Swine flu

22 May 2009

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Influenza is caused by infection with a virus. There are many types of flu virus and these are constantly changing, which makes it hard for the human immune system to deal with it. The particular type of influenza virus that causes swine flu is known as H1N1, which is a type of Influenza A (there are also influenza B and C viruses).

It is the same group of H1N1 viruses which cause the regular yearly flu outbreaks in humans, but swine flu is caused by different strains of H1N1. The current strain of swine flu has not previously been found in pigs or humans, and contains a mixture of genetic material from human, pig and bird flu. In other words it is a new variety of flu which people are therefore unlikely to have much immunity to.

Outbreaks of flu tend to follow certain patterns. Every year the number of flu cases rise during the winter months - this is seasonal flu. If a lot of people are affected it may be said to be an epidemic, and about one per cent of those affected are likely to die from it, usually due to complications such as a secondary bacterial infection of the lungs.

But every so often, with a major change in the flu virus, it spreads rapidly around the world causing a high number of deaths. This is pandemic flu. One of the factors that scientists look out for is when a new strain of flu arises among animals and spreads to humans, changing again to develop the ability to spread from human to human without any contact with the animals.

Initally all the confirmed cases of swine flu in the UK were in people who had visited Mexico where the outbreak was first reported, but recent cases have been confirmed as being the result of virus transmission within the UK.

In the early stages it can be difficult to predict how severe a pandemic will be. However, initial laboratory studies of the strain of H1N1 causing the Mexico outbreak suggest that it is a relatively mild strain and that further mutation of the virus would be needed to cause severe problems.

It is thought to be far less dangerous than the H5N1 strain of bird or avian flu which has caused so much worry in the past few years.

Although anyone can catch flu, seasonal flu tends to cause more problems among the elderly, the very young and the chronically ill. However pandemic flu tends to affect previously healthy adults - this has been the case in Mexico where young men and women have been the main victims of swine flu.

Initially in this outbreak of swine flu humans picked up the virus through contact with pigs but now it is being passed from human to human in some countries. This is probably occurring in the same way as seasonal flu, through coughing and sneezing which generates an aerosol of virus particles into the atmosphere that can spread for up to a metre, or through contact with contaminated surfaces such as door handles.

Those living in close proximity to someone with the infection are at greatest risk.

Swine flu is highly contagious. Symptoms begin within two days of exposure, at which point the person is most infectious. They are very similar to those produced by seasonal flu, including :

High fever (usually above 38 oC)
Cough
Sore throat
Headache
Aching muscles
Chills and shivers despite fever
Exhaustion or fatigue

It can be difficult to tell mild flu from a cold but it is usually more generalised (the symptoms of a cold tend to be limited to the head while the flu causes aching and fatigue all over) and more severe with higher fever.

If complications such as pneumonia develop there may be other symptoms such as difficulty breathing or diarrhoea, a symptom which appears to be a particular feature of the swine flu outbreak in Mexico.

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