Clinical Signs and Symptoms


Influenza Clinical Features

  • Incubation period 2 days (range, 1–4 days)
  • About 8% of U.S. population gets sick each season
  • Sudden onset of symptoms
    • Respiratory: cough, sore throat, runny or stuffy nose
    • Systemic: fever, chills, headache, malaise, myalgia
    • Gastrointestinal: vomiting, diarrhea
  • Rapid recovery

Source: CDC - The Pinkbook. Chap. 12. Influenza

Uncomplicated flu illness is characterized by the abrupt onset of: fever, bodyaches, headache, severe fatigue, sore throat, runny nose, and nonproductive cough. Among children with influenza illness, otitis media, nausea, and vomiting are also commonly reported.

The incubation period for influenza is usually 2 days but can vary from 1 to 4 days. Influenza illness can range from asymptomatic to severe infection. On average, about 8% of the U.S. population gets sick from influenza each season (range between 3% and 11%).

Flu is not just a common cold. (think F. A.C.T.S.):

  • Fever
  • Aches (muscle, body, and headaches)
  • Chills
  • Tiredness (fatigue)
  • Sudden onset
  • Cough, runny or stuffy nose, and/or sore throat
  • Vomiting and diarrhea (more common in children than adults)

Onset of influenza symptoms is sudden. Respiratory symptoms include cough, sore throat, and runny or stuffy nose. Systemic symptoms generally include fever, chills, headache, malaise, and myalgia. Vomiting and diarrhea may also occur, especially in children. Recovery is rapid; fever usually resolves within 3 to 4 days and other symptoms within approximately 7 days. Some patients may have a cough and lingering asthenia (lack of strength or energy) for several weeks.

Influenza can be difficult to diagnose based on clinical symptoms alone. Diagnosis is complicated because the initial symptoms of influenza can be similar to disease caused by other infectious agents. These agents include, but are not limited to: Mycoplasma pneumoniae, adenovirus, respiratory syncytial virus, rhinovirus, parainfluenza viruses, and Legionella spp.

Treatment

Antiviral Drug Options

Prevention

Vaccination is highly recommended at the start of the winter season. The flu vaccine recommendations include:

Complications

Influenza Complications

  • Secondary bacterial pneumonia
  • Exacerbations of underlying respiratory conditions
  • Otitis media
  • Laryngotracheobronchitis
  • Bronchitis
  • Other less common complications may occur

People most at risk of developing serious influenza-related complications include people age 65 years and older, people with chronic medical conditions (e.g., heart disease or diabetes), pregnant women, and young children, especially those younger than age 2 years. More common complications of influenza include secondary bacterial pneumonia (e.g., Streptococcus pneumoniae, Haemophilus influenzae, or Staphylococcus aureus), exacerbations of underlying respiratory conditions, otitis media, laryngotracheobronchitis, and bronchitis.

Other complications may include primary pneumonia, encephalitis, aseptic meningitis, transverse myelitis, myocarditis, pericarditis, Guillain-Barré syndrome, and Reye Syndrome. Reye syndrome is a complication that occurs almost exclusively in children taking aspirin, primarily in association with influenza B virus (or varicella zoster virus), and presents with severe vomiting and confusion, which may progress to coma due to swelling of the brain.


Instant feedback:

Abrupt onset of severe fever, combined with severe aches & pains, is most likely:

A cold
The flu

References

Boktor S, Hafner JW. (2019) StatPearls. Influenza. Treasure Island (FL).

Centers for Disease Control and Prevention. (2020). Clinical signs and symptoms of influenza. Centers for Disease Control and Prevention. Retrieved August 1, 2022, from https://www.cdc.gov/flu/professionals/

Centers for Disease Control and Prevention. (2021). Pinkbook: Influenza. Centers for Disease Control and Prevention. Retrieved September 21, 2021, from https://www.cdc.gov/vaccines/pubs/pinkbook/flu.html#Pathogenesis.

CDC, & Hall, E. (n.d.). Influenza. CDC-Epidemiology and Prevention of Vaccine-Preventable Diseases. https://www.cdc.gov/vaccines/pubs/pinkbook/flu.html


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