Transmission occurs in one of two ways. Transmission can be by direct contact between an infected bird and healthy one. Transmission can also occur through indirect contact with virus-contaminated material. Bird flu virus is present in the alimentary canal, respiratory tracts and visceral organs of affected birds and can be dispersed via ocular, nasal and oral secretions and the feces of live birds or in carcasses. Physical transmission can then occur through such avenues as feedstuffs, water supplies, henhouses, coops, cages, handling facilities, vehicles, insects, wild birds and in the air. Healthy poultry can then contract the disease via the respiratory tract and alimentary canal. As airborne transmission is possible, migratory birds such as wild ducks can become infected and spread the virus from one place to another. This can lead to large-scale infection and morbidity of domestic poultry populations as the contamination is dispersed through water supplies etc. In addition, movements of virus-hit poultry and poultry products also transmit bird flu.
(China.org.cn February 13, 2004)