Avian (bird) flu

Avian influenza

What is influenza?

Influenza (most commonly known as 'flu') is an acute, usually short-term illness involving mainly the respiratory tract (nose, throat, lungs and breathing tubes) and which also causes generalised symptoms (eg fever, muscle aches and joint pains) throughout the body. Flu, which occurs throughout the world, is usually not medically serious but on occasions can cause other serious health problems and, in some cases, can be fatal.

Who gets influenza?

Anyone from children to adults can get flu and it does not have serious consequences in most people. It has been estimated that up to 15 per cent of the population may develop the illness in an average year. So it is fortunate that, generally speaking, most people recover on their own. However, the problem arises when the elderly or those whose resistance is weakened by other medical conditions (such as emphysema or asthma) catch flu. In these cases it can cause more severe illness. In addition, some strains of flu are more harmful than others and, if a completely new strain of flu develops to which no-one is immune, this has the potential to cause a significant epidemic or pandemic (an epidemic across the world).

What is the influenza virus?

In fact there are thousands of different strains of influenza virus, which is why the human body has such difficulty developing resistance to the infection. They can all be grouped into one of three categories (called serotypes):

Type A: Influenza A viruses are the most virulent (powerful) and are responsible for most major epidemics or pandemics.

Type B: Influenza B is less powerful and usually causes less severe illness although it can still result in widespread or local outbreaks.

Type C: This type of influenza virus usually only causes very mild illnesses, which are very similar to the common cold.

The coating of the flu virus has a number of protein molecules on it, which our body's immune systems detect and attack. If this coating remained the same, the next time the virus tried to infect the body, it would be immediately recognised by the body's defences as an infection it had dealt with before. This would trigger an immune response straight away that would kill the virus so that the person would probably not even know they had been exposed to the infection.

The reason why the flu virus is of such concern is, because it can easily change its protein makeup, the body's defence mechanisms have to keep learning new variations of the virus against each of which it takes time to mount a fresh defence. Our immune systems can 'remember' our previous encounters with viruses and we mount rapid vigorous 'memory-based' responses when we re-encounter a virus of a type that we have previously repelled. However, even small changes in the makeup of the virus mean that the body does not recognise it and therefore has to start from scratch when it tries to fight off the infection.

So what is avian flu?

Avian or bird flu is a type of influenza A virus which generally only affects birds and sometimes pigs. Previous outbreaks have occurred and resulted in the first known human case in 1997.

The current outbreak of avian flu began in South-east Asia in 2003 causing the deaths of thousands of chickens. What makes this outbreak remarkable is that it has spread to many other countries, possibly partly because of the movement of migratory birds.

A similar outbreak in 1997 resulted from infected birds in a particular market. Although it caused several people to die, no human-to-human transmission took place and no cases occurred after all the birds in the market were slaughtered.

What is special about the H5N1 strain of avian flu?

Influenza A viruses have many different so-called subtypes which are referred to by different H and N numbers to distinguish one from another. It has been found that the H5 and H7 subtypes frequently cause highly dangerous types of bird flu and what seems to be special about the H5N1 subtype is that it appears to be especially resistant to the usual methods of control. For instance, despite the culling of many millions of birds, the virus has still managed to continue to survive and spread throughout many Asian and Indonesian countries.

From the initial outbreak of H5N1 in December 2003 the virus has gradually spread to many countries across the world including Turkey and Romania.

Why is it a problem for humans?

The reason it is relevant to humans is firstly that when it passes from birds to humans (something that so far has happened very rarely) it causes a very serious illness resulting in the death of about one third of those who develop it. In January 2004 the virus was found in people in Vietnam who had died of flu. Since then human cases have also been found in Cambodia, Indonesia and Thailand resulting in further deaths.

Secondly, so far, the H5N1 flu virus has not developed the ability to transfer itself from one human to another other than in one or two rare instances. Therefore, the biggest worry is that it will in some way alter itself and become infectious and able to spread easily from person to person. The way this may happen is if someone catches an ordinary type of human flu and, simultaneously, catches avian flu. This person may then become a 'melting pot'in which the two viruses may somehow combine to form a completely new flu virus that is as deadly as avian flu and has the ability to spread readily from human to human.

Should person-to-person spread more easily, a pandemic could result.

How is it caught?

So far most cases have been as a result of a person being in direct contact with infected birds or their droppings and, as mentioned, only very rarely has human-to-human transmission so far been reported. The virus is not easy for a human to catch since there have been only a small number of reported cases of the disease in humans compared to the large number of infected birds, many of which live in close contact with their human owners.

Is it possible for avian flu to be caught by eating chicken or eggs?

It is not thought that eating properly cooked chicken or eggs poses any risk because the illness is transmitted via contact with live birds that have the flu virus or the faeces of infected birds. It is thought that the killing, plucking or preparation of infected birds for eating is a potential source of infection but this tends to occur more in those areas of the world where families live in close contact with their poultry and kill them to sell or to eat for themselves.

What are the symptoms?

The symptoms of avian flu are similar to other forms of severe influenza namely high fevers, sweats, muscle pains, cough, and a runny nose. It is impossible to distinguish avian flu from other forms of influenza without a specific blood test or by analysing a sample of infected tissue. It should be remembered that, so far, all cases have related to contact with infected birds in the countries mentioned above; at the time of writing (November 2005) there have been no cases of avian flu in humans in the UK.

Is there a treatment for avian flu?

There are two types of drugs available for prevention and treatment of flu. These types of drugs are called M2 inhibitors (amantadine and rimantadine) and neuraminidase inhibitors (oseltamivir -commercially known as Tamiflu- and zanimivir). However, initial analysis of viruses isolated from the recently fatal cases in Vietnam indicates that the viruses are resistant to the M2 inhibitors although further testing is underway to determine the effectiveness of amantadine.

It is thought that the H5N1 virus may respond to treatment with the neuraminidase inhibitors if given within 48 hours of the start of symptoms and as a result may improve the chances of survival of the individual.

Treatment of avian flu also includes the prevention or treatment of the complications of the illness, such as pneumonia, by administering antibiotics, if appropriate, and generally providing supportive medical care which may include admission to an Intensive Care Unit.

Medications

Symmetrel (Amantadine), Tamiflu (Oseltamivir)

Is there a vaccine against avian flu?

At the time of writing, there is no effective vaccine against avian flu although one is under development. The usual flu vaccine given to certain groups of people, such as the elderly and those with lung and heart conditions, does not protect against avian flu. However, it is thought that preventing as many people as possible from developing the usual types of flu may reduce the chances of one person catching both human flu and bird flu simultaneously and therefore make the chances of the emergence of a human form of bird flu less likely.

Work is underway to develop a vaccine against avian flu but it takes time to produce a new flu vaccine, not least because it has to be properly tested before large amounts being licensed for widespread use.

Another reason why a vaccine is not available is that the makeup of the vaccine has to be based on the very particular virus causing the illness and, as yet, the human-to-human form of virus does not exist. To a certain extent the scientists have to wait for the virus to emerge before they can develop a fully effective vaccine against it.

Last updated 7 August 2011


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