What are the signs and symptoms of Influenza H1N1?

Signs and Symptoms of Influenza H1N1

Influenza H1N1 is characterized by the abrupt onset of constitutional and respiratory symptoms including fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis, with symptoms varying by age group. 1

General Clinical Presentation

• Incubation period is 1-4 days, with an average of 2 days 1
• Uncomplicated influenza typically resolves after 3-7 days, although cough and malaise can persist for >2 weeks 1
• Adults are typically contagious from the day before symptoms begin through 5-6 days after symptom onset 2
• Children can be infectious for up to 10 days after symptom onset 2
• Immunocompromised persons may shed virus for weeks or months 1, 2

Common Symptoms by Age Group

Neonates and Infants

• May present with non-specific signs resembling sepsis 1
• Fever may be the only presenting feature 1
• Other signs include:
  • Pallor, floppiness (poor peripheral circulation, poor tone) 1
  • Lethargy, poor feeding, episodes of apnea 1
  • Irritability and appearing toxic 1
  • Gastrointestinal symptoms (diarrhea and vomiting) more common than in adults 1
  • Febrile seizures (reported in 6-20% of hospitalized children) 1
  • Otitis media is a common complication 1

Children

• Presentation similar to adults in older children 1
• Common symptoms include:
  • Sudden onset of high fever, chills 1
  • Cough, headache, sore throat 1
  • Fatigue, nasal stuffiness, conjunctivitis 1
  • Otitis media, nausea, and vomiting 1
• The triad of cough, headache, and pharyngitis has shown 80% sensitivity and 78% specificity for influenza in children during an epidemic 1

Adults

• Abrupt onset of constitutional and respiratory signs and symptoms 1
• Key symptoms include:
  • Fever and chills 1
  • Myalgia (muscle aches) 1
  • Headache 1
  • Malaise (general discomfort) 1
  • Nonproductive cough 1
  • Sore throat 1
  • Rhinitis (nasal inflammation) 1
• Mild illness without fever can occur in 6-33% of H1N1 cases 1

Specific H1N1 Presentations

• H1N1 influenza may present with gastrointestinal symptoms more frequently than seasonal influenza 3, 4
• Symptoms commonly reported in H1N1 cases include:
  • Fever and cough (most common, reported in 55% of hospitalized patients) 4
  • Vomiting (55%) 4
  • Rhinorrhea (80%) 4
  • Body ache/myalgia (40%) 4
  • Sore throat (35%) 4
  • Abdominal pain (25%) 4
• Laboratory findings may include leucopenia and thrombocytopenia 4

Complications and Severe Presentations

• Primary influenza viral pneumonia 1
• Exacerbation of underlying medical conditions (e.g., pulmonary or cardiac disease) 1
• Secondary bacterial pneumonia, sinusitis, or otitis media 1
• Coinfections with other viral or bacterial pathogens 1
• Severe complications in children may include:
  • Bacterial pneumonia (commonly caused by S. pneumoniae or S. aureus) 1
  • Encephalopathy or encephalitis presenting as seizures or altered mental status 1
  • Acute respiratory distress syndrome 1
• Patients at higher risk for severe disease include:
  • Children under 5 years 1, 3
  • Pregnant women 1, 3
  • Individuals with underlying chronic cardiopulmonary conditions 1
  • Individuals with certain neuromuscular conditions 1
  • Immunocompromised people 1, 3

Clinical Pearls

• Respiratory illness caused by influenza is difficult to distinguish from illness caused by other respiratory pathogens based on symptoms alone 1
• Viral shedding decreases rapidly by 3-5 days after illness onset in most adults 2
• During the 2009 H1N1 pandemic, diffuse viral pneumonitis was the most common cause of hospitalization in severe cases 1
• Secondary or concomitant bacterial pneumonia has been identified in 20-38% of severe H1N1 cases requiring intensive care 1
• The diagnosis should be made clinically, and treatment decisions should not be delayed for laboratory confirmation 5