Avian Influenza is an important disease that has severe health and economic consequences. It is also a global problem against which all countries suffering outbreaks of infection have to fight against together. Avian influenza is produced by type A influenza viruses and can affect different species of poultry, pet birds and wild birds. Avian influenza viruses have been isolated occasionally also in mammals and man. There are two types of infections produced by different subtypes of viruses, which are referred to as “high pathogenicity” and “low pathogenicity” viruses. The “high pathogenicity” strains produce a generalized infection and fast mortality, which can reach up to 100% of the susceptible birds. In poultry farms, it is a “transboundary”, highly contagious disease, which constitutes a sanitary emergency.
To date, viruses with H5 and H7 hemagglutinins are the only ones responsible for the high pathogenicity avian influenza. Among them, H5N1, which emerged in East Asia in 1997, stands out because it has produced various epizootic diseases in more than 60 countries in Asia, Africa, Europe, and more recently, in North America. New subtypes have emerged in the avian domestic and wild populations, which have produced outbreaks of different impact and magnitude. These viruses include H5N2, H5N3, H5N6, H5N8, H5N9, H7N2, H7N3, H7N7, H7N8 and H7N9. High pathogenicity avian influenza is a major risk to human health, since it is a zoonosis, although only on few occasions avian viruses have been able to cross the species barrier and infect humans.
Transmission to humans only happens in close contact with infected birds or in environments with high levels of viral contamination or viral load. The person-to-person transmission of these viruses is not consistent. Cases of human disease by high pathogenicity viruses tend to be associated with ongoing outbreaks of disease in birds. They should never be confused with the seasonal flu that we have already seen and that usually is produced by viruses with H1 or H3 hemagglutinin. Wild aquatic birds, including ducks, are natural reservoirs of the so-called “low pathogenicity” viruses, which typically cause a subclinical infection or a mild respiratory illness and low mortality, with the exception of secondary complications. These avian viruses of wild aquatic birds are shed through the stools.
This is the main route of contamination and spread of the infection. While wild birds normally do not get sick, and there are no clinical signs, they can transmit the virus to susceptible domestic species, such as chickens, domestic turkeys or ducks. The low pathogenicity viruses have H5 or H7 hemagglutinins also. Thanks to the OIE surveillance programs in different countries of the world, outbreaks by H5N2, H7N7, H7N8 and H7N9 subtypes have been reported in 2016. In poultry farms, the primary introduction of avian influenza viruses probably occurs by direct or indirect contact with wild birds.
Poultry species may be more or less receptive to the infection and the incubation period will vary slightly, from five to six days, or from hours to up to a week, depending on the host and the viral strain. The clinical signs, in general, can be sudden deaths in a high percentage of animals,
severe depression, loss of appetite, diarrhoea, regurgitation, sneezing, congestion and haemorrhage in the trachea, loss of weight, abnormal plumage, swelling of the head, and neurological disorders. In farms of laying hens, there is a rapid reduction of feed intake and, therefore, of egg production, with a mortality rate between 35 and 70%. In broiler farms, mortality ranges between 5-80%, depending on the conditions of the farm and the viral strains. The disease is controlled by reinforcing the biosecurity measures in the farms, to prevent the entry and spread of virus.
In addition, it is based on the adoption of programmes of epidemiological surveillance, disease control between countries and global health policies, contingency plans and also, the improvement of communication and information flow within the affected country itself and between countries at risk. Lastly, avian influenza has a major economic impact as it negatively disturbs trade, affecting the stability of poultry markets at national and international levels.