Avian Influenza

For the latest information on Avian Influenza issued by the European Centre for Disease Prevention and Control (ECDC) please click here.

What is the Avian Flu?

Avian Influenza, or “bird flu”, is a virulent and highly contagious viral disease, which occurs in poultry and other birds, and, less commonly, pigs.  While all bird species are thought to be susceptible to infection, domestic poultry flocks are especially vulnerable to infections that can rapidly reach epidemic proportions.

The disease in birds has two forms.  The first causes mild illness.  Of greater concern is the second form, known as “highly pathogenic avian influenza”. It was first identified in Italy in 1878.  It is extremely contagious and rapidly fatal, with a mortality approaching 100%.  Birds can die on the same day that symptoms first appear.  Avian influenza can occasionally spread to humans and other animals, usually following direct contact with infected birds.

What are the Control Measures in Birds?

The most important control measures are rapid destruction (“culling” or “stamping out”) of all infected or exposed birds, proper disposal of carcasses, and the quarantining and rigorous disinfection of farms.

The virus is killed by heat (56ºC for 3 hours or 60ºC for 30 minutes) and common disinfectants, such as formalin and iodine compounds.

The virus can survive, at cool temperatures, in contaminated manure, for at least three months.  In water the virus can survive for up to 4 days at 22ºC and more than 30 days at 0ºC.  For the highly pathogenic form, studies have shown that a single gram of contaminated manure can contain enough viruses to infect 1 million birds.

Restrictions on the movement of live poultry, both within and between countries, are another important control measure.

How do Outbreaks of Influenza Spread Within a Country?

Within a country the disease spreads easily from farm to farm.  Dust and soil are contaminated by virus secreted in bird droppings whilst airborne virus can spread the disease, causing infection when it is inhaled.  Contaminated equipment, vehicles, feed, cages or clothing – especially shoes – can carry the virus from farm to farm.  The virus can also be carried on the feet and bodies of animals, which act as “mechanical vectors” for spreading the disease.

How does the Disease Spread from One Country to Another?

The disease can spread from country to country through international trade in live poultry.  Migratory birds can carry the virus for long distances and have, in the past, been implicated in the international spread of highly pathogenic avian influenza.  Migratory waterfowl – most notably wild ducks – are the natural reservoir of bird flu viruses, and these birds are also the most resistant to infection – they only develop a mild and short-lived illness.

Domestic ducks, however, are susceptible to lethal infections, as are turkeys, geese, and several other species raised on commercial or backyard farms.

What is the Present Situation?

Since mid-December 2003, a growing number of Asian countries have reported outbreaks of highly pathogenic avian influenza in chickens and ducks.  Infections in several species of wild birds and in pigs have also been reported.

The rapid spread of highly pathogenic avian influenza, with outbreaks occurring at the same time in several countries, is historically unprecedented and of great concern for human health as well as for agriculture.

In the case of human beings, it is particularly alarming that a highly pathogenic strain of the virus –H5N1 – has jumped the species barriers causing severe disease in humans, on two occasions in the past, and is now doing so again, in gradually growing numbers, in Viet Nam and Thailand.

Why is there so much concern about the Current Outbreaks?

Public Health officials are alarmed by the unprecedented outbreaks in poultry for several reasons.

Firstly, most, but not all, of the major outbreaks recently reported in Asia have been caused by the highly pathogenic H5N1 strain.  There is mounting evidence that this strain has a unique capacity to jump the species barrier and cause severe disease, with high mortality in humans.

A second, even greater concern is the possibility that the present situation could give rise to another influenza pandemic in humans.  Scientists know that avian and human influenza viruses can exchange genes when a person is simultaneously infected with viruses from both species.  This process of gene swapping inside the human body can give rise to a completely new subtype of the influenza virus to which few, if any, humans have natural immunity.  Moreover, existing vaccines, which are developed each year to match presently circulating strains and protect humans during seasonal epidemics, could not be effective against a completely new influenza virus.

If the new virus contains enough human genes, transmission directly from one person to another (instead of from birds to humans only) can occur.  When this happens, the conditions for the start of a new influenza pandemic will have been met.  Most alarming would be a situation in which person-to-person transmission resulted in successive generations of severe disease with high mortality.

Is there evidence of Efficient Human-to-Human Transmission now?

No.  However, in Thailand in September 2004, the Ministry of Health announced possible human-to-human transmission in a family cluster where a mother could have acquired the infection either from some environmental source or while caring for her daughter.  While the investigation of this family cluster provides evidence that human-to-human transmission may have occurred, evidence to date indicates that transmission of the virus among humans has been limited to family members and that no wider transmission in the community has occurred.  Continued vigilance is needed to determine whether the epidemiological situation in humans remains stable.

Are all of the Currently Reported Outbreak in Birds Equally Dangerous for Humans?

No, only outbreaks caused by the H5N1 strain are presently of the greatest concern for human health.

However, urgent control of all outbreaks of avian influenza in birds – even when caused by a strain of low pathogenicity – is of utmost importance.  Research has shown that certain avian influenza virus strains, initially of low pathogenicity, can rapidly mutate (within 6 to 9 months) into a highly pathogenic strain if allowed to circulate in poultry populations.

Can a Pandemic be Averted?

Based on historical patterns, influenza pandemics can be expected to occur, on average, three to four times each century when new virus subtypes emerge and are readily transmitted from person to person.  However, the occurrence of influenza pandemics is unpredictable.  In the 20^th century, the great influenza pandemic of 1918-1919 was followed by pandemics in 1957-1958 and 1968-1969.

Experts agree that another influenza pandemic is inevitable and possibly imminent.

However, it is hoped that, if the right actions are taken quickly, an influenza pandemic can be averted.

• The first priority, and major line of defense, is to reduce the opportunities for human exposure to the largest reservoir of the virus – infected poultry.  This is achieved through the rapid detection of poultry outbreaks and the emergency introduction of control measures, including the destruction of all infected or exposed poultry stock, and the proper disposal of carcasses.
• Also, vaccination of persons at high risk of exposure to infected poultry, using existing vaccines effective against currently circulating human influenza strains, can reduce the likelihood of co-infection of humans with avian and influenza strains, and thus reduce the risk that genes will be exchanged.  Workers involved in the culling of poultry flocks must be protected, by proper clothing and equipment, against infection.  These workers should also receive antiviral drugs as prophylactic measure.

All available evidence points to an increased risk of transmission to humans when outbreaks of highly pathogenic avian H5N1 influenza are widespread in poultry.  As the number of human infections grows, the risk increases that a new virus subtype could emerge, triggering an influenza pandemic.  This link between widespread infection in poultry and increased risk of human infection is being demonstrated right now in Asia.  All human cases and deaths detected so far are in to countries – Viet Nam and Thailand – with very widespread outbreaks in poultry.

When cases of avian influenza in humans occur, information on the extent of influenza infection in animals as well as humans and on circulating influenza viruses is urgently needed to aid assessment of risk to public health and to guide the best protective measures. 

It is necessary to stress the importance of rapid action in the animal and agricultural sectors.

Each country should have a Pandemic Plan, which should have defined national goals, and priorities must be set – e.g. reduction of death, reduction of serious illness, limiting social disruption and limiting economic losses.

The plans must focus on building routine prevention activities, stockpiling vaccines and antiviral agents, increasing vaccine coverage of risk groups and enhancing surveillance efforts.

Is it Reassuring that so Few Human Cases have Occurred?

Yes it is.  There is evidence that the H5N1 strain may have been circulating in birds since April 2003.  The detection so far of only a few human cases suggests that the virus may not easily be transmitted from birds to humans at present.  However, the situation may change quickly, as the H5N1 strain has been shown to mutate rapidly and has a documented propensity to exchange genes with influenza viruses from other species.

What are the Symptoms of Avian Influenza in Humans?

Published information about the clinical course of human infection with H5N1 avian influenza is limited to studies of cases in the 1997 Hong Kong outbreak.  Patients developed symptoms of fever, sore throat, cough, and, in several of the fatal cases, severe respiratory distress secondary to viral pneumonia.  Previously healthy adults and children, and some with chronic medical conditions were affected.

Are there any Tests Available?

Laboratory tests for diagnosing all influenza strains of animals and humans are rapid and reliable.  Rapid bedside tests for the diagnosis of human influenza are also available in some countries, but do not have the precision of the more extensive laboratory testing that is currently needed to fully understand the most recent cases and to determine whether human infection is spreading, either directly from birds or from person to person.

Are Presently Available Vaccines Useful in Averting an Influenza Pandemic?

Yes they are, but in a precisely targeted way.  Current vaccines, when administered to high-risk groups, such as poultry cullers, protect against circulating human strains and thus reduce the risk that humans at high risk of exposure to the bird virus might become infected with human and avian viruses at the same time.  Such dual infections give the avian and human viruses an opportunity to exchange genes, possibly resulting in a new influenza virus subtype with pandemic potential.

Are There Drugs Available for the Prevention and Treatment?

Two classes of drugs are available:

• M2 inhibitors – amantadine and rimantadine
• Neuraminidase inhibitors – oseltamivir and zanimivir

These drugs have been licensed for the prevention and treatment of human influenza in some countries, and are thought to be effective regardless of the causative strain.

However, initial analysis of viruses isolated from the recently fatal cases in Viet Nam indicates that the viruses are invariably resistant to M2 inhibitors.   Network laboratories are also conducting studies to confirm the effectiveness of neuraminidase inhibitors against the current H5N1 strains.