The clinical spectrum of influenza ranges from asymptomatic infection to primary viral pneumonia that may progress to death.
Patients presenting with influenza-like illness (ie temperature of 37.8ºC, cough and/or sore throat and absence of a known cause other than influenza) might be infected with different types of influenza virus [eg avian influenza (H5N1)] as well as other respiratory pathogens.
A high index of suspicion is needed to recognize influenza in hospitalized patients.
Pneumonia is the most common complication of influenza virus.


  • Influenza is an acute febrile respiratory viral illness that usually occurs in annual outbreaks of varying severity and occasionally occurs in worldwide epidemics (pandemics) and as sporadic zoonotic infection
  • It infects the respiratory tract, highly contagious, and typically produce prominent systemic symptoms early in the illness


Seasonal Influenza

  • The types of influenza virus are types A, B and C
  • Types A and B are the main causes of influenza outbreaks
Avian Influenza
  • Also known as bird flu, caused by type A strains of the influenza virus
  • Influenza A (H5N1) is a subtype of the type A influenza virus and is the cause of the current outbreak of the avian influenza worldwide
  • Although it is generally considered to be a disease that affects only birds, avian influenza viruses can infect several animal species (eg pigs, horses, seals, whales) but do not usually directly infect humans or circulate among humans
  • Human transmission can be caused by exposure to droplet and direct/indirect contact with feces or nasal secretion of the sick animal
  • Human-to-human transmission can possibly occur among close contacts

Signs and Symptoms

Influenza-Like Illness (ILI)

An acute respiratory infection with:

  • Temperature of ≥37.8°C and
  • Cough and
  • Onset within the last 10 days

Seasonal Influenza

Typical signs and symptoms

  • Abrupt onset of fever
  • Severe myalgia and/or arthralgia
  • Severe dry cough
  • Loss of appetite
  • Headache
  • Malaise
  • Sore throat
  • Rhinitis

Zoonotic Influenza

Avian influenza A (H5N1) should be suspected in the following:

  • Documented temperature of >38°C and
  • ≥1 of the following: Cough, sore throat and/or shortness of breath and
  • Contact history
    • History of contact with sick or dead poultry, family members with suspected or confirmed H5N1 infection, healthcare workers of suspected or confirmed H5N1 patients, lab workers handling specimens or viral cultures of H5N1 viruses
    • History of living in an endemic area or an environment with sick or dead poultry
    • If the patient only has a history of travel to endemic areas, risk assessment will need to be individualized

Other reported presentations of avian influenza in humans:

  • Typical influenza-like symptoms (abrupt onset with headache, high-grade fever, sore throat, chills, dry cough, malaise, myalgia, anorexia)
  • Dyspnea or bloody sputum
  • Chest pain
  • Bleeding from the nose and gums
  • GI upset (vomiting, diarrhea, abdominal pain)
  • Neurological symptoms may occur (eg stupor, convulsions)
  • Conjunctivitis
  • Pneumonia/Pneumonitis
  • Acute respiratory distress
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