In the mild form, signs of illness may be expressed
only as ruffl ed feathers, reduced egg production, or
mild effects on the respiratory system.
In the severe form of the disease, the virus not only
affects the respiratory tract, as in the mild form, but
also invades multiple organs and tissues that can
result in massive internal haemorrhaging.
Some or all of the following clinical signs are evident
in birds infected with a highly pathogenic strain
of AI (including H5N1 strain):