Wild aquatic birds – natural hosts
Avian influenza is caused by influenza A viruses. These viruses occur naturally and are found in many different varieties in wild birds, especially in ducks, gulls and waders. They rarely spread to other animal species and humans.
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 from wild birds to other species
Like many other viruses and bacteria, influenza viruses sometimes also infect other species than usual. When transmitted to a new host species, influenza viruses can evolve and develop new properties such as a greater ability to cause disease. 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 infections occur 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 a few years ago 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 a different mammal, such as pigs. However, this theory is debated.
Avian influenza virus infections in 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 serious illness.
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 removal 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, Europe 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. From 2003 to December 2013, about 648 laboratory-confirmed cases of A(H5N1) from 15 countries were reported by the World Health Organization (WHO). Of these, 384 died. In January 2014, Canada reported a death in a patient who was diagnosed with influenza A(H5N1) after traveling to China. No new cases have been linked to the patient in Canada.
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, especially among those who had been exposed to sick birds, but also among their family members.
In 2013, a new avian influenza virus, A(H7N9), appeared in the eastern parts of China that is also transmitted to humans. 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. H6, H9 and H10 are examples of this.
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. Theoretically, some viruses can also infect humans via meat or eggs, but 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.
The symptoms of avian influenza in humans vary with the virus variant and may have different severity. 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 can also occur. A serious infection of the lower respiratory tract with subsequent respiratory problems may appear relatively early in the course, and may result in respiratory failure and death. Sometimes brain inflammation (encephalitis) also occurs.
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 influenza outbreak, called a 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 infectious 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. These viruses 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).
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. People who travel to countries with outbreaks of avian influenza should avoid contact with infected birds. It is also important to prevent concurrent infection with avian influenza viruses, human influenza viruses and influenza viruses from other species.
- Avoid contact with live tame and wild birds
- Avoid visiting markets where live birds and animals are sold, or poultry farms
- Avoid contact with surfaces that are soiled with faeces or secretions from birds and other animals
- Do not touch dead birds
- Avoid touching untreated feathers
- Ensure that meat and eggs from birds are heat treated to over 70 °C (well fried / grilled / cooked)
- Wash hands thoroughly and regularly with soap and use hand disinfectant
- Do not attempt to bring live poultry or poultry products back to Norway
- Follow the advice of local and Norwegian authorities
If you suspect that you have been infected with avian influenza, contact the local health service. If you are in doubt about where to seek treatment, contact your nearest Norwegian embassy or consulate for advice. The seasonal influenza vaccine does not protect against avian influenza. It is not necessary to bring an emergency package of influenza medicines when traveling.
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 but this is currently not commercially available. Some countries, such as USA, have contingency stocks of H5 vaccine. Norway has no inventory or agreement to purchase an H5 vaccine. There is no commercial vaccine against A(H7N9), but several companies are working to develop a vaccine. Antiviral agents are commercially available to treat people who have been infected with avian influenza. These drugs may also be appropriate for preventive use.
Following the avian influenza outbreak in Hong Kong in 1997, many countries intensified efforts to finalise a plan against pandemic influenza. In Norway, this was completed in 2001 and last revised in February 2006. An interim plan was completed in 2013 and a new revision is expected in 2014. It examines the preparedness work in all parts of the health service.
There is a close national and international collaboration between the various authorities under the leadership of the Ministry of Agriculture and Food and the Ministry of Health and Care Services.
Norwegian Institute of Public Health: responsible for monitoring human disease, giving advice on infectious disease control for people and vaccine preparedness. The NIPH participates in international surveillance networks in infectious disease control and provides advice and information to health authorities, the health service and the public about disease, infection, infectious disease control and vaccination.
Norwegian Directorate of Health: responsible for emergency preparedness, action and response if avian influenza virus spreads to humans. The Directorate has authority in health regulations and can introduce preventive measures in the community.
Norwegian Food Safety Authority: responsible for emergency preparedness, risk management and control of avian influenza in animals. They introduce surveillance and control, import bans and participates in international forums.
Norwegian Veterinary Institute: responsible for risk assessment associated with avian influenza in animals, they analyse samples taken in Norway and are responsible for the monitoring of avian influenza in poultry and wild birds.
2013: Influenza A(H7N9) in China
Influenza A(H7N9) is the first avian influenza virus we are aware of that shows few symptoms in poultry but that has led to deaths in humans. Many of the people diagnosed with A(H7N9) virus were reported to have been in contact with poultry or live bird markets. Contact tracing indicated that the virus did not spread easily between humans. Although some examples of possible infection within the same family were seen, so far there is no evidence of effective transmission between humans.
The majority of cases are in the eastern and southern parts of China in 12 different provinces. The virus was detected in both sexes and in all age groups.
Analysis of A(H7N9) virus shows that it is composed of components from other avian influenza viruses from Asia. Unlike the A(H5N1) virus, the A(H7N9) virus causes little disease among poultry and it is therefore difficult to identify how far the virus has spread, the transmission routes and the source of infection.
The virus was detected in poultry and environmental samples taken at live bird markets in the same areas where infection has been confirmed in humans. Analysis shows that these viruses were genetically similar to those found in humans. The virus was detected in samples from ducks, pigeons and chickens. The occurrence of specific antibodies against A(H7N9) virus was higher in blood samples from poultry workers than in samples taken from the general population. Handling of poultry at poultry markets is considered to be the greatest risk factor for human infection. It is unknown whether wild birds played a role in the spread of the virus.
Symptoms of A(H7N9)
The time between infection and appearance of symptoms (incubation period) is believed to be about six days but can vary from one to 10 days. The A(H7N9) virus can cause influenza-like symptoms such as high fever, cough and shortness of breath. Diarrhoea and vomiting may more rarely occur. Some people get complications like rapidly developing pneumonia and severe respiratory failure and possibly other organ failure. Several of the reported infections have led to hospitalisation but there may also be a milder course of the disease.
Laboratory tests indicate that oseltamivir (Tamiflu) and zanamivir (Relenza) work against influenza A(H7N9) virus. These medicines can shorten the course of the disease. They must be given within 48 hours of onset of symptoms to maximise their effect but can also be given in the case of a severe infection.
Past outbreaks of H7 viruses
Other avian influenza viruses of the H7 subtype have also previously sporadically infected humans. Transmission to humans associated with outbreaks in birds was seen in the Netherlands in 2003, in the United Kingdom in 2006, Canada in 2004 and Mexico in 2012. In these cases, conjunctivitis was a distinctive feature. The infections gave mild symptoms with the exception of one fatal case during the H7N7 outbreak in the Netherlands. The new A(H7N9) virus is different from previously known H7 and H9 viruses, and may cause other clinical outcomes.
Does A(H7N9) have pandemic potential?
As the influenza A(H7N9) virus does not usually occur in humans, it can be assumed that immunity against the virus in the population is low. Any influenza viruses from birds and mammals (e.g. pigs) that develop the ability to infect humans involve a theoretical pandemic risk. Whether influenza A(H7N9) virus could cause a pandemic is uncertain. Currently, it does not seem to transmit easily between humans. However, genetic analysis and experimental studies suggest that the virus shows signs of adaptation to mammals and therefore also humans. In addition, it is possible that the virus could mix with seasonal influenza viruses, giving rise to a virus that spreads easily between people.