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Diseases

Influenza (Flu)

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What is Influenza?


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Influenza


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Influenza is an acute respiratory illness caused by the influenza virus which exists as three types: A, B or C.

Influenza type A is found in humans as well as other mammals and birds. It is the only type that has been known to cause influenza pandemics (widespread outbreak of disease). 

Influenza type B occurs in humans and seals.

Type C occurs in humans and pigs. Influenza type C occurs commonly in children under 6 years of age, and causes a mild illness. Most humans are exposed to influenza type C as children and therefore develop immunity to it. 

Tiny proteins are present on the envelope that surrounds the influenza virus particles. These are known as the hemagglutinin (H) and the neuraminidase (N). Several types of H and N exist, and the combination of these changes over time. For example bird flu is H5N1, while swine flu is H1N1. Influenza vaccines are designed to prevent viruses with particular H and N combinations, which is why a vaccine that works one year will not be effective for preventing flu the following year.


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Influenza is a highly contagious infection, with outbreaks of the illness being commonplace, particularly during winter months. Large numbers of virus particles are present in respiratory tract secretions (such as sputum/phlegm or saliva), resulting in transmission of small aerosolised particles that spread though sneezing, coughing and talking and are subsequently inhaled by the next person to become infected.

Alternatively, contact transmission may take place, when infected individuals touch mucous membranes (soft skin in the mouth or nose) or respiratory secretions (cough or sneeze into their hand), before direct or indirect (touching common surfaces) interpersonal contact with a new host who deposits the virus on their own mucous membranes.

Uncomplicated influenza infection occurs when virus particles attach to the upper respiratory tract (windpipe and upper airways). If the virus particles extend further down into the lungs, they cause the most common complication of influenza; pneumonia. The lungs may then become infected by bacteria, and this is known as secondary bacterial pneumonia. Other rarer complications may occur, such as spread of the virus to cause disease in the heart, muscles or brain.

Who gets Influenza?

During influenza outbreaks, 10-20% of the general population may become infected. During pandemics, up to 50% of the population may become infected. Seasonal influenza causes 200,000 hospitalisations and 41,000 deaths in the US every year. It is the seventh leading cause of death in the US.  

In Australia, around 20-43% of all children are infected during typical influenza seasons, with the highest incidence amongst children less than 2 years old.

Deaths due to influenza are not common, but the rates are highest in elderly patients, followed by infants.

Influenza is the cause of 5 to 12% of pneumonia that occurs in the community.

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Predisposing Factors

Like other respiratory viruses, influenza is spread through tiny particles of respiratory secretions in the air therefore exposure is more likely in overcrowded or poorly ventilated environments.

The influenza vaccine is advised for the following list of people, due to their increased risk of exposure to the virus, or their increased risk of serious complications if exposure does occur. In Australia, immunisation is recommended for:

  • All adults aged over 65 years;
  • Infants and children aged from 6 months to 4 years;
  • Children on long term aspirin therapy;
  • Pregnant women (especially those who are in their second or third trimester between June and October);
  • Anyone with a chronic medical condition (e.g. asthma, diabetes or heart, kidney or lung disease);
  • Anyone with a weakened immune system or who is undergoing immunosuppressive treatments ;
  • Residents of nursing homes or long term care facilities;
  • Child care workers, health care workers, or anyone living with or looking after someone at high risk of developing flu-related complications; and
  • Anyone visiting parts of the world where influenza is circulating.

Progression

When an individual is exposed to the influenza virus, viral particles enter the lining of their respiratory tract and begin to multiply. The newly formed viral particles then begin ‘shedding’ off the lining and entering secretions such as phlegm and saliva. Healthy adults began shedding the flu virus one half to one day after they were exposed, with a sharp increase to peak shedding on the second day, followed by a rapid decline. Average duration of shedding is 4.8 days, though it continues for 10 days in some individuals. Children, the elderly, those with chronic illnesses and immuno-compromised hosts may shed for longer. While someone is shedding the virus, they are able to spread it to other people.

The incubation period (during which a person is infected with the virus but doesn’t experience any symptoms) is 1 to 3 days. Symptoms then start suddenly, as discussed below.

In the case of uncomplicated influenza, these symptoms generally improve over two to five days, though may last one or more weeks. Some patients experience postinfluenzal asthenia (persistent weakness or becoming tired easily) which may be present for several weeks following the illness. A dry cough (post viral cough syndrome) may also linger for several weeks.

In particularly nasty (virulent) influenza strains, or in susceptible hosts, complicated influenza may occur. The major complication of influenza is pneumonia.

Pneumonia

Pneumonia occurring as a complication of influenza infection may be due to the virus itself (primary viral pneumonia) or a bacterial infection following the influenza (secondary bacterial pneumonia). Primary viral pneumonia is the least common and the most severe of these categories. Secondary bacterial pneumonia, though less severe, contributes to approximately 25% of all influenza-associated deaths.

Influenza related pneumonia occurs most commonly in the following high risk groups:

  • Those with respiratory or cardiovascular co-morbidities;
  • Those who are immunosuppressed, or have a diagnosis of diabetes  mellitus, renal (kidney)disease or hemoglobinopathy;
  • Individuals aged over 50; and
  • Individuals who reside in care facilities such as nursing homes. 

Myositis and rhabdomyloysis

Muscle pain is commonly associated with influenza, however true myositis, in which the virus damages muscle tissue is a rare complication, and tends to occur in children.

Other complications

Many other less well documented complications of influenza infection have been reported. Such complications include  encephalitis affecting the brain, Guillain-Barre syndrome, aseptic meningitis, transverse myelitis, toxic shock syndrome, and myocarditis or pericarditis affecting the heart.

Children

Young children may have a wide range of symptoms that resemble other conditions such as bronchiolitis, croup, or otitis media (middle ear infection). Rarely febrile convulsions (fitting associated with high temperature) may occur. Viral particles have been known to travel to muscles resulting in myositis.

Reye’s syndrome is a rare complication that occurs principally in children with viral illness who are administered aspirin. Presenting features of Reye’s syndrome are vomiting and confusion progressing to coma. Brain injury or death may ensue. Aspirin should therefore not be used in children who have influenza.

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Article Dates:

calendar icon Created: 21/2/2006 calendar icon Modified: 1/6/2009 calendar icon Reviewed: 3/5/2009