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Introduction[edit | edit source]

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Influenza is a communicable viral disease that affects the upper and lower respiratory tract. Symptoms of influenza include: Runny nose; High fever; Cough; Sore throat. A wide spectrum of influenza viruses causes it. Some of these viruses can infect humans, and some are specific to different species. WHO estimates that seasonal influenza may result in 290 000-650 000 deaths each year due to respiratory diseases alone.[1]

Influenza viruses:

  • Transmissible through respiratory droplets expelled from the mouth and respiratory system during coughing, talking, and sneezing.
  • Can be transmitted by touching inanimate objects soiled with the virus and touching the nose or eye.
  • Can be transmitted before the patient is symptomatic and until 5 to 7 days after infection.
  • Take a few days for most of the healthy patients to recover fully from.
  • Complications (eg pneumonia and death) are common in certain high-risk groups eg young children, the elderly, immunocompromised, and pregnant females.
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  • Influenza spreads rapidly and efficiently in seasonal epidemics. Flu epidemics occur every autumn and winter in temperate regions and affect a significant portion of adults and children, but seasons differently impact age groups and severity[2]
  • An influenza pandemic occurs when a novel influenza virus emerges with the ability to cause sustained human-to-human transmission, and the human population has little to no immunity against the virus. With the growth of global travel, a pandemic can spread rapidly globally with little time to prepare a public health response.[3]
  • H1N1 influenza A was the cause of the Spanish flu, one of the worst global pandemics in recorded human history[4]According to the CDC, an estimated 500 million people — or 1/3rd of the world’s population — caught the virus during the pandemic and between 50 million and 100 million people were killed.[5]
  • Image: Hospital during Spanish Flu

Etiology[edit | edit source]

Hemagglutinin .jpg

There are four types of influenza viruses, A, B, C, and D. Influenza viruses have receptors responsible for making them species-specific. Influenza types A and B cause human infection annually during the epidemic season. There are many different strains and they can change every year.[6]

Influenza A

  • Has several subtypes according to the combination of hemagglutinin (H) and the neuraminidase (N) proteins that are expressed on the surface of the viruses.
  • Image: Hemagglutininan protein (such as an antibody or viral capsid protein) that causes hemagglutination ie agglutination of red blood cells[7]
  • There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1-18 and N1-11).
  • Influenza A viruses can be characterized by the H and N types such as H1N1 and H3N2[2].

Influenza B viruses

  • Not classified into subtypes, but can be broken down into lineages.
  • Currently circulating influenza type B viruses belong to either B/Yamagata or B/Victoria lineage.

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

Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.[1][6].

Epidemiology[edit | edit source]

  • All age groups can be affected but there are groups that are more at risk than others.
  • People at greater risk of severe disease or complications when infected are: pregnant women, children under 59 months, the elderly, individuals with chronic medical conditions (such as chronic cardiac, pulmonary, renal, metabolic, neurodevelopmental, liver or hematologic diseases) and individuals with immunosuppressive conditions (such as HIV/AIDS, receiving chemotherapy or steroids, or malignancy).
  • Health care workers are at high risk acquiring influenza virus infection (due to increased exposure to the patients) and risk further spread particularly to vulnerable individuals.
  • Seasonal influenza spreads easily, with rapid transmission in crowded areas eg schools and nursing homes.
  • In temperate climates, seasonal epidemics occur mainly during winter, while in tropical regions, influenza may occur throughout the year, causing outbreaks more irregularly.
  • The time from infection to illness, known as the incubation period, is about 2 days, but ranges from one to four days[1].

Pathophysiology[edit | edit source]

Influenza is an acute disease that targets the upper respiratory tract causing inflammation of the upper respiratory tree and trachea.

  • The acute symptoms persist for seven to ten days, and the disease is self-limited in most healthy individuals.
  • The immune reaction to the viral infection and the interferon response are responsible for the viral syndrome that includes high fever, coryza, and body aches.
  • High-risk groups (eg chronic lung diseases, cardiac disease, and pregnancy) are more prone to severe complications such as primary viral pneumonia, secondary bacterial pneumonia, hemorrhagic bronchitis, and death. These severe complications can develop in as little as 48 hours from the beginning of symptoms.
  • The virus replicates in the upper and lower respiratory passages starting from the time of inoculation and peaking after 48 hours, on average[2].

Signs and Symptoms[edit | edit source]

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Seasonal influenza is characterized by:

  • A sudden onset of fever, cough (usually dry), headache, muscle and joint pain, severe malaise (feeling unwell), sore throat and a runny nose.
  • The cough can be severe and can last 2 or more weeks.

Most people recover from fever and other symptoms within a week without requiring medical attention. But influenza can cause severe illness or death especially in people at high risk.

  • Illnesses range from mild to severe and even death.
  • Hospitalization and death occur mainly among high risk groups.
  • Worldwide, these annual epidemics are estimated to result in about 3 to 5 million cases of severe illness, and about 290 000 to 650 000 respiratory deaths.
  • In industrialized countries most deaths associated with influenza occur among people age 65 or older.
  • Epidemics can result in high levels of worker/school absenteeism and productivity losses. Clinics and hospitals can be overwhelmed during peak illness periods.
  • The effects of seasonal influenza epidemics in developing countries are not fully known, but research estimates that 99% of deaths in children under 5 years of age with influenza related lower respiratory tract infections are found in developing countries[1]

Treatment[edit | edit source]

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Mild flu gets better on its own without any treatment. Ways to relieve the symptoms include:

  • resting
  • drinking fluids, particularly water
  • taking paracetamol to reduce pain and fever
  • using decongestant medicines.

People with a serious case of the flu may need to go to hospital (even with treatment, some people with severe flu may die).

  • Antiviral medications can be used to treat or prevent influenza infection, especially during outbreaks in healthcare settings such as hospitals and residential institutions. Neuraminidase inhibitors family and can be used for the treatment of influenza A and B. The adamantanes antiviral family has two medications which are effective against influenza A, but not influenza B. Resistance to these antivirals has been low in recent influenza seasons[2], but the virus may mutate and develop resistance at any time

Antibiotics should not be used to treat colds or the flu, which are viral — not bacterialinfections[6].

Prognosis/Complications[edit | edit source]

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Influenza has enormous morbidity. Many individuals have to take time off work and school. In children and seniors, the infection can lead to death. Mortality rates are higher in people with pre-existing lung disease and diabetes. Overall, the condition can seriously limit the quality of life for 1-2 weeks each season[2].

Complications include: Secondary bacterial pneumonia; Acute respiratory distress syndrome;(ARDs) Myositis; Myocarditis; Multiorgan failure[2].

Image: Prone positioning for ARDs

Prevention[edit | edit source]

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Vaccination is the best protection against the flu.

  • The flu vaccines protects individuals from getting infected and prevent serious disease. The flu strains constantly change so people need a new vaccine every year to make sure they stay protected.[6]
  • Among healthy adults, influenza vaccine provides protection, even when circulating viruses do not exactly match the vaccine viruses.
  • Among the elderly, influenza vaccination may be less effective in preventing illness but reduces severity of disease and incidence of complications and deaths.
  • Vaccination is especially important for people at high risk of influenza complications, and for people who live with or care for the people at high risk.

WHO recommends annual vaccination for: pregnant women at any stage of pregnancy; children aged between 6 months to 5 years; elderly individuals (aged more than 65 years); individuals with chronic medical conditions; health-care workers.[1]

Influenza (Avian and other zoonotic)[edit | edit source]

  • Viruses-10-00497-g004.png
    Humans can be infected with avian, swine and other zoonotic influenza viruses eg avian influenza virus subtypes A(H5N1), A(H7N9), and A(H9N2) and swine influenza virus subtypes A(H1N1), A(H1N2) and A(H3N2). A (H1N1) influenza notably resulted in a pandemic in 2009. It was referred to colloquially as 'swine flu' due to the origin of the virus. The CDC estimates that between 151,700 - 575,400 people died worldwide during the first year that the (H1N1) virus circulated. About 80% of those deaths are believed to have been people younger than 65, which is unusual. During typical seasonal influenza epidemics, 70-90% of deaths occur in people over 65[5].
  • Human infections are primarily acquired through direct contact with infected animals or contaminated environments, these viruses have not acquired the ability of sustained transmission among humans.
  • Avian, swine and other zoonotic influenza virus infections in humans may cause disease ranging from mild upper respiratory tract infection (fever and cough), early sputum production and rapid progression to severe pneumonia, sepsis with shock, acute respiratory distress syndrome and even death[3].[4]

Image:  Timeline showing influenza pandemics and epidemics caused by influenza A Viruses (IAVs). The “Spanish Flu” of 1918 was the most devastating influenza pandemic in the 20th century and was likely caused by a zoonotic transmission of an H1N1-type IAV from poultry to humans.

References[edit | edit source]

  1. 1.0 1.1 1.2 1.3 1.4 WHO Flu Available from: (last accessed 7.12.2020)
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Boktor SW, Hafner JW. Influenza [Internet]. StatPearls.21.11.2020 Available from: (last accessed 7.12.2020)
  3. 3.0 3.1 WHO Influenza (Avian and other zoonotic) Available from; (last accessed 8.12.20200
  4. 4.0 4.1 Radiopedia H1N1 influenza Available from: (last accessed 8.12.2020)
  5. 5.0 5.1 The Weather Channel Flu epidemics Available from: (last accessed 8.12.2020)
  6. 6.0 6.1 6.2 6.3 Australian Govt. Flu Available from; (last accessed 7.12.2020)
  7. hemagglutination Available from: (last accessed 8.12.2020)