Swine Flu

Dr Trisha Macnair
Swine influenza (or flu) is normally found in pigs, but does occasionally occur in humans. There had been no human cases detected in the UK for ten years until the outbreak that started in Mexico in spring 2009.

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. 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 2009 strain of swine flu, now known as novel influenza A (H1N1), 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.

Although the outbreak of novel influenza A H1N1 first appeared in Mexico, there has since been a growing number of cases reported around the world including the UK. Initially all the confirmed cases of swine flu in the UK were in people who had visited Mexico but these were followed by many more cases that have been the result of virus transmission within the UK.

By the beginning of July 2009, there were over 9,700 confirmed cases in the UK (these deaths have been in people with other concurrent medical issues). The number of confirmed cases around the world has now passed 94,000 (it is likely that that are many many more unconfirmed cases). Numbers continue to rise in July 2009, which is unusual for this time of year which isn’t typically the flu season, and doctors suspect that widespread transmission throughout the general community is likely in the next few weeks.

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.

In the early stages it can be difficult to predict how dangerous a new strain of flu is and whether is could cause a pandemic. It’s still unclear exactly how severe this novel H1N1 flu strain is going to prove to be but as numbers of cases rise, the World Health Organization (WHO) has declared it a pandemic.

However, initial laboratory studies suggest that novel H1N1 is a relatively mild strain and that further mutation of the virus would be needed to cause severe problems. This is being confirmed by the clinical picture as most people with swine flu are experiencing a relatively mild respiratory illness. It’s not clear why it caused particular problems in Mexico where a higher proportion of people died. Many of these people were previously fit young adults, and the WHO have suggested that pre-existent immunity to this new virus is low or non-existent, or limited to older people (who don’t seem to be so badly affected by the virus). Fortunately there have so far only been a small number of deaths outside Mexico, although some experts suggest that there could be a second, more deadly, wave of the virus next winter.

The virus does seem to be more contagious than seasonal influenza, with up to twice as many contacts of those affected developing the disease. But overall, novel H1N1 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 with novel H1N1 (swine flu) where many victims have been younger adults.

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.
• Diarrhoea or a stomach upset have been a particular feature of novel H1N1

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.

The symptoms and signs in an affected person are usually enough to suggest that flu is to blame, but it can be confused with other viral illnesses. Laboratory tests are needed to formally confirm the diagnosis and, more specifically, identify exactly which strain of flu it is.

A number of tests can be done in the laboratory to identify flu, using swabs or washings from the nose or throat for example. These can give results within 15 minutes but may not identify the exact strains of flu involved, only that influenza type A or B is present.

These rapid tests are only 50-70 per cent sensitive for detecting flu and so may give a negative result when a person does in fact have flu.

More reliable tests, which also identify exactly which sub-type or strain of flu is present, may involve growing or culturing the virus in the laboratory, and can therefore take a couple of days to give a definite result. These tests may be needed to confirm a specific strain such as swine flu.

However, as the infection has become more widespread the Department of Healthhas advised doctors to stop testing and simply treat those with symptoms straight away with anti-viral drugs.

If you think you have developed flu-like symptoms, you should stay at home and avoid contact with other people. Call your doctor to discuss your symptoms. It’s important to tell them if you have recently been in contact with someone else with flu or if you have recently travelled abroad. Your GP may then decide to come and check you if they are worried but it is more likely that if they suspect swine flu they will explain how you can get a relative or friend to pick up anti-viral medication for you at a local centre. This medicine includes Tamiflu (Oseltamivir) or Relenza (Zanamivir). Novel H1N1 (swine flu) seems to respond to these medicines in the same way as seasonal flu.

The treatments are not a cure but can reduce the risk of infection for those who have been exposed to the virus, reduce the ability of a person with the virus to pass it on, and shorten the course of the illness when someone has flu. Most importantly these drugs treatments reduce the risk of serious complications and death. For maximum effect, the drugs should be given as early as possible to someone who has the flu.

Treat symptoms as they arise. Try to keep well hydrated with plenty of clear fluids. Take paracetamol or ibuprofen (children should not be given aspirin because of the risk of Reyes syndrome) for aches and pains, and use simple remedies such as cough syrups and vapour rubs. Try to rest as much as possible.

Although research is well underway, there is not yet a vaccine specifically for the strain of H1N1 swine influenza virus which has caused the 2009 Mexico outbreak, and it will take several months to develop one. It’s not yet clear whether current flu vaccines may offer a degree of protection.

As with all infections, good hygiene is absolutely essential to reduce the risk of spread or contact with the virus:

• Wash your hands regularly.
• Cover your mouth and nose when you cough or sneeze.
• Use tissues and dispose of them carefully.
• Clean hard surfaces regularly with a standard cleaning fluid.
• Avoid close contact with people who have fever and a cough.
• The effectiveness of wearing a face mask in preventing transmission of the flu virus isn’t yet clear, although it may give some protection to those people such as health professionals who work closely with someone with symptoms of flu.

Don’t worry about eating pork products – there is no evidence that swine flu can be picked up that way. Even if virus is present in the meat, thorough cooking will destroy it.

If you want further information on swine flu, visit the Swine flu page on the NHS Choices site or the websites of leading health and research organisations such as theWorld Health Organization or the UK Health Protection Agency.

This article was last medically reviewed by Dr Trisha Macnair in July 2009.