Avian influenza is an infection caused by type A influenza viruses, especially subtypes H5, H7 and H9. It can affect almost all species of birds, both wild and domestic. It is generally asymptomatic in wild birds but can become highly contagious and cause extremely high mortality in factory farms of chickens and turkeys, earning it the nickname "fowl plague" or "chicken Ebola". The avian influenza virus can sometimes infect other animal species such as pigs and other mammals, including humans.
H5N1: the risks in France and worldwide
The H5N1 avian influenza virus was first identified in 1997 during an outbreak in Hong Kong which resulted in the deaths of six people. It re-emerged in late 2003, firstly triggering epizootics (diseases affecting only animals) in poultry in several countries in Asia, and then causing the first human cases.
Human-to-human transmission via the emergence of a new subtype
In all known cases of avian 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. But the threat remains: the spread of infection among birds increases the probability of the emergence of a new influenza virus in the human population. Moreover, as with all type A influenza viruses, subtype H5N1 is highly capable of mutating over time, and also of exchanging its genes with influenza viruses of other subtypes that infect other species. The potential emergence of a new virus capable of human-to-human transmission cannot be ruled out.
A virus that would evade our immune system
If an influenza virus belonging to a viral subtype that is totally unknown to the human population, such as H5N1, were to emerge, the immune memory accumulated among the general population during seasonal outbreaks caused by traditional influenza viruses (currently for type A influenza: H3N2 and A(H1N1)pdm09) would become ineffective. This would create the conditions for a pandemic – the rapid global spread of the virus.
Probable scenarios for the emergence of a new subtype
How could a sudden change in subtype occur? There are two possibilities:
The first is that an existing subtype of the virus in the human population may stop circulating among humans for several years but remain present in an animal population. If this happens and the animal population comes into direct contact with a human, it could transmit the virus to this individual once again. For example, the H1N1 subtype that caused Spanish influenza disappeared from the human population in around 1957. But it continued to circulate in pigs, enabling it to re-emerge in humans 20 years later, in 1977.
The second possibility is that a new viral subtype may be created by genetic reassortment.
This would occur if a host were co-infected with two different viruses – in this case an avian virus and a virus infecting mammals (humans). Within the same cell, the two viruses would multiply, producing several copies of their genomes. During virus assembly, reassortant viruses that have randomly incorporated segments from the genomes of one or other of their parental viruses are formed. If one of these new viruses were to contain segments of the proteins H5 and N1 that are found in avian influenza, it would fail to be recognized by the human immune system, falling completely under the radar. If it also had genes enabling it to multiply effectively in mammals, it would be capable of spreading between humans as easily as "traditional" influenza.
Monitoring and prevention
To prepare for the risk of an influenza pandemic, the World Health Organization (WHO) emphasizes the importance of monitoring the emergence of outbreaks in populations of poultry and migratory birds and any respiratory diseases in individuals exposed to infected poultry. It also highlights the need for rapid adoption of the measures recommended by the Food and Agriculture Organization (FAO) and the World Organization for Animal Health (OIE) and for the identification of viruses in reference laboratories.
The avian influenza virus generally spreads through contact with infected birds. One of the main safety measures to contain the disease is therefore to observe good hygiene practices (regular hand-washing, wearing a mask, etc.).
Surveillance in France
In France, the mission of Santé publique France is to monitor the health of the population as a whole and to alert the public authorities in the event of any public health threats.
As part of the National Influenza Pandemic Prevention and Response Plan, the French General Directorate of Health (DGS) has drawn up a government action plan and asked Santé publique France to estimate both the likely scale of such a pandemic in France and the epidemiological impact of various strategies that could be adopted in the event of a pandemic. Like many other European countries, France has stepped up surveillance of wild birds to get a clearer picture of the circulation of avian influenza viruses.
Measures in France in the event of a pandemic
The government action plan drawn up by the DGS identifies various phases for the progressive implementation of measures to combat a pandemic, with the aim of slowing down the spread of the outbreak to give the authorities time to adapt to conditions on the ground and reduce the risk of inefficiency in healthcare structures.
The first measure would be to eradicate all avian reservoirs that may contribute to the spread of the outbreak (systematic culling of chickens and other poultry). The first industrial farm affected would be emptied and left vacant. The same would apply to other poultry farms within a perimeter set by veterinary authorities.
If the pandemic risk were confirmed, extreme measures could be imposed: restrictions on movement, border closures, public transport closures, etc. Some of these measures could be taken as soon as there are any confirmed cases of human-to-human transmission.
In countries affected by the virus
When an animal outbreak is identified, 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 July 2005, an international conference under the aegis of WHO, the OIE and the FAO ruled on the measures needed to stop such a virus spreading. It particularly focused on the need to segregate different animal species, avoiding all contact between poultry and pigs, and to encourage farmers to report any suspected cases of influenza to the authorities.
As well as recommendations for farmers, personal protective measures are recommended for individuals exposed to infected poultry. Anyone traveling to areas with animal outbreaks should also take certain precautions.
A vaccine to prevent the disease
Each year, the pharmaceutical industry produces vaccines which target the most recent strains of human influenza viruses. For countries in the northern hemisphere, the composition of these vaccines is decided by WHO in February, to make sure the vaccines are available in October, before the start of the new influenza season.
WHO has also launched an initiative for the development of a pandemic influenza vaccine. This vaccine candidate, still under development, is based on a strain isolated in Vietnam in 2004. But it raises a number of problems, the main one being that the pandemic virus does not yet actually exist.
WHO-led epidemiological surveillance has confirmed that the evolution of the most recent strains of H5N1 virus has not cast doubt on the efficacy of the vaccine – unlike in 2004, when development of the vaccine produced using the 2003 strain had to be stopped and the vaccine program was relaunched using a strain isolated in 2004. In any case, it takes between 6 and 8 months to develop a vaccine, which is why antiviral treatments are so important for initial efforts to combat a pandemic.
Vaccination involves introducing an agent (a virus, bacterium or molecule) into the body which stimulates the immune system but is not itself pathogenic. The vaccinated individual begins to produce antibodies against these foreign molecules, and if the body is infected by the same infectious agent at a later date, it has the tools it needs to combat the infection.
Although there is currently no vaccine available, there are two effective antiviral drugs for "traditional" or avian influenza viruses. These molecules inhibit the activity of a viral enzyme known as neuraminidase. They can both be used as a curative treatment, and one is also effective as a preventive treatment. In the event of a pandemic, these antiviral drugs will mainly be used to protect healthcare professionals and others whose jobs are vital for the continued operation of national infrastructures.
It is worth pointing out that antibiotics have no effect on viruses and that they are only recommended in the event of a secondary bacterial infection.
At the Institut Pasteur
At the Institut Pasteur, the National Reference Center for Virus respiratory infections (including influenza) (page in French), directed by Sylvie van der Werf, is also a Collaborating Center in the WHO H5 Reference Laboratory Network. In this capacity, in 2003 it characterized strains of the H5N1 virus that it received from the various institutes in the Institut Pasteur International Network and sent the results in real time to WHO, thereby contributing to the ongoing epidemiological surveillance of avian influenza. The CNR is also involved in the French network monitoring the emergence of the virus in wild birds by studying samples from two sensitive sites, the Camargue and Dombes. This surveillance is vital to monitor the development of H5N1 viruses and to highlight any antigenic drift or viral reassortment that could result in adaptation of the H5N1 virus to humans.