Influenza Virus: Introduction, Properties, Pathogenicity, Lab Diagnosis, Treatment and Prevention

Influenza Virus: Introduction, Properties, Pathogenicity, Lab Diagnosis, Treatment and Prevention

Introduction of Influenza Virus

Influenza virus infection is a highly contagious airborne disease. It causes acute febrile respiratory illness and results in varying degrees of systemic symptoms, ranging from mild fatigue to respiratory failure and even death. It is responsible for infecting 5–15% of the global population annually. Influenza is a viral infection and commonly called the flu.

Family: Orthomyxoviridae

Types: A, B, and C

Influenza B and C viruses have a very limited host range and are mainly present in humans whereas influenza A reservoirs are human as well as wild aquatic birds and other animal species like birds, pigs, ferrets, horses, seals, whales, mink, giant anteaters, cats, and dogs.

Properties of Influenza Virus

  • Single straned RNA virus
  • Spherical and pleomorphic particles with helical capsid symmetry, 80-120 nm in diameter.
  • The envelope of this virus has many projections, knowns as peplomere.
  • These peplomeres contain haemagglutinin and attach to the host cells.
  • These peplomeres are antigenic.
  • Influenza viruses are able to change their surface antigenic structure; this is particularly true in type -A.
  • The virus has got an antigenic shift and antigenic drift.
  • The antigenic shift is used to describe major change whereas antigenic drift is used to describe when there is only a minor change in the antigenic structure.
  • Only type -A Influenza virus is capable of antigenic shifts with the production of new subtypes whereas B and C have the antigenic drift.
  • Antigenic shifts and drifts have been the cause of widespread and severe influenza epidemics and pandemics. This has also caused difficulties to prepare vaccines for prophylaxis measures.
  • Influenza viruses also contain inner stable antigens. These antigens do not cross-react and undergo antigenic changes. Therefore new influenza virus can always be typed serologically.
  • Influenza viruses haemagglutinate red cells of various animal species and can be cultured in embryonated eggs or in monkey tissue.

Pathogenicity  of Influenza Virus

Influenza virus was the causative agent of following-

“Spanish” flu in 1918–1919

“Asian” flu in 1957–1958 (H2N2 subtype

“Hong Kong” flu in 1968–1969 (H3N2 subtype)

1957 pandemic was caused by the H3N2 influenza virus and originated in China

2009 Pandemic H1N1, the virus of swine-origin emerged in Mexico and the USA in early April 2009.

Transmission: Infection due to inhaling the viruses through droplet infection.

Incubation period: 1–5 days

Following groups are more prone to infection-

  • Young children under 5 years of age, especially those under 6 months of age
  • Adults older than 65 years of age
  • Residents of nursing homes and other long-term care facilities;
  • Pregnant women and women up to two weeks after birth
  • People with a weakened immune system
  • People with chronic diseases such as asthma, heart disease, kidney disease, liver disease, and diabetes
  • People who are very obese with a BMI of 40 or more

Common signs and symptoms

  • Fever Fever
  • Muscles Aching
  • Chills and sweating
  • Headaches
  • Dry, persistent coughing
  • Breath shortness
  • Tiredness and weakness:
  • Runny nose or stuffy nose
  • Sore throats
  • Vomiting and diarrhea
  • Pain in the eyes

Emergency signs and symptoms may include:

  • Difficulty in breathing or shortness of breath
  • Chest of pain
  • The Continuing Dizziness
  • Seizes
  • Worse of existing medical conditions
  • Severe weakness or pain in the muscle

Laboratory diagnosis of Influenza Virus

  • Collection of samples:  Specimens may be nasopharyngeal aspirate, throat gargle, or blood depending on the nature of the test.

For the demonstration of the Virus: 

  • Centrifuge the specimen
  • Make the smear from the deposit to obtain virally infected cells.
  • Perform a direct fluorescence test to demonstrate the viral antigen into the cells. e.g. rapid antigen test (RIDT—rapid influenza diagnostic test), direct immunofluorescence assay (DFA) test

Culture: Egg inoculation, tissues culture, and monkey kidney cell lines.

Molecular Test: Reverse transcriptase-polymerase chain reaction (RT-PCR)


Two classes of antiviral compounds approved by the US FDA are Adamantane group drugs (amantadine and rimantadine) that block the activity of the viral M2 proton channel, preventing the virus from uncoating and inhibiting the release of the viral genome into host cells, while the second group of dugs is NA inhibitors (Oseltamivir/Tamiflu and zanamivir/Relenza) that bind and block enzymatic activity.

Prevention and Control

The vaccine is available. Influenza vaccine is not 100% effective, so it is also important to take a number of measures to reduce the spread of infection, including:

  • Wash your hands.
  • Don’t touch your face.
  • Cover the coughs and the sneezes.
  • Clean surfaces.
  • Evite the crowds.


  • All three subtypes of influenza virus (A, B, C) are similar in structure but differ antigenically.
  • 16 serologically variant HA and 9 NA have been identified.

PB1: Polymerase basic protein 1

PA: Polymerase acidic protein (PA)

HA: Haemagglutinin

NA: Neuraminidase

NP: Nucleoproteins

M1: Matrix protein 1

NS1: non-structural proteins 1

SO-IVA: Swine origin influenza virus A

vRNP: Viral ribonucleoprotein particles

Further Reading

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