What are the typical signs and symptoms of influenza, including gastrointestinal manifestations and associated warning signs?

Signs and Symptoms of Influenza with Gastrointestinal Manifestations

Influenza typically presents with abrupt onset of fever, cough, myalgia, headache, and malaise, but gastrointestinal symptoms such as vomiting and diarrhea are uncommon in adults (occurring in <10%) and more frequent in young children. 1

Core Clinical Features in Adults

The hallmark presentation includes:

• Fever (68-100% of cases): Usually ranges between 38-40°C, peaking within 24 hours of onset and lasting 1-5 days (typically 3 days) 1
• Cough (85-93% of cases): Generally nonproductive, though may be productive in up to 40% of patients, especially those with underlying chronic lung disease 1, 2
• Myalgia (~53% of cases): Primarily affects the back and limbs 1
• Headache (~65% of cases) 1
• Malaise (~80% of cases) 1
• Sore throat (~50% of cases) 1
• Chills (~70% of cases) 1
• Rhinitis/nasal congestion (~60-91% of cases) 1, 2

The combination of cough and fever together has a positive predictive value of 79% for influenza when the virus is circulating in the community. 2

Gastrointestinal Symptoms: Age-Dependent Patterns

Adults

Gastrointestinal symptoms are rare in adults with influenza, occurring in less than 10% of cases. 1 When present:

• Vomiting and diarrhea are uncommon 1
• Abdominal pain is rare 1

Infants and Young Children (Under 2 Years)

Gastrointestinal symptoms are significantly more common in young children and may be the predominant presenting feature:

• Diarrhea and vomiting are common presenting symptoms 1, 3
• Fever may be the only presenting feature in neonates and infants 1, 3
• Non-specific signs resembling sepsis (pallor, floppiness, lethargy, poor feeding, apnea) are typical in neonates 1, 3
• Irritability and appearing toxic are frequent 1, 3

Older Children

• Sudden onset of high fever, chills, cough, headache, sore throat, fatigue, and nasal stuffiness 1, 3
• Nausea and vomiting may occur 3
• The triad of cough, headache, and pharyngitis has 80% sensitivity and 78% specificity for influenza 1

Warning Signs and Complications

Respiratory Complications

The most common serious complications include:

• Secondary bacterial pneumonia (common): Typically occurs 4-5 days after illness onset with pathogens including S. pneumoniae, S. aureus, and H. influenzae 1, 3
• Primary viral pneumonia (uncommon): Onset within 48 hours of fever start 1
• Acute bronchitis (common): More frequent in elderly and those with chronic conditions 1

Neurological Complications

• Encephalopathy/encephalitis (rare in adults, more common in children): Presents with seizures, altered mental status, extreme irritability, lethargy, or confusion 1, 3, 4
• Febrile seizures: Occur in up to 20% of hospitalized children with influenza and are more likely to be recurrent compared to other causes 4
• Guillain-Barré syndrome, transverse myelitis (very rare) 1

Other Complications

• Otitis media (common in children): Occurs in approximately 25% of children under 5 years 1, 3, 4
• Myositis, myocarditis, pericarditis (rare) 1

Red Flags Requiring Immediate Medical Attention

Children should be evaluated immediately if they develop: 1, 3

• Breathing difficulties or respiratory distress
• Cyanosis
• Severe dehydration
• Altered consciousness or extreme lethargy
• Signs of septicemia
• Fever persisting beyond 4 days (suggests bacterial superinfection)
• Initial improvement followed by fever recurrence (hallmark of bacterial superinfection) 3

Clinical Pearls

• The incubation period is 1-4 days (average 2 days), with adults infectious from the day before symptoms begin through approximately 5 days after onset 1, 3
• Children can be infectious for up to 10 days, and immunocompromised persons may shed virus for weeks 3
• Respiratory illness caused by influenza is difficult to distinguish from other respiratory pathogens based on symptoms alone, with reported sensitivity of 63-78% and specificity of 55-71% for clinical definitions 1
• Uncomplicated influenza typically resolves after 3-7 days, though cough and malaise can persist for more than 2 weeks 1, 3
• Children with recent influenza are 12 times more likely to develop severe pneumococcal complications compared to those without influenza 3, 4