About avian influenza
Article
|Updated
Avian influenza (also known as bird flu), is a contagious viral disease that mainly affects birds. In some cases, other animals and people have become infected after close contact with infected birds.
Avian influenza is caused by influenza A viruses. These viruses occur naturally and are found in many different strains among wild birds, especially in ducks, gulls and waders. They rarely spread to other animal species and humans.
When chickens are infected by influenza virus from wild birds, the virus can appear in two main forms: a low-pathogenic form that causes little disease, and a highly pathogenic form that almost 100% fatal for them. The highly pathogenic form can occur if the low pathogenic viruses of the H5 and H7 subtypes develop new properties. Both low-pathogenic and highly pathogenic viruses in chickens can, rarely, infect other animals and humans and cause a severe disease course.
In recent years, there have been extensive outbreaks of highly pathogenic avian influenza among birds in Europe. The viruses belong to a special subgroup of H5 viruses (2.3.4.4b). The outbreaks were dominated by H5N8 in 2020 and then by H5N1 in 2021-2022. H5N1 has also spread to birds in Asia, Africa and the Americas.
Highly pathogenic avian influenza was detected for the first time among wild birds in Norway in 2020. The virus was of the H5N8 subtype. The following year, highly pathogenic avian influenza was detected for the first time in commercial poultry farming in Rogaland, where H5N1 was detected among two flocks of laying hens. In the spring and summer of 2022, highly pathogenic avian influenza H5N1 and H5N5 were detected among wild birds (including birds of prey and seabirds) in large parts of Norway, including on Svalbard and Jan Mayen. The H5N1 virus was also detected in a small number of sick red foxes. In autumn 2022, H5N1i was detected in two poultry flocks in Rogaland.
Influenza in wild aquatic birds
Although influenza viruses are commonly found in humans and some other mammals, it is in wild birds, especially birds that live near water, that a large variety of influenza A viruses are found. All influenza A viruses we have in humans can be traced back to viruses among birds.
Influenza A viruses are divided into subtypes according to the H and N proteins on their surface. 16 different H proteins and 9 different N proteins have been detected in influenza viruses from wild birds. All possible combinations of the two can occur in ducks, gulls and waders without them becoming particularly ill. These birds are therefore considered as natural hosts for influenza A viruses.
Transmission to other species
Like many other viruses and bacteria, influenza viruses sometimes also infect other species, where they may evolve to develop new properties such as the ability to cause a more severe disease course. This can happen, for example, if a virus from wild birds begins to infect chickens. Among chickens, two subtypes of influenza, A, H5 and H7, are especially prone to changing properties under such conditions, where they can go from being relatively innocuous to become deadly viruses.
Since these viruses are basically adapted to birds, it is difficult for these viruses to transmit to different mammals - including humans, so such transmission occurs relatively infrequently. If an animal or human does become infected by an avian influenza virus, it is not easy for the virus to transmit to new individuals.
Until the discovery of the transmission of A(H5N1) virus directly from birds to humans in 1997, it was commonly assumed that the transition to humans was so difficult for avian influenza viruses that they first had to circulate for a while in another mammal, for example pigs. There is still much we do not know about the prerequisites for an influenza virus from birds to be fully adapted to humans.
Avian influenza outbreaks with transmission to humans
Among the cases of avian influenza viruses detected in humans, viruses of the subtypes H5 and H7 dominate, but other subtypes have also occurred. Both viruses that are pathogenic in chickens can occasionally infect humans and cause a severe disease course.
Influenza A(H5)-virus
Cases of infection directly from birds to humans with H5N1 virus were first detected in Hong Kong in 1997. At that time there were 18 cases, six of them fatal. Genetic analysis determined that the virus came from an outbreak of deadly avian influenza in Hong Kong's poultry stocks. Prompt culling of approximately 1.5 million birds probably prevented a larger number of cases among humans. From 2003-2006 the H5N1 virus spread to parts of Asia and Africa, where it caused sporadic infections and deaths in humans and other animal species. Since then, the A(H5N1) virus has become established in poultry populations in parts of Asia, Africa and the Middle East. The virus occasionally infects humans in these areas. In January 2014, Canada reported a death in a patient who was diagnosed with influenza A(H5N1) after traveling to China. From 2003 to January 2023, about 868 laboratory-confirmed cases of A(H5N1) from 21 countries were reported by the World Health Organization (WHO). Of these, 457 died.
In 2013, a new A(H5) virus, called A(H5N6), was detected among birds in China which could sporadically infect humans and cause a severe disease course and death. The virus is related to H5N1 and since 2013 has also been detected among wild birds and poultry in South Korea and Japan. During the period 2014 to January 2023, the WHO reported 83 laboratory-confirmed cases in humans, most of them in China. Of these, 33 died.
Despite extensive outbreaks among birds of highly pathogenic avian influenza belonging to H5 subgroup 2.3.4.4b, the virus has only been detected in a small number of people. In February 2021, Russian authorities reported to the WHO the discovery of A(H5N8) in 7 workers at a poultry farm in Astrakhan. The infection occurred in connection with combating an outbreak of avian influenza in a large commercial poultry flock in December 2020. In January 2022, H5N1 was detected in a person in England who had close and long-term contact with infected domestic poultry. The virus was also detected in one person in Colorado, USA, in April 2022, and two people in Spain in September/October 2022. All had been involved in combating outbreaks in commercial poultry flocks. The people with the virus had no or mild symptoms. In autumn 2022, WHO reported that H5 virus belonging to 2.3.4.4b had been detected in one person in Vietnam and one person in China. Both became seriously ill, and one of them died. There have been no signs of further transmission between people.
Influenza A(H7)-virus
In the Netherlands in 2003, a veterinarian died during an outbreak of H7N7 avian influenza, while 83 people had mild influenza-like illness and conjunctivitis. Antibody studies have subsequently shown that considerably more had been infected without becoming ill. Those who became ill had been exposed to sick birds, or were family members of those who were infected, probably by person-to-person transmission.
In 2013, a new avian influenza virus, A(H7N9), that is also transmitted to humans appeared in eastern China. During the period from 2013 to January 2023, 1,568 laboratory-confirmed cases were reported by the WHO, most of them from China. Of these, 615 died. Most people who have been diagnosed with A(H7N9) have been in contact with poultry or live bird markets. The virus causes little disease in poultry (i.e., it is low pathogenic). In February 2017, the WHO reported that A(H7N9) virus had been detected in parts of China that had evolved from being less pathogenic to being more pathogenic for poultry. This change has so far not led to an increased risk for people. In its current form, the influenza A(H7N9) virus does not spread easily to or between people.
Over a hundred people were diagnosed with the virus in April-May 2013. During the summer, there were few cases. From October 2013 and early 2014, the number of detections in humans has again increased significantly. It is reported that most people diagnosed with A(H7N9) have been in contact with poultry or markets where live birds are sold. As the virus causes little disease in poultry, it is difficult to identify the virus distribution and transmission routes. In its current form the A(H7N9) virus is not readily transmitted between humans.
Other avian influenza viruses than H5 and H7 have also occasionally infected humans, examples include H3N8, H6N1, H9N2, H10N8 and H10N3.
The two subtypes that are currently established in humans, H1 and H3, also originated in viruses from birds 50-100 years ago.
Transmission route and infectivity
Depending on the virus, avian influenza viruses can pass from one bird to another through respiratory secretions or faeces.
Human infection may occasionally occur by direct or indirect contact with infected birds or their respiratory secretions or faeces. It is usually people who handle poultry who are infected. Heat treatment at 70 °C (cooking, frying and grilling) of eggs and meat kills the virus.
Avian influenza virus spreads easily from bird to bird, but with difficulty to humans. Our mucous membranes are usually not very receptive to such viruses. Rarely, people still become infected. A person who is infected with avian influenza virus will rarely infect others.
Disease
In case of transmission to humans, the incubation period for avian influenza is approximately 2-5 days (up to 17 days), which is somewhat longer than for regular seasonal influenza, but it can vary somewhat with the virus variant. The symptoms in humans also vary with the variant and may have different severity. Some people may experience no symptoms. Some viruses cause conjunctivitis and mild respiratory disease. Others give typical influenza symptoms such as high fever, muscle aches, headache, dry cough, fatigue and malaise. Diarrhoea, vomiting, abdominal pain, chest pain and bleeding from the nose and gums may also occur. A severe infection of the lower respiratory tract with subsequent respiratory problems may appear relatively early in the disease course and may result in respiratory failure and death. Sometimes brain inflammation (encephalitis) also occurs.
Prevention
The most important measures to prevent avian influenza in humans are to monitor, prevent and control the occurrence of avian influenza virus in the world's poultry flocks. This varies from country to country.
Culling of infected birds on farms or markets, closure of markets, vaccination of poultry and increased biosecurity (e.g., prevent contact between wild birds and poultry) are examples of control measures.
Avoid touching sick and dead birds and other animals that may be infected. People who must handle birds or other animals with suspected or confirmed infection should use protective equipment.
People who travel to countries with outbreaks of avian influenza should avoid contact with infected birds.
Vaccine and treatment
Vaccines against influenza must be made for each virus variant. This means that the seasonal influenza vaccine does not protect against avian influenza viruses. After the avian influenza outbreak in Hong Kong in 1997, work began to develop a vaccine against the H5N1 virus. There are now several approved vaccines against H5N1 but they are currently not commercially available. Some countries, such as USA, have contingency stocks of H5N1 vaccine. Currently, Norway has no inventory but has an agreement to reserve production capacity or purchase of a vaccine in the case of a pandemic. There is no commercial vaccine against A(H7N9), but several companies are investigating the effect and safety of vaccines against this virus.
Antiviral agents are commercially available to treat people who have been infected with avian influenza. These drugs may also be appropriate for preventive use. Norway has a stock of these medicines.
Laboratory testing
People who have had contact with birds or other animals with suspected or confirmed infection, and who develop influenza-like symptoms (e.g., fever and cough), conjunctivitis, vomiting, diarrhoea, or a severe disease course in the 10 following days, should be tested for influenza A virus. In certain situations, it may be relevant to take samples from people without symptoms who have been exposed to infection.
The Norwegian Institute of Public Health has specific tests (PCR) for H5 and H7 viruses. If an influenza A virus that cannot be subtyped is found, the sample should be forwarded to the reference laboratory for influenza at the Institute of Public Health, where subtype identification and further testing will be done, and also possibly sent to the WHO reference laboratory.
Notification of suspected cases
Healthcare personnel who suspect infection with avian influenza (all types) in patients are asked to report this (in accordance with the MSIS regulations and the IHR regulations) to the Duty Medical Officer at the Norwegian Institute of Public Health on telephone 21 07 63 48.
Suspicion of avian influenza in animals must be reported to the Norwegian Food Safety Authority.
Travel advice
Travellers to countries and areas where human infection with avian influenza has been detected should take certain precautions.
Virus mixing - creating a new influenza virus
Since avian influenza viruses are not very contagious between humans, it is very unlikely that they will cause a new global pandemic. However, influenza viruses are unpredictable and are constantly mutating when they reproduce. Some of these changes may result in viruses with new properties. For example, if a person is infected with regular influenza and avian influenza viruses simultaneously, the infected cells could produce a cross between the two viruses. This may result in a new virus with the pathogenic properties of the avian influenza virus and the transmissive properties of the human influenza virus. This will be a new virus to which few have immunity.
Epidemics of "new" influenza viruses in humans - influenza pandemics
We know that new influenza viruses have spread and created pandemics among humans in the past, and we expect that this will happen in the future. When the Asian influenza spread in 1957 it was with a cross of influenza virus from the circulating human H1N1 virus and H2N2 avian influenza virus. Likewise, Hong Kong influenza appeared ten years later when H2 was substituted by H3 - also from avian viruses. Many believe that the Spanish influenza of 1918-1919, which killed 40 million people in the course of a year, emerged in a similar way or that the entire virus originated directly from an avian virus.
When these "new" influenza viruses appeared there was almost no immunity to these viruses in the human population and they could therefore spread virtually unhindered. Such unusually large global influenza epidemics are called influenza pandemics. From the 1900s to the present day, this has happened four times: 1918-20: Spanish influenza, 1957-1958: Asian influenza, 1968-1970: Hong Kong influenza and 2009-10: Swine influenza A(H1N1).
Preparedness
Norway has contingency plans against pandemic influenza. In addition, we have agreements on the delivery of vaccine to the entire population in the event of a new pandemic. The Norwegian Directorate of Health also has a large stockpile of antiviral agents that can be used in the event of an outbreak of pandemic influenza. The Norwegian Food Safety Authority and the Veterinary Institute have comprehensive routines and plans for the prevention, detection and control of avian influenza among poultry.
