Usually, wild birds are resistant to the disease but do carry and secrete the virus, transmitting it to domesticated birds (chicken, duck, and turkey) that are susceptible and can become sick and die. The influenza A virus is found in the saliva, nasal secretions, and faeces of the birds. There are several subtypes of influenza A virus, based on differences in two proteins on the virus membrane: hemagglutinin (HA) and neuraminidase (NA). There are 17 known HA subtypes (H1 to H17) and 9 NA subtypes (N1 to N9) and therefore, there are many possible combinations of HA and NA. Some of the strains are of the “low pathogenic” form causing mild, if any, clinical signs in poultry and others are “highly pathogenic” causing high morbidity and mortality, e.g. the H5N1 that killed millions of birds and forced the culling of several hundreds of millions more in recent years, especially in Asian regions.
Since March 31, 2013, an H7N9 avian influenza A virus has been detected in four provinces of eastern China and has infected 1584 people of which 616 have died, some 1000 contacts are under medical observation (situation as of 21 September 2017). Importantly, this is the first time that this subtype has been found in humans. The virus is classified as low pathogenic (i.e. infected birds look healthy even though they carry the H7N9 virus and therefore are a potential threat to human health). It is a new subtype found in human and it derives from the combination of genes from 3 different avian influenza viruses. While there is clear indication of the transmission of the H7N9 virus from poultry to humans, there has been no indication of human to human transmission and therefore, at present, the risk to public health seems to be low. No cases of this new H7N9 subtype have been reported outside China.
