Global epidemiological cycle and transmission of flu viruses

The Influenza viruses circulate in migratory birds in a natural, temporary form and also, obviously, in the habitat and environment that they frequent. In general, these viruses are naturally well adapted to these species and, therefore, the infection has no implications from a clinical point of view. Occasionally, these viruses can “jump” from these birds (what is known as “spill over”), which are “virus reservoirs”, to other animal species, including domestic birds, swine, horses, and a great variety of other mammals, both carnivorous and marine. Each species has its own viruses (as we have seen when speaking of taxonomy), which circulate naturally and seasonally in the particular species. The global influenza cycle has as its central axis in nature, aquatic and marine birds, (i.e.

Charadriiformes), which carry virus enzootically and distribute it along their migrations, through the water, or simply as they fly. Other waterfowl, such as ducks or the anseriformes, can become infected by these viruses and serve as a virus reservoir for the former, where genetic reorganization events can take place and new viruses can appear. The ducks can transmit influenza virus to domestic birds, due to their proximity to them. In turn, domestic birds can be sources of virus to mammals, such as horses or pigs. Since these species have their own influenza viruses, genetic reassortments may happen that give viruses a greater potential of infection for humans.

For example, the genome of the pandemic virus that spread in 2009, had a triple origin, from bird, pig and human viruses. For the transmission to different species from the wild reservoirs

to occur, it is necessary: the contact between them, that the new species becomes infected, that there is a good adaptation to the new species, and that the virus is able to spread effectively. In a previous activity, we have seen how new variants and new viruses generate, but we have to bear in mind that not all viruses are equal, and are capable of adapting equally well to the circulation within the new species. When a new virus is very efficient in its replication in the new species, we say that it has pandemic potential, due to its rapid spread, either within the human species, or within other species throughout the world.

Domestic birds in semi-freedom have a greater risk of becoming infected with influenza viruses, either from wild birds along their migratory routes, or from wild waterfowl that have become infected by other aquatic birds. Influenza viruses are shed from infected animals primarily through the air-borne route. In the case of the enzootic cycle in wild birds, the transmission is also faecal-oral. However, interspecies transmission has other additional sources of infection such as direct contact, inhalation of aerosols, or indirect contact with the equipment, the staff that takes care of the animals, etc.

Other factors that also influence the ease of transmission are: animal trade, transport, markets, or animal density. In these situations, the disease will acquire the form of “outbreaks”. In the wild, the circulation of avian viruses is enzootic, and also stays connected to different environments such as coasts, lakes, also associated with the migratory routes of wild birds. Stagnant water and the load of virus in droppings will influence the circulation of virus in different parts of the world. On the circulation and distribution of the virus at a geographical level, species, the migratory routes and interior wetlands play a very important role. The great danger of avian influenza is that it is rapidly transmitted between wild and domestic species.

In the avian hosts, the virus is excreted through faeces, conjunctiva, and respiratory secretions. The most relevant species in the avian enzootic cycle are the Charadadriiformes, the Anseriformes, and other birds, such as Galliformes and Psittaciformes From the point of view of biological risk of transmission, the Anseriformes are the most important, since they are very susceptible to infection, they do not become ill and do not show signs of infection. Due to the huge variety of species that can be infected by influenza viruses, the possibility of genetic reorganization in different hosts, and the permeability between wild and domestic species, influenza is a disease that cannot be eradicated, although it may be controlled.

In the environment of a farm, it is a lot easier to control the infection than in the wild, as we can vaccinate or eliminate infected animals. We should not forget that by controlling the infection in the animal populations, we help prevent the possible transmission to human populations.