Influenza (flu) is an acute air-borne viral infection of the upper respiratory tract. The name originates from the Italian word meaning “Influence”, as the disease was once believed to have been “influenced by the stars”.

The Influenza Virus

With permission from ESWI Flucentre . Orginally published in Rapid Reference to Influenza, Second Edition, Elsevier, 2006

Influenza viruses are classified into the A, B and C groups. Influenza A and B are structurally similar surrounded with glycoprotein spikes of Hemagglutinin (HA) and Neuraminidase (NA), whereas the Influenza C virus has a single surface glycoprotein known as Hemagglutinin-esterase-fusion.

Group A and B viruses cause regular epidemics of flu (seasonal flu) in humans and Group A viruses occasionally cause pandemics. 

Influenza viruses continuously evolve through:

  -  antigenic drift (point mutations in genes that code the surface proteins hemagglutinin and neuraminidase). This process results in new strains of flu that emerge from year to year

  -  antigenic shift (genetic reassortment, where the influenza strain acquires the hemagglutinin and neuraminindase segment from an influenza virus of a different subtype). This occurs very occasionally and can result in a pandemic if the new virus is transmissible in humans. This occurred 3 times in the 20th Century. On the 11th of June the WHO declared a new global pandemic of a new variant of H1N1 (swine flu), which was first isolated in humans in Mexico in the spring.

Influenza A Virus

Influenza A viruses are classified further based on their surface glycoproteins HA and NA, making up several genetically distinct subtypes

Over the last 30 years human flu epidemics have been caused by Influenza A H1N1 and H3N2, and Influenza B.

Transmission

Direct human-to-human transmission occurs by ingestion/inhalation of contaminated respiratory secretions. When an infected individual coughs/sneezes the virus escapes via aerosols thereby enabling a healthy individual to become infected when he comes into contact with the contaminated air or surface. Using a handkerchief can transfer virus to the hands, and so it can be passed on by touch.

Symptoms

Most people recover from Influenza within 1-2 weeks. Infection is often characterised by

  Chills
  Sudden onset of high fever (38 – 39 ºC)
  Headache
  Cough
  Sore throat
  Runny nose
  Muscle pain

Death from influenza can occur, usually in the elderly and frail. It is often associated with secondary bacterial infections caused by Staphylococcus aureus, Streptococcus pneumonia and Haemophilus influenza.

Control  

Sunlight, disinfectants and detergents can inactivate the influenza virus. Frequent washing of hands is thereby recommended to reduce the risk of transmission.

Treatment

The symptoms and duration of infection can be reduced using antiviral drugs, such as

  Amantadine
  Zanamivir (Relenza)
  Oseltamivir (Tamiflu)

Oseltamivir and Zanamivir are Neuraminidase Inhibitors and Amantadine is an M2 inhibitor. The influenza virus quickly develops resistance to amantadine (and related drugs), and it has been associated with a range of side-effects. These M2 inhibitors are seldom used to treat flu these days.

The Neuraminidase Inhibitors have been shown to be effective at reducing the severity and duration of illness, particularly if given early. Many countries have stockpiled these drugs for use in a pandemic. The UK has a stockpile of over 30 million courses of these drugs (enough to treat half the population). This should be plenty to treat everyone who is likely to be sick during a pandemic. 

Vaccine 

Vaccines are developed every year against seasonal flu. They need to be regularly updated because of the virus's ability to undergo continuous evolution (see above).

In the UK the elderly and people in certain risk groups (such as those with chronic heart disease) are targeted to be vaccinated.

They need to be vaccinated every year, as the strains that are ciculating change over time. The vaccine consists of three strains an H1N1 strain, an H3N2 stain and a strain of Influenza B.

It takes roughly 6 months to develop new stocks of vaccine.