Infection of the endothelium by influenza viruses.

Highly pathogenic avian influenza viruses are not only the cause of devastating "bird flu" outbreaks in domestic fowl, but are also occasionally the cause of human disease with a high mortality rate as is being currently observed with the H5N1 viruses in South-East Asia. Infection in birds is systemic with hemorrhages and edema as characteristic symptoms, and virus replication in the endothelium appears to play an important role in pathogenesis. Some of the factors determining endotheliotropism have been elucidated at the molecular and cellular level. They include proteolytic activation of the hemagglutinin, polarity of virus budding, and tissue specific expression of virus receptors.