Wild birds worldwide carry
avian influenza viruses in their intestines, but usually do not get sick from them.
Avian influenza is very contagious among birds and can make some domesticated birds,
including chickens, ducks, and turkeys, very sick and kill them.
Infected birds shed influenza
virus in their saliva, nasal secretions, and feces. Domesticated birds may become
infected with avian influenza virus through direct contact with infected waterfowl
or other infected poultry, or through contact with surfaces (such as dirt or cages)
or materials (such as water or feed) that have been contaminated with the virus.
Avian influenza infection in domestic poultry causes
two main forms of disease that are distinguished by low and high extremes of virulence.
The "low pathogenic" form may go undetected and usually causes only mild symptoms
(such as ruffled feathers and a drop in egg production). However, the highly pathogenic
form 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 90-100%,
often within 48 hours. It is the highly pathogenic form of H5N1 that concerns scientists.