Influenza A Viruses and Zoonotic Events-Are We Creating Our Own Reservoirs?

Abstract

Zoonotic infections of humans with influenza A viruses (IAVs) from animal reservoirs can result in severe disease in individuals and, in rare cases, lead to pandemic outbreaks; this is exemplified by numerous cases of human infection with avian IAVs (AIVs) and the 2009 swine influenza pandemic. In fact, zoonotic transmissions are strongly facilitated by manmade reservoirs that were created through the intensification and industrialization of livestock farming. This can be witnessed by the repeated introduction of IAVs from natural reservoirs of aquatic wild bird metapopulations into swine and poultry, and the accompanied emergence of partially- or fully-adapted human pathogenic viruses. On the other side, human adapted IAV have been (and still are) introduced into livestock by reverse zoonotic transmission. This link to manmade reservoirs was also observed before the 20th century, when horses seemed to have been an important reservoir for IAVs but lost relevance when the populations declined due to increasing industrialization. Therefore, to reduce zoonotic events, it is important to control the spread of IAV within these animal reservoirs, for example with efficient vaccination strategies, but also to critically surveil the different manmade reservoirs to evaluate the emergence of new IAV strains with pandemic potential.

Keywords: animal-human interface; avian influenza; equine influenza; influenza A viruses; livestock farming; pandemic; swine influenza; zoonosis.

Figure 1

Poultry and swine livestock increased worldwide since 1961, as well as zoonotic transmission events and the establishment of IAV lineages. (A) The amount of poultry that were produced per year is shown for different global regions as share of the worldwide production on the left y-axis. The right y-axis indicates the number of humans that were infected with AIVs (H5N1 cases in yellow, H7N9 cases in light blue). The red arrow indicates the start of the H5/H7 poultry vaccination campaign. (B) On the left y-axis, the amount of swine livestock in different global regions as share of the worldwide production per year is shown. The number of humans that were infected with swine IAV are shown on the right y-axis (yellow line). The establishment of major IAV lineages in swine is indicated. The data on livestock were retrieved from FAOSTAT.