Virus transmission
How are influenza viruses transmited from human to human?
Seasonal and pandemic influenza are highly contagious diseases and are spread primarily by coughing and sneezing. When they sneeze, people release tiny droplets of fluid into the air. Those tiny droplets can fly up to 1 metre and infect persons nearby by reaching susceptible cells of the respiratory tract. The virus can also be spread indirectly when a person touches objects such as door knobs, tap handles or phones that are contaminated with the influenza virus through sneezing or touch. The influenza virus may remain infectious on surfaces for up to three months in cool temperatures, and in water at 0 °C for more than 30 days or in water at 22 °C for up to 4 days.
People who are infected with an influenza virus can pass along the virus even before the symptoms kick in, and may remain infectious for about five days after they start sneezing. Young children can be infectious even longer and seven days is no exception.
How are influenza viruses transmited from animals to humans?
Many different animals, including birds such as ducks, geese, gulls and domestic poultry, and mammals such as pigs, whales, horses, dogs and seals, can be infected with influenza viruses. Remarkably, however, certain subtypes of influenza are solely or predominantly found in certain animal species. Birds, of course, are hosts to all known subtypes of influenza A virus, but also in birds a preference of certain influenza viruses for certain species does exist.
Occasionally, an influenza A virus normally seen in one species crosses the species barrier and causes illness in another species. In humans, this may for instance happen after close contact with birds infected with avian influenza viruses. Notoriously, sporadic infections with highly pathogenic avian influenza (HPAI) H5N1 viruses transmitted from birds to humans in Asia and in Europe have caused at least hundreds of serious infections reported in the past few years, the majority of which were associated with a fatal outcome. Significant human to human transmission of this virus was not observed. Obviously, the main concern is that this could happen and a pandemic outbreak of influenza would thus emerge. It cannot be predicted whether an HPAI-H5N1 virus will indeed be at the basis of a future pandemic. Other influenza A virus subtypes such as the H1, H2, H7, and H9 subtypes are also likely candidates. They may spill over directly from birds, but also from other mammals previously infected by birds, such as pigs, horses, dogs or cats. The adaptation to transmission among humans or other mammals may either happen by acquiring sequential mutations, or by a re-assortment event.
What are the most important factors that determine influenza virus transmission to humans?
Whether an influenza A virus will be transmitted from birds to humans primarily depends on two factors: the nature of the exposure to infected birds or their products and the susceptibility to infection with the influenza virus concerned. Initially, it was speculated that humans are relatively unsusceptible to direct infection with avian influenza viruses. However, in the past decade it has become clear that humans, like other mammals, may be susceptible to direct infection with several avian influenza A viruses. These infections may directly spill over from birds or via an intermediate mammalian host. Although to date several hundreds of people have been hospitalized with an HPAI-H5N1 virus infection that in most cases spilled over directly from domestic birds, this number is relatively low given the enormous number of poultry that have been infected in Europe, Asia and Africa to date. Apparently, one only becomes infected after close contact with infected birds. Slaughtering and de-feathering as well as consumption of insufficiently heated or raw meat or blood are important risk factors. Swimming in contaminated water is also a risk factor.