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Influenza ('flu) is a viral infection of the airways that also has severe general symptoms. It is a seasonal illness that affects large numbers of people during winter.
Summary
Description
Influenza ('flu) is a viral infection of the airways that also has severe general symptoms. It is a seasonal illness that affects large numbers of people during winter.
Cause
Influenza is caused by a group of influenza viruses. There are three of these viruses, named A, B and C. It is impossible to tell the difference between influenza A and B from the symptoms that you have, but influenza C causes a much milder illness resembling a cold. An infected person sheds virus from the nose and throat and the virus is aerosolised by speaking, sneezing or coughing. It can be left on hands, surfaces and objects such as handkerchiefs. Thus you can be infected with 'flu viruses via the air, or via an infected hand or object touching your nose or mouth.
Avian influenza ("bird 'flu") and the origins of human influenza viruses
Many people are surprised to learn that the natural home of the influenza viruses is in freshwater birds, such as ducks. In these birds, influenza viruses usually cause a harmless infection of the gut and the birds have no symptoms of being ill. Viruses are shed in the bird droppings and other birds get infected by drinking infected water.
There is great variety of water bird influenza viruses; some of the strains are able to infect land birds such as poultry, sea mammals such as seals, and land mammals such as pigs and humans. In all these instances the virus will infect the respiratory system rather than the gut, and cause respiratory disease.
Some bird 'flu strains are extremely virulent, such as the 'highly pathogenic' H5N1 avian 'flu that emerged in China in 1996, re-emerged in wider Asia in 2003, spread to parts of Europe and the middle East and reached North Africa in 2006.
As well as causing the death of millions of birds, this virus has infected more than two hundred people who have been in contact with sick birds, and has resulted in the death of a large number of them due to the severe form of the influenza virus. Luckily, up until now, this virus has shown very limited, if any, ability to spread from human to human.
In the distant past, some influenza viruses established themselves permanently in humans and became human viruses: influenza B and C are now strictly human viruses.
Human influenza A, however, is still closely related to the natural avian 'flu viruses and can even swop genes with avian viruses. This occurs when a human influenza virus and a bird influenza virus infect a human or an intermediary animal (such as a pig) at the same time.
The process of exchanging genetic material is known by scientists as "reassortment" and has been an important mechanism in generating "new" influenza viruses that have infected human populations in global epidemics of the past (for example, the 1918 'Spanish' 'flu that killed millions of people at the end of World War I).
Experts are concerned that the highly pathogenic H5N1 virus currently circulating in birds will recombine with a contemporary human influenza A virus and create a virus that is not only very virulent, but that is also able to very easily spread from human to human.
For this reason, the World Health Organisation and other health authorities are trying to contain the spread of the virus by monitoring its movement and culling infected flocks of birds. It is also very important that people in affected areas avoid contact with sick or dead birds.
Horse influenza
Horse 'flu viruses were only isolated in 1956. There are two main types of virus called equine influenza-1 (H7N7) and equine influenza-2 (H3N8). Donkeys and mules can also be infected with the equine influenza viruses.
Signs of illness in horses are the same as those in humans: cough, nasal discharge, high fever, loss of appetite and muscle soreness. A common complication is secondary bacterial pneumonia.
Infected horses should not be exercised for at least three weeks after infection because this can lead to permanent disability. Horse 'flu spreads very easily between horses, especially with transport of horses over large distances for sporting events and breeding. Affected stables should be placed under quarantine for at least four weeks.
There is an effective inactivated vaccine to prevent horse 'flu, but it only produces immunity that lasts for approximately three months.
Symptoms and course
From this point onwards, we will only consider influenza A and B, since the influenza C virus does not cause significant illness. The disease known as influenza, which is caused by the A and B viruses, has the following features:
sore throat, runny nose and sore, red eyes are often present
These symptoms begin very abruptly, so much so that people may know the exact hour of the day that the symptoms began, generally between 24 hours to four days after exposure to a person with 'flu. The fever usually departs within 48 hours and other symptoms resolve within a week.
Influenza is a distinct illness, much more severe than an ordinary cold. Cold symptoms, like a blocked or runny nose, may be present in 'flu, but the high fever in particular distinguishes 'flu from a cold. The fever is highest in children and least marked in the elderly. High fever in children can cause "febrile convulsions".
In children, vomiting and diarrhoea often occur in influenza, but these symptoms are infrequent in adults. Gastroenteritis with a fever, which is sometimes labelled 'gastric 'flu', is not caused by influenza but may well be caused by a gastroenteritis virus.
A well-recognised consequence of a bout of 'flu is the fairly long convalescence. A person may feel weak and mildly depressed for one to three weeks afterwards, this usually occurs in the elderly.
Risk factors and complications
The majority of cases of 'flu in young healthy people will not have complications. However, the following people are at risk for well-recognised complications:
The most severe disease is seen in individuals with no protective antibodies, either from vaccination or previous infection with a similiar type of influenza virus.
Children in their first year or two of life are very susceptible to 'flu because they have no prior immunity. In the elderly, prior immunity can wane.
Possible complications of 'flu include:
Prevalence
Infection with influenza at various points in your life is inevitable and unavoidable, unless you happen to live in an extremely remote and isolated community. These viruses are highly infectious (easily spread). In the general community, pre-school and school children are the chief disseminators of 'flu. This is because children have little pre-existing immunity and are highly susceptible to the viruses, which they then bring home to their families. Closed communities, such as homes for the elderly, university campuses and military bases, are prone to outbreaks of influenza that run their course over a few weeks.
Everyone is aware that influenza infections are far more frequent in winter. It is not known for certain why this is so, but reasonable theories are:
Assisted by modern air travel, 'flu viruses can easily migrate between the Northern and Southern hemispheres according to the season. Influenza viruses do, however, occur even in tropical countries where there is little change in climate all year round.
It is possible to be infected with influenza A, B and C in the same winter, or in successive winters, as these viruses are significantly different from one another. In addition, Influenza A and B are continually undergoing slight changes through what is technically known as 'genetic drift'. (This is due to random copying errors in the viruses' genetic material during replication.) Thus despite having had a previous bout of 'flu, about every three years the viruses have changed sufficiently so that they are no longer properly recognised by our immune systems, and you can be re-infected with 'flu and all its accompanying symptoms. This is why there are regular epidemics of winter 'flu.
Occasionally, there is a drastic change in the influenza A virus when it replaces one of its genes with a new gene from a bird influenza virus; this is technically known as 'genetic shift'. (See Origins of Human Influenza Viruses). When this happens, no-one has appropriate immunity to the altered influenza A, and a pandemic of influenza follows. (A pandemic is a world-wide epidemic.)
The 20th century saw pandemics of influenza A in 1918, 1957, 1968 and 1977. It is impossible to predict when the next pandemic will occur, but most scientists believe it is 100 percent certain that there will be one sometime.
A brief history of influenza, including the 1918 'flu pandemic
Historians have recognised descriptions of 'flu in records dating back to approximately 400BC. It is identifiable in the historic record because of its distinctive clinical picture, its seasonal pattern, and because it is an illness that affects a large proportion of the population simultaneously (an epidemic disease). It causes outbreaks of varying intensity which end as suddenly as they begin, and generally cause mortality in the elderly.
The name 'influenza' was coined by the Italians in the 14th Century, and referred to the belief that the illness was due to the adverse 'influence' of the weather or stars. 'Flu is believed to have been introduced into the Americas by Columbus' second expedition.
The most devastating 'flu pandemic occurred in 1918, at the end of World War I. This virus killed between 20 and 40 million people, many more than the war itself. It was known as the 'Spanish 'flu' (though it definitely did not originate in Spain) or later as 'the Great 'Flu'. It was unusual because very many young healthy adults died in the pandemic, most from overwhelming pneumonia.
At the time of the 1918 'flu, neither the knowledge nor the tools to analyse the virus responsible were available. In 1998, 80 years later, fragments of the 1918 'flu virus were recovered from stored pathology samples.
Subsequently, work was done on tissue collected from 1918 'flu victims buried in the permafrost in the Arctic Circle. Although this exciting work provided some information about the virus, it has not solved the riddle of what made this particular virus so very virulent.
The possibility of another such pandemic still exists.
When to call a health professional
If you have a high fever for more than a few hours, and are not certain that it is from 'flu, it is advisable to see a health care professional for an opinion. Any signs of one of the complications of 'flu (see "Risk Factors and Complications") should prompt a visit or call to a health care professional.
Diagnosis
Influenza is usually recognisable to health care professionals by the symptoms and signs alone. If it is known that 'flu is active in a community, then the diagnosis becomes even more probable. It is uncommon for laboratory tests to be done. However, in young children influenza is easily confused with infection by respiratory syncitial virus (RSV), the parainfluenza viruses or adenovirus. Laboratory diagnosis of a viral respiratory illness might be attempted for one of the following reasons:
Influenza viruses have traditionally been cultured in hen's eggs (actually in the chick embryo inside the egg). These days most laboratories use cell cultures to grow the virus. There are some rapid diagnostic tests available that do not involve culture at all. A throat swab or nose swab would usually be collected for any of these tests.
Treatment
Home
There are numerous over-the-counter medicines and alternative remedies available for colds and 'flu. Sales of these medications boom during outbreaks of 'flu, but there is little, if any, scientific evidence that they are beneficial. There are three basic ingredients for home treatment of 'flu:
Note that aspirin or any medication containing aspirin should not be given to children of 15 years or younger during a viral illness (see Reye's syndrome under "Risk Factors and Complications").
An uncomplicated bout of 'flu will put a person in bed for two to three days and they will need to take it easy for a few days afterwards. It is not at all a good idea to exercise during 'flu, especially while a fever is present. You should slowly return to your normal activities as your energy levels allow. Loss of appetite during a viral illness is to be expected, but the ill person should be encouraged to drink plenty of fluids, such as water and fruit juice, since a fever increases loss of body fluids.
Specific anti-influenza drugs
Amantadine and rimantadine are two similar drugs that have been available for many years. They are effective against influenza A only, and work by preventing the virus from uncoating inside cells, with the effect that it is unable to multiply. Amantadine and rimantadine are useful for preventing 'flu in unvaccinated, at-risk people during outbreaks, and are of some benefit as treatment if started early. They have significant side-effects. These drugs are very seldom used in South Africa.
Very recently, a new class of anti-influenza drugs, known as neuraminidase inhibitors, has become available. They block an important enzyme carried by both influenza A and B, which usually allows them to spread from cell to cell through sticky secretions. One of the available neuraminidase inhibitors is inhaled (zanamivir), and the other is taken by mouth (oseltamivir). These drugs shorten the course of 'flu by a day or two if taken early on, and have few side effects. They appear to be useful for preventing 'flu as well. They are already used extensively in general practice in the United States. Zanamivir was registered for use in South Africa in 2001.
Prevention
The influenza vaccine is still the most important way to either avoid 'flu altogether or to reduce its severity.
How is the vaccine made?
The World Health Organisation (WHO) does constant surveillance of human influenza strains circulating around the world.
Virology laboratories, including South African laboratories, send 'flu viruses recovered from patients to the WHO reference laboratories. Based on this up-to-date collection, the WHO gives vaccine manufacturers the 'seed stock' of the viruses that should be used to produce the next vaccine. Since 1977 there have been two strains of influenza A circulating around the world, along with the single influenza B strain. Therefore there are three viruses in the 'flu vaccine.
To increase the accuracy of 'flu vaccines, there are now separate vaccines produced every six months for the winter seasons of the Northern and Southern hemispheres. The vaccine is composed of the current strains of 'flu that are grown in hens' eggs and then chemically inactivated. 'Split' vaccines have had further treatment to break down the virus, and 'subunit' vaccines have been partially purified so that the more important 'flu proteins are concentrated in the vaccine. Split and subunit vaccines are recommended for children since they have reduced side effects.
Who should have the vaccine?
These groups of people will benefit especially from the 'flu vaccine and it may save their lives. In the case of HIV, people with a CD4 count of less than 200 per microlitre are unfortunately unlikely to respond to the vaccine. Vaccination is also important for the caregivers and close contacts of any of the above!
Ultimately it is a good idea for all adults to have the 'flu vaccine, especially if they are sportsmen, or self-employed, or have other strong reasons to avoid being ill! Although it is not widely practised in South Africa, one can give all children the 'flu vaccine from about six months of age. Despite the significant illness caused by influenza, the vaccine is underutilised. This is especially true in South Africa, where only about 2% of people eligible for the vaccine are actually vaccinated.
Are there any medical reasons to avoid the 'flu vaccine?
When should you be vaccinated and what is the dose?
How quickly does the vaccine take effect?
It takes about two weeks for the immune system to respond to the vaccine and for one to become protected against 'flu.
What are the side effects of the 'flu vaccine?
How effective is the vaccine and how long do you benefit from it?
Are there any new types of vaccine on the way?
There have been successful trials with a new type of 'flu vaccine that is a live virus vaccine, given in the form of nasal drops. This live 'flu virus has been made harmless, but through infection of the upper airways it causes the body to produce the correct immune response to counter the real influenza virus. This vaccine has fewer side effects than existing vaccines, and this, together with the way it is given, will make it a welcome improvement for protecting children.
In South Africa, you should not be unduly concerned about receiving the correct vaccine formulation. This is because all vaccines sold in South Africa are regulated by the Medicines Control Council. It may, however, be worth checking that you are receiving the vaccine issued for the current year. Last year's vaccine may not contain the appropriate strains for the current year.
"I got a 'flu shot, so why did I still get the 'flu?'"
(Reviewed by Dr Jane Yeats)
(Health24.com)