What are the causes?
Wild aquatic birds are a reservoir for influenza A viruses. These viruses do not typically cause disease in birds and do not generally infect mammals. But they are frequently introduced into and circulate among domestic poultry. Moreover, some of these viruses of avian origin may occasionally cross the species barrier and infect mammals, including humans.
Certain influenza A viruses (subtypes H5, H7 and H9) may cause severe disease in birds, with a high fatality rate, especially in domestic poultry (chickens, turkeys and ducks). These outbreaks are known as epizootics.
Between 2013 and 2017, the intense circulation of influenza virus A(H7N9) in domestic poultry in China was associated with more than 1,500 cases of zoonotic influenza in humans.
H5N1-subtype viruses are currently of most concern. They were detected for the first time in humans in Hong Kong in 1997 during a widespread epizootic of influenza A(H5N1) virus.
Influenza A(H5N1) virus has continued to circulate throughout South-East Asia, giving rise to further human cases between 2003 and 2005. Despite mass culling, the virus has continued to circulate in domestic poultry and wild birds. As of 2024, it is currently circulating intensely worldwide (including in Europe and the United States) in wild birds, with outbreaks in domestic poultry and occasional cases of transmission to mammals. Since the beginning of 2024, influenza A(H5N1) virus has also been circulating among dairy cattle in the United States, with transmission events to humans, poultry and cats. The number of cases of infection in mammals and the variety of species affected have sparked fears that a virus will emerge that is better adapted to mammals and capable of spreading among the human population.
How does the virus spread?
The virus spreads through airborne transmission. Humans are generally infected if they come into close contact with infected animals, for example while slaughtering or plucking poultry, or their secretions or feces. It is theoretically possible to be infected through contact with a contaminated environment. The virus may also be found in the milk of infected dairy cattle.
In all known human cases of zoonotic influenza to date, the individuals were in direct contact with infected poultry, and the very rare cases of human-to-human transmission of the H5N1 virus have been episodic.
Avian influenza | Santé publique France (santepubliquefrance.fr) (in French)
What are the symptoms?
Infection with influenza A virus is generally asymptomatic in wild birds and causes few or no symptoms in poultry. Some viruses may be extremely pathogenic for domestic poultry and result in high mortality in factory farms of chickens, turkeys or ducks, earning them the nickname "fowl plague" or "chicken Ebola." These same viruses are sometimes also pathogenic for wild birds.
Avian influenza viruses can sometimes infect other animal species such as pigs and other mammals, including humans. In humans, the first symptoms of zoonotic influenza viruses are similar to those caused by seasonal influenza: fever, cough, etc. These symptoms can worsen and patients may have difficulty breathing. Gastrointestinal symptoms such as diarrhea, nausea and abdominal pain, as well as neurological symptoms, may occur, leading to complications such as secondary infections and multiple organ dysfunction syndrome. It is worth noting that they may also cause (mild) conjunctivitis in humans, probably when infected individuals touch their eyes with contaminated hands.
How is infection diagnosed?
There are tests to diagnose infection in humans:
- Laboratory RT-PCR tests
- RIDTs: commercially available rapid influenza diagnostic tests
These tests detect influenza infections in general (human influenza and zoonotic influenza) but most of them do not identify whether the virus is of zoonotic origin.
What treatments are available?
Each year, the pharmaceutical industry produces vaccines which target the latest strains of human influenza viruses. WHO identifies vaccine strains for viruses with zoonotic potential each year so that effective vaccines can be produced rapidly if a virus capable of human-to-human transmission emerges.
There are also two effective antiviral drugs for "conventional" or avian influenza viruses:
- neuraminidase inhibitors
- endonuclease inhibitors
In the event of a pandemic, these antiviral drugs can treat patients and protect healthcare professionals and others whose jobs are vital for the continued operation of national infrastructure.
How can zoonotic influenza be prevented?
Prevention of avian-origin zoonotic influenza involves several measures:
- Monitoring wild birds
- Preventing circulation in domestic poultry
- Early detection of outbreaks in domestic poultry
- In the event of suspected cases (domestic poultry, mortality among wild birds, etc.), personal protection measures: avoiding exposure, washing hands
- In the event of contact with infected animals, wearing personal protective equipment (masks, goggles, gloves, etc.)
- Monitoring cases of respiratory infection in individuals in contact with poultry or sick wild birds
- Vaccination of individuals whose occupation exposes them to domestic poultry or water birds
Seasonal influenza vaccination is recommended for professionals exposed to poultry farms.
Cases of avian-origin zoonotic influenza must be reported to WHO.
How to respond to an avian influenza outbreak in domestic poultry?
If an animal source is identified (with identification of the virus in a reference laboratory), measures generally involve quarantine followed by the slaughter of infected animals and any other animals that may have been exposed to the virus. Decontamination procedures for any equipment used must be applied to avoid contamination between farms.
In France, the Ministry of Agriculture monitors the domestic situation regarding avian influenza and has established a number of control measures to be taken in the event of an outbreak.
The World Organization for Animal Health (WOAH) has also established measures to prevent, tackle and control avian influenza: Avian influenza – WOAH – World Organization for Animal Health (woah.org)
Which regions are affected?
The widening geographical spread of H5N1 viruses and the growing number of cases of infected mammals and humans is currently a cause for concern.
These viruses circulate all over the world, including in polar regions. Human cases have been reported in recent years in South-East Asia, the Middle East, and more lately on the American continent.
Since 1997, 2,723 cases of zoonotic human infection with avian influenza A viruses have been reported. Of the many subtypes responsible, the vast majority of reported cases have been caused by subtypes H7N9, H5Nx and H9N2.
The Food and Agriculture Organization (FAO) has a regularly updated page on avian influenza viruses (AIV) with zoonotic potential worldwide: Global AIV with Zoonotic Potential (fao.org)
The World Organization for Animal Health publishes regular situation reports: Avian influenza – WOAH – World Organization for Animal Health (woah.org)
Lastly, the European Food Safety Authority (EFSA), the European Center for Disease Prevention and Control (ECDC) and the European Union Reference Laboratories (EURLs) are also monitoring the global situation: Avian flu: cases on the decline in Europe, surveillance recommended in view of upcoming season | EFSA (europa.eu)
Santé publique France monitors mainland France: Avian influenza: strengthening human surveillance for early detection of any transmission to humans (in French) | Santé publique France (santepubliquefrance.fr)
Other sources:
Influenza (Avian and other zoonotic) (who.int)
Avian influenza (ECDC - europa.eu)
November 2024