Introduction

Influenza is a contagious viral respiratory illness caused by influenza viruses that infects the nose, throat and sometimes the lungs. It can cause mild to severe illness, and at times can lead to significant morbidity and mortality.

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  There are four types of seasonal influenza viruses, types A, B, C and D.

  
  
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  Influenza A and B viruses circulate and cause seasonal epidemics of disease.

  
  
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  Influenza A virus subtypes include: subtype A (H1N1) and A (H3N2). The A (H1N1) caused the pandemic in 2009. Only influenza type A viruses are known to have caused pandemics.

  
  
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  Influenza B viruses are not classified into subtypes.

  
  
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  Influenza C virus is detected less frequently and usually causes mild infections; it does not present public health importance.

  
  
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  Influenza D viruses primarily affect cattle and are not known to infect or cause illness in humans.¹

Prevalence

Worldwide, annual epidemics are estimated to result in about 3 to 5 million cases of severe illness, and about 290 000 to 650 000 deaths.

In the USA, flu-related hospitalisations since 2010 ranged from 140 000 to 710 000, while flu-related deaths are estimated to have ranged from 12 000 to 56 000.²

The effects of seasonal influenza epidemics in developing countries are not fully known. Research estimates that 99 per cent of deaths in children under five years of age with influenza-related lower respiratory tract infections are found in developing countries.

Signs and Symptoms

There is usually a sudden onset of fever, dry cough, headache, muscle and joint pain, severe malaise, sore throat and a runny nose.

Influenza illnesses range from mild to severe and even death.

Transmission and Infectivity

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  Infection is transmitted when an infected person coughs or sneezes.

  
  
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  Infection droplets containing viruses are dispersed into the air and can spread up to 1 metre. Persons breathing in the proximity will inhale the infected droplets and become infected.

  
  
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  The virus can also be spread by hands contaminated with influenza viruses.

The incubation period is from one to four days.

Diagnosis of Influenza

There are a number of diagnostic influenza tests available. They detect the virus in the respiratory system.

Differential Diagnosis

Maternal Implications

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  Influenza (flu) is more likely to cause severe illness in pregnant women than in women who are not pregnant.

  
  
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  Changes in the immune system, heart and lungs during pregnancy make pregnant women (and women up to two weeks postpartum) more prone to severe illness from flu, including illness resulting in hospitalisation.

  
  
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  Evidence suggests that pregnant women are even more vulnerable during pandemics.

  
  
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  Pregnant women with influenza were five times more likely to be hospitalised than non-pregnant women, and the risks increase with advancing gestations.⁴ [EL 2]

  
  
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  The risks of influenza in pregnant women have been observed in pandemic and non-pandemic seasons.

  
  
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  Influenza in pregnant women may be complicated by acute severe cardiopulmonary complications. [EL 1]

  
  
  
  
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    ○ During the H1N1 influenza pandemic of 2009–10, pregnant women were more likely to be hospitalised or admitted to intensive care units (ICUs), and were at higher risk of death than non-pregnant adults.⁵

    
    
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    ○ In one study of the 1918, H1N1 influenza pandemic, half of the pregnant women had pneumonia and there was a 27 per cent case fatality rate.

    
    
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    ○ In the H2N2 influenza pandemic in 1957–58, one-half of women of reproductive age who died from pandemic influenza were pregnant.^{6, 7}

    
    
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    ○ In a systematic review of 120 studies reporting on 3110 pregnant women from 29 countries with H1N1 influenza, pregnant women accounted for approximately 6 per cent of individuals who were hospitalised, were admitted to an ICU and died as a result of H1N1 2009 pandemic influenza A (H1N1).⁸ [EL 1]

  
  

Fetal Implications

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