[Inter-species transmission of the influenza virus]

Influenza is an infection of human beings and several animal species. It is caused by influenza viruses which belong to the Orthomyxoviridae family. Type A influenza viruses are the most important as they cause severe epidemics and are responsible of important pathological troubles. Type A influenza viruses are classified in different sub-types depending of the nature of their surface glycoproteins: haemagglutinin (H) and neuraminidase (N). The nature of the genome and the mode of replication of influenza viruses account for the high variability of these two proteins which are responsible for the immunity to the virus. The continuous appearance of point mutations in the gene coding for the H protein, leads to the progressive emergence of new viral strains. This event which is called antigenic drift makes it necessary to annually assess the composition of the human flue vaccine. Genetic reassortment is another mechanism of antigenic variation. When the gene coding for the H protein, or when both genes coding for H and N proteins are involved in genetic reassortment, a new viral sub-type occurs which replace the precedent. This event, which is termed antigenic shift, occurs occasionally every 10 to 30 years, and it is responsible of the great human pandemics. The role of the animals and particularly the importance of pigs and poultry in the emergence of these new viruses is discussed.