What are the clinical features of influenza?

Clinical Features of Influenza

The most common presenting features of influenza are fever, cough, and rhinorrhea, with the hallmark being abrupt onset of fever accompanied by respiratory symptoms. 1

Core Clinical Presentation in Adults

Fever is the paramount symptom, typically ranging between 38°C and 40°C (though may reach 41°C), with peak occurring within 24 hours of onset and lasting typically 3 days (range 1-5 days). 1

The classic symptom constellation includes:

• Cough (~85%) - generally dry, though may be productive in up to 40% of cases 1
• Headache (~65%) 1
• Myalgia (~53%) - affecting mainly the back and limbs 1
• Malaise (~80%) 1
• Chills (~70%) 1
• Sore throat (~50%) 1
• Coryzal symptoms/rhinorrhea (~60%) 1
• Anorexia (~60%) 1

The combination of cough and fever together has a positive predictive value of 79% when influenza is circulating in the community. 2 The three-fold combination of fever, cough, and acute onset is the most predictive clinical definition. 1

Clinical findings on examination include a toxic appearance initially, hot and moist skin, flushed face, injected eyes, and hyperemic mucous membranes. 1

Gastrointestinal symptoms (vomiting, diarrhea) are uncommon in adults (<10%), and abdominal pain is rare. 1 This is an important distinguishing feature from influenza in children.

Clinical Presentation by Age Group in Children

Neonates (0-60 days)

Fever may be the only presenting feature, making influenza the most common reason for hospital admission in this age group during epidemics. 1

Non-specific signs resembling sepsis include:

• Pallor and floppiness (poor peripheral circulation, poor tone) 1
• Lethargy and poor feeding 1
• Episodes of apnea 1

Infants and Very Young Children (Under 2 Years)

Fever remains the predominant feature, but these children are more likely than older children to present with gastrointestinal symptoms. 1

Key features include:

• Irritability and toxic appearance 1
• Diarrhea and vomiting (much more common than in adults) 1
• Febrile convulsions, particularly repeated convulsions, are positively associated with influenza A 1
• Otitis media is a common complication 1

Admission rates for children under 2 years are 12 times higher than children aged 5-17 years. 1

Older Children (≥5 Years)

The presentation does not differ significantly from adults. 1

Common features include:

• Sudden onset of high fever 1
• Chills 1
• Cough 1
• Headache 1
• Sore throat/pharyngitis 1
• Fatigue 1
• Nasal stuffiness 1
• Conjunctivitis 1

The triad of cough, headache, and pharyngitis has a sensitivity of 80% and specificity of 78% for laboratory-confirmed influenza in children. 1

Fever typically settles 2-4 days later, though a dry cough and clear nasal discharge last for 1-2 weeks. 1

Time Course and Duration

Uncomplicated influenza typically resolves after 3-7 days, although cough and malaise can persist for more than 2 weeks. 3 The incubation period is 1-4 days, with an average of 2 days. 3

Adults are typically contagious from the day before symptoms begin through 5-6 days after symptom onset, while children can be infectious for up to 10 days. 3

Severe and Life-Threatening Complications

The severe complications most likely to cause mortality and morbidity are:

• Bacterial pneumonia (most common complication in adults) 1
• Acute respiratory distress syndrome (ARDS) 1
• Encephalopathy or encephalitis presenting as seizures or altered mental status 1

Pneumonia Patterns

Primary viral pneumonia is uncommon but has onset within 48 hours of fever start. 1

Secondary bacterial pneumonia is common and typically occurs 4-5 days after illness onset, most commonly caused by Streptococcus pneumoniae or Staphylococcus aureus. 1 Children with recent influenza-like illness are 12 times more likely to develop severe pneumococcal pneumonia. 1

Other Complications

Cardiovascular complications include:

• ECG abnormalities (common) - non-specific T wave and rhythm changes, ST segment deviation 1
• Myocarditis and pericarditis (rare) 1

Neurological complications (rare in adults):

• Encephalitis/encephalopathy - occurs within first week of illness, more common in children 1
• Transverse myelitis (very rare) 1
• Guillain-Barré syndrome (very rare) 1

Musculoskeletal complications:

• Myositis (uncommon) - occurs during early convalescence 1
• Myoglobinuria and renal failure (rare) 1

Otitis media is much more common in children than adults. 1

High-Risk Populations

Children with underlying respiratory or cardiac disease, immune compromise, or who are non-ambulant are more likely to be severely affected. 1 These children experience substantial morbidity with a disproportionate number requiring inpatient care and ventilatory support. 1

The younger the child, the more likely hospital admission will be needed. 1

Critical Clinical Pitfalls

Influenza is difficult to distinguish from other respiratory pathogens based on symptoms alone. 4, 3 Other viruses (RSV, adenovirus, rhinovirus, parainfluenza) and bacterial pathogens (Mycoplasma pneumoniae, Streptococcus pneumoniae, Chlamydia pneumoniae, Legionella sp.) can present with identical influenza-like illness. 1

In young children presenting to primary care during non-pandemic influenza season, there are no specific clinical features that distinguish influenza from other winter viruses. 1

If fever persists beyond 2-4 days or worsens after initial improvement, strongly suspect bacterial superinfection, particularly pneumonia or otitis media. 4 This is a critical decision point that changes management from supportive care to antibiotics.

Do not miss bacterial complications: examine the ears in all children, assess work of breathing, and maintain high suspicion for pneumonia if fever duration exceeds the expected 2-4 days. 4