Clinical Presentation, Gastrointestinal Manifestations, and Complications of Influenza

Epidemiology and Typical Presentation

Influenza usually begins abruptly with fever, cough, myalgia, headache, and malaise; gastrointestinal symptoms are uncommon in adults (<10 %) but occur more frequently in young children. [@1]
Fever is present in 68–100 % of adult cases, typically 38–40 °C, peaks within 24 h of onset and lasts 1–5 days (median 3 days). [@1][@2]
Cough occurs in 85–93 % of adults, generally non‑productive; up to 40 % may develop a productive cough, especially in patients with chronic lung disease. [@1]
Myalgia affects roughly 53 % of adults, mainly the back and limbs. [@1]
Headache is reported in about 65 % of adult cases. [@1]
Malaise is experienced by approximately 80 % of adults. [@1]
Sore throat is seen in ~50 % of adults. [@2]
Chills occur in ~70 % of adult patients. [@1]
Nasal congestion/rhinitis is present in 60–91 % of adults. [@1]

Gastrointestinal symptoms (vomiting, diarrhea, abdominal pain) are rare, occurring in <10 % of adult influenza cases. [@1]

Diarrhea and vomiting are common presenting features. [@4]
Fever may be the sole presenting sign in neonates and infants. [@4]
Neonates often display sepsis‑like signs (pallor, floppiness, lethargy, poor feeding, apnea). [@4]
Irritability and a toxic‑appearing demeanor are frequent. [@4]

Typical presentation includes sudden high fever, chills, cough, headache, sore throat, fatigue, and nasal congestion. [@4]
Nausea and vomiting may accompany the illness. [@5]
The triad of cough, headache, and pharyngitis yields 80 % sensitivity and 78 % specificity for influenza. [@4]

Secondary bacterial pneumonia is common, usually developing 4–5 days after illness onset; common pathogens include Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. [@1]
Primary viral pneumonia is uncommon and typically appears within 48 h of fever onset. [@1]
Acute bronchitis is common, especially in the elderly and those with chronic medical conditions. [@1]

Encephalopathy/encephalitis is rare in adults but more frequent in children, presenting with seizures, altered mental status, extreme irritability, or lethargy. [@1]
Febrile seizures occur in up to 20 % of hospitalized children with influenza and are more likely to recur than seizures from other causes. [@6]
Guillain‑Barré syndrome and transverse myelitis are very rare complications. [@2]

Otitis media affects roughly 25 % of children younger than 5 years. [@4]
Myositis, myocarditis, and pericarditis are rare cardiac and muscular complications. [@2]

Respiratory distress or difficulty breathing. [@4]
Cyanosis. [@4]
Severe dehydration. [@4]
Altered consciousness or profound lethargy. [@6]
Clinical signs suggestive of septicemia. [@4]
Fever persisting beyond 4 days (suggesting possible bacterial superinfection). [@4]
Initial improvement followed by recurrence of fever (classic biphasic pattern of bacterial superinfection). [@4]

The incubation period ranges from 1–4 days (average 2 days); adults are infectious from the day before symptom onset through approximately 5 days after onset. [@2]
Children may remain infectious for up to 10 days, and immunocompromised individuals can shed virus for several weeks. [@5]

Symptom‑based clinical definitions differentiate influenza from other respiratory pathogens with a sensitivity of 63–78 % and specificity of 55–71 %. [@2]

Uncomplicated influenza typically resolves within 3–7 days, although cough and malaise may persist for more than 2 weeks. [@2]

Children who have recently had influenza are 12 times more likely to develop severe pneumococcal complications compared with children without recent influenza. [@5]