Avian influenza: mini-review, European control measures and current situation in Asia.

Avian influenza (AI) is a highly contagious disease for birds, which can easily take epidemic proportions when appropriate and efficacious measures are not taken immediately. Influenza viruses can vary in pathogenicity from low to medium or highly pathogenic. A low pathogenic strain can become highly pathogenic by introduction of new mutations (insertions, deletions or substitutions) in the cleavage site of the haemagglutinin during circulation in chickens. Up till now only H5 and H7 strains gave rise to highly pathogenic strains in this manner. At present the avian H5N1 influenza virus is endemic in Southeast Asia (47) and is expanding westward. In addition, its virulence is extremely higher than other HPAI, like H7N7. Moreover, the avian host range is expanding, as species previously considered resistant, now get infected and can contribute to the dissemination of the virus. In the context of H5N1, all movements (trade, high international mobility, migration and smuggling) can become high risk factors of spreading the disease. In most European countries eradication measures are applied when an outbreak occurs. But such measures have great economical and social implications, and are no longer generally accepted. The combination of prophylactic measures (vaccination and medicines), hygienic measures and surveillance could offer an acceptable alternative.