virus, is endemic in much of Asia and has recently spread into Europe. H5N1 infections have recently killed poultry and other birds in a number of countries. Strains of H5N1 bird flu may infect various types of animals, including wild birds, pigs, and tigers. Symptoms of H5N1 in birds and other animals vary, but virulent strains can cause death within a few days.
H5N1 Bird Flu in Humans
H5N1 bird flu in humans is currently limited and not considered a pandemic. H5N1 influenza infection in humans was first recognized in 1997, when this virus infected 18 people in Hong Kong, causing 6 deaths. Since 2003, more than 100 human H5N1 cases have been diagnosed in:
Of those cases, more than half have died as a result of the bird flu virus.
Currently, close contact with infected poultry has been the primary source of human infection with bird flu. Though rare, there have been isolated reports of human-to-human transmission of the disease. Genetic studies confirm that the influenza A virus H5N1 mutates rapidly, which means that should it adapt to allow easy human-to-human transmission, a pandemic could ensue (it has not done so to date). At this time, it is uncertain whether the currently circulating H5N1 virus will lead to a global outbreak of the disease in humans -- a pandemic.