Avian influenza, also known as ‘avian flu’ or ‘bird flu’ is an infection- usually of wild birds- but sometimes also of commercial or domestic poultry. Water birds are the natural host of avian influenza. Avian influenza viruses are within the group of ‘influenza A viruses’ and are classified into subtypes according to two proteins found on the surface of the virus: haemagglutinin (H) and neuraminidase (N).
There are 18 haemagglutinin subtypes and 11 neuraminidase subtypes. These proteins determine the kind of animal (birds, pigs, or humans) that the virus can infect. Avian influenza and human influenza viruses are the same virus: they have the same basic viral structure, but have different H and N proteins.
Human influenza is caused by ‘influenza A(H1N1)’ and ‘influenza A(H3N2)’ and influenza B. Many different avian influenza strains are responsible for bird flu outbreaks, such as ‘influenza A(H5N8)’, ‘influenza A(H5N2)’. Rarely, avian influenza strains can cause disease in humans. Avian influenza strains that have caused disease in humans are ‘influenza A(H5N1)’ and ‘influenza A(H7N9)’
Most viruses cause only mild disease in poultry, and are called low pathogenic avian influenza (LPAI) viruses. Highly pathogenic avian influenza (HPAI) viruses can develop from certain LPAI viruses, usually while they are circulating in poultry flocks. HPAI viruses can kill up to 90-100% of the flock, and cause epidemics that may spread rapidly, devastate the poultry industry and result in severe trade restrictions. In poultry, the presence of LPAI viruses capable of evolving into HPAI viruses can also affect international trade.
Who can get avian influenza?
Human infections with avian influenza are uncommon and usually occur in individuals who have close contact with birds, either live or dead, that are infected with avian influenza viruses. Human infections have occurred in persons who visit or work in live poultry markets or commercial farms where an avian influenza strain is circulating. Avian influenza viruses do not usually spread from person-to-person.
The species in the orders Anseriformes (ducks, geese, swans) and Charadriiformes (shorebirds, waders, gulls) are regarded as important reservoir hosts and disseminators of AI viruses, but rarely display clinical signs of infection
Mode of transmission
Bird to bird
Infected birds may shed virus in their saliva, nasal and respiratory secretions, and faeces depending on many factors such as the type of bird, the virus sub-type and the presence of other diseases. Faeces of infected birds can contain large amounts of virus with fecal-oral transmission the predominant mode of spread between birds. Asymptomatic waterbirds may directly or indirectly introduce the virus into poultry flocks via contaminated excretions from infected birds or via contaminated environments. Secondary dissemination is by fomites, movement of infected poultry, and possibly airborne. LPAI infection is primarily a localised infection in poultry and HPAI infection typically presents as a more systemic infection.
Bird to person
Transmission of AI infection from birds to humans is rare. When it has occurred, it is believed to have resulted from close contact with infected poultry or breathing in dust contaminated with their excretions. The virus can survive on poultry products (including eggs and blood), however no infection has been documented from eating properly cooked eggs and meat from infectious birds. Transmission has been thought to occur by ingesting uncooked poultry products (including raw blood) from H5N1 infected poultry.
Person to person
The spread of AI viruses from one ill person to another through prolonged, unprotected, close contact has been reported very rarely, and has been limited, inefficient and not sustained.
The incubation period for AI in humans may be longer than that for normal seasonal influenza, which is around two to three days. Current data indicate an incubation period will typically (and for public health purposes should be considered to) range from one to ten days. This may vary depending on the AI strain.
Influenza A viruses are susceptible to a wide variety of disinfectants including sodium hypochlorite, 60% to 95% ethanol, quaternary ammonium compounds, aldehydes (glutaraldehyde, formaldehyde), phenols, acids, povidoneiodine and other agents. Influenza A viruses can also be inactivated by heat of 56-60°C (133-140°F) for a minimum of 60 minutes (or higher temperatures for shorter periods), as well as by ionizing radiation or extremes of pH (pH 1-3 or pH 10-14).
No detailed studies have been conducted of infectivity of AI viruses in humans. Viral shedding of H5N1 has been detected in some patients up to 21 days after symptoms begin, and up to 20 days after symptoms begin for H7N9 patients, however the low number of secondary cases detected indicates that viral shedding is not an accurate reflection of AI infectivity in humans.
Based on data on human influenza subtypes:
• Adults and children older than 12 years of age are thought to be infectious typically from the day before (and up to a maximum of 5 days before) symptoms begin until usually 7 days after (and up to a maximum of 14 days after) symptoms begin. For practical public health purposes, adults and children older than 12 years of age should be considered infectious from 1 day before symptoms begin until 7 days after symptoms begin.
• Children up to 12 years of age are thought to be infectious typically from the day before (and up to a maximum of 6 days before) symptoms begin until usually 7 days after (and up to a maximum of 27 days after) symptoms begin. For practical public health purposes, children up to 12 years of age should be considered infectious from 1 day before symptoms begin until 7 days after symptoms begin.
• Severely immunocompromised persons can shed virus for weeks or months, and should be considered on a case-by-case basis.
Asian lineage H5N1 HPAI viruses
Most infections with Asian lineage H5N1 HPAI viruses have been severe. The initial signs are often a high fever and upper respiratory signs resembling human seasonal influenza, but some patients may also have mucosal bleeding, or gastrointestinal signs such as diarrhea, vomiting and abdominal pain.
Respiratory signs are not always present at diagnosis; two patients from Vietnam had acute encephalitis without symptoms to indicate respiratory involvement. Similarly, a patient from Thailand initially exhibited only fever and diarrhea. Lower respiratory signs (e.g., chest pain, dyspnea, tachypnea) often develop soon after the onset of the illness. Respiratory secretions and sputum are sometimes blood-tinged. Most patients deteriorate rapidly, and serious complications including heart failure, kidney disease, encephalitis and multiorgan dysfunction are common in the later stages. Milder cases have been reported occasionally, particularly among children.
Zoonotic H7N9 LPAI viruses in China, 2013-2014
Most clinical cases caused by H7N9 viruses in China have been serious, to date. The most common symptoms were fever and coughing, but a significant number of patients also had dyspnea and/or hemoptysis, and severe pneumonia (frequently complicated by acute respiratory distress syndrome and multiorgan dysfunction) developed in most laboratory-confirmed cases. A minority of patients had diarrhea and vomiting, but nasal congestion and rhinorrhea were not common initial signs. Conjunctivitis (which is a common sign with some other avian influenza viruses) and encephalitis were uncommon. In most cases, patients deteriorated rapidly after the initial signs. Concurrent bacterial infections were identified in some patients, and may have contributed to the clinical picture.
How can avian influenza infection in humans be prevented?
Do not touch dead birds, especially where large numbers of dead birds are found.
Where a suspected or confirmed outbreak of avian influenza has occurred, only limited numbers of persons should be exposed, and all persons should use appropriate personal protective equipment.
Surveillance for avian influenza amongst animal populations, and occupationally exposed humans should be ongoing. There is ongoing adaptation of avian influenza viruses, and it is possible that viruses with the ability to infect humans, and be transmitted from person-to-person could emerge.
All suspected cases of potential transmission of avian influenza virus from infected birds/ poultry or ostriches to humans should be investigated.
Clusters (e.g., 3 or more cases in 72 hours, or 5 or more cases in a 5-day period) of severe respiratory illness (hospitalised or warranting hospitalisation or ICU admission or death) with evidence of common exposure or epidemiologic link
How is avian influenza infection treated?
Severe infections with avian influenza (for example due to H5N1) should be treated with supportive care, and individuals should be isolated to prevent secondary cases. In addition, avian influenza infections in humans can be treated with the same anti-viral agents used to treat human influenza