Influenza: Definition and Clinical Features
Influenza is a contagious respiratory illness caused by influenza viruses, characterized by the abrupt onset of fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis. 1
Viral Biology and Classification
- Influenza A and B are the two types of influenza viruses that cause epidemic human disease 1
- Influenza A viruses are categorized into subtypes based on two surface antigens: hemagglutinin (H) and neuraminidase (N) 2
- Since 1977, influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses have been in global circulation 1
- Influenza A undergoes frequent antigenic changes through:
- Antigenic drift: point mutations during viral replication
- Antigenic shift: major changes in surface antigens through genetic reassortment 2
Transmission and Incubation
- The virus spreads primarily through respiratory droplets from coughing and sneezing of infected persons 1
- The incubation period ranges from 1-4 days, with an average of 2 days 1
- Persons can be infectious starting the day before symptoms begin through approximately 5 days after illness onset 1
- Children can be infectious for longer periods, up to 10 days after onset of symptoms, and young children can shed virus for up to 6 days before illness onset 1
- Severely immunocompromised persons can shed virus for weeks or months 1
Clinical Manifestations
Common Symptoms
- Fever (up to 41°C, typically 38-40°C, lasting 1-5 days) 1
- Cough (approximately 85% of cases) 1
- Headache (approximately 65% of cases) 1
- Myalgia/muscle aches (approximately 53% of cases) 1
- Malaise (approximately 80% of cases) 1
- Sore throat (approximately 50% of cases) 1
- Chills/rigors (approximately 70% of cases) 1
- Coryzal symptoms/rhinitis (approximately 60% of cases) 1
Presentation in Children
- Children may present with otitis media, nausea, and vomiting in addition to general symptoms 1
- In infants, influenza may initially present with signs similar to bacterial sepsis, high fever, or febrile seizures 1
- Up to 20% of children hospitalized with influenza experience febrile seizures 1
Disease Course
- Uncomplicated influenza illness typically resolves after 3-7 days for most people 1
- Cough and malaise can persist for more than 2 weeks 1
- The risk for complications, hospitalizations, and deaths is higher among:
- Persons aged >65 years
- Young children (especially those aged 0-1 years)
- Persons with underlying health conditions 1
Complications
Respiratory Complications
- Primary viral pneumonia (uncommon, onset within 48 hours of fever) 1
- Secondary bacterial pneumonia (common, typically occurs 4-5 days after onset) 1
- Acute bronchitis (common, especially in elderly and those with chronic conditions) 1
- Croup and bronchiolitis in children 1
Cardiovascular Complications
- ECG abnormalities (common, non-specific T wave and rhythm changes) 1
- Myocarditis (rare) 1
- Pericarditis (rare) 1
Neurological Complications
- Encephalopathy/encephalitis (rare, occurs within first week) 1
- Transverse myelitis (very rare) 1
- Guillain-Barré syndrome (very rare) 1
- Febrile seizures (in children) 1
Musculoskeletal Complications
Diagnosis
- During influenza season, a clinical diagnosis based on symptoms has moderate sensitivity (63-78%) and specificity (55-71%) compared to viral culture 1
- The presence of fever, cough, and acute onset is the most predictive clinical definition 1
- Laboratory confirmation may not be necessary for most outpatient cases but can be useful for hospitalized patients 3
- Rapid molecular assays are the preferred diagnostic tests due to their high accuracy and fast results 3
Treatment
- Antiviral medications may be considered if started within 48 hours of symptom onset 3
- Treatment benefit is greatest when started within 24 hours of symptom onset 3
- Antiviral drugs decrease illness duration by about 24 hours in otherwise healthy patients 3