Irish Thoracic Society

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Influenza

What is Influenza?

Influenza (more commonly known as ‘flu’) is a highly contagious infection of the lungs and upper airways that is caused by catching one of the three flu viruses, influenza A, B or C. The virus spreads in small droplets of saliva which are coughed or sneezed into the air by infected people.
According to the World Health Organisation (WHO), influenza infects between 5-15% of the world population and causes severe illness in up to 5 million people each year. It is responsible for between 250,000 and 500,000 deaths per year, worldwide.

After a short incubation period (1-4 days), the symptoms begin to appear, often quite suddenly. People are usually infectious for about a day before symptoms are shown and remain infectious for 5 days after the onset of symptoms. Children and the elderly may be infectious for longer periods if they have a lowered immune system. Infectious people should try and avoid contact with others until they feel better.
Influenza epidemics usually occur during winter months (October – April in the northern hemisphere) due to cold, damp conditions that allow it to survive longer outside the body. In equatorial climates, this seasonal effect is less pronounced.

Symptoms usually peak after 2-3 days and last for 5-8 days Flu symptoms can be confused with having a heavy cold. Both flu and the common cold are caused by viruses but flu has a greater impact on the lungs and is associated with a more severe general illness. There may be less nasal congestion than occurs with a cold.

Main symptoms:

High temperature (fever) that develops quickly (38?C/ 100.4?F or above)

  • Sweating
  • General aches and pains all over body
  • Dry, chesty cough
  • Sore throat
  • Sneezing
  • Loss of appetite
  • Difficulty sleeping
  • Vomiting and diarrhoea (particularly in children).


Although the fever, sore throat and general illness get better after a week, symptoms such as cough and tiredness may last up to 3 weeks.

The ‘flu virus changes its characteristics frequently. Usually these changes are small, and the new virus is similar to previous strains. Although these new strains cause epidemics every few years, many people will have some immunity so only a minority of people will be susceptible and catch ‘flu.

Occasionally the ‘flu virus undergoes a big change resulting in a completely new strain which causes a pandemic of ‘flu. Pandemic flu occurs every few decades and rapidly spreads across most countries because populations have no immunity to new strains of the virus. Symptoms are the same as normal flu but may be more severe.

Increased use of air travel makes it easier for viruses to spread around the world at a faster rate than they would have done during previous outbreaks.

What causes influenza?

The flu virus is made up of 3 main types (A, B and C) which vary in their severity. Flu strains are named after the place where they are first identified and are given a unique number. Flu viruses are known to change to prolong their survival. Populations do not have any resistance to new strains.

Influenza A

  • Most serious type of the virus
  • Outbreaks usually seen every 2-3 years
  • Likely to mutate to produce a new strain
  • Severe symptoms with high fever
  • Can cause death, especially in vulnerable patients

Influenza B

  • Less severe than type A
  • Tends to affect children who have never been exposed to the virus
  • Causes smaller outbreaks of the virus
  • Immunity develops after being infected
  • Rarely mutates so little risk of being infected more than once

Influenza C

  • Mildest form of the virus
  • Symptoms are similar to a common cold

Type A influenza viruses can infect several animals, including birds, pigs & horses. All known subtypes of the virus circulate among wild birds, which are considered the natural hosts for influenza A viruses and are known as 'avian influenza viruses'. Avian influenza viruses (bird flu) are different from regular flu viruses because they only spread from bird to bird and occasionally bird to person, but not person to person.

Avian flu has mostly infected birds in Asia. Once a bird is infected, they become sick and often die. Some people who handle the birds, such as farmers, have caught avian flu from the animals.
So far, about 100 people who have been infected with bird flu and more than half have died from it. With the regular flu, most healthy people will recover after a week or two of feeling sick. They usually don't even need special medicine. The young and elderly are more vulnerable and should have a flu jab that offers protection against the virus.

Treatment of influenza

People with flu should rest for as long as they have symptoms of fever and feel generally ill. A cough sometimes continues for some time after other symptoms have gone. The symptoms should be treated accordingly. This can be done with remedies regularly found in pharmacies. GPs will not prescribe antibiotics (unless further complications develop) because they have no effect on viruses. In severe cases, complications of the virus (bronchitis and pneumonia) will also need to be treated.

Anti-viral drugs

Anti-viral drugs are available to treat patients most at risk of serious illness. They reduce the severity of symptoms and prevent the virus from multiplying throughout the body. For these drugs to be effective, they must be given to patients within 48 hours of showing symptoms. Although symptoms of flu are reduced on this treatment, it does not stop infectious people passing on the virus. Stockpiles of anti-viral drugs are being built up in the event of a pandemic; however, flu viruses may become resistant to these drugs over time.

  • Amantadine is an anti-viral drug that was used to treat Influenza A, however, it is no longer recommended because of its side effects and because influenza A easily develops resistance to amantadine.
  • Neuraminidase inhibitors (Zanamivir and Oseltamivir) are a new class of anti-virals that are used to treat Influenza A and B. They are thought to reduce the duration of uncomplicated flu by a day, have fewer side effects than amantadine and the virus is less likely to develop resistance. However, these drugs are not widely used because they are expensive and not available in many countries.

Immunisation

To avoid catching flu, doctors recommend a yearly influenza vaccine instead of avoiding the cold weather.
Flu jabs are usually given in autumn before the virus starts circulating. Vaccines change from year to year as they are prepared from viruses that are most likely to cause flu during the following season. It is necessary to get a flu jab every year to have protection against new emerging strains of the virus.
The vaccine does not contain any live virus so it will not cause flu but will stimulate the body’s defences to protect against the virus present in the vaccine. The body starts making antibodies to the vaccine virus about a week after the injection is given. Minor side effects of the vaccine include fever, shivering, sweating, tiredness and headaches but these should be mild and go away within a few days. If symptoms are severe, or do not settle, contact a doctor for advice. Even low cost precautions such as hand-washing, covering your nose and mouth when coughing or sneezing and getting rid of dirty tissues can help reduce the spread of the virus.

Influenza viruses alter their characteristics (mutate) to help them survive. This makes it very difficult for our bodies to fight infection. Vaccines against the virus can only be made once new strains emerge. Researchers continue to find faster ways of developing vaccines in case a new flu strain appears.

Reproduced by kind permission of the European Lung Foundation