What are the typical symptoms, risk factors, and recommended treatment for Influenza A infection?

Influenza A: Clinical Presentation, Risk Factors, and Treatment

Typical Symptoms

Influenza A characteristically presents with abrupt onset of high fever (38-40°C, occasionally reaching 41°C) accompanied by constitutional and respiratory symptoms that distinguish it from other respiratory infections. 1

Cardinal Features in Adults:

• Fever is the paramount symptom, peaking within 24 hours of onset and typically lasting 3 days (range 1-5 days) 1
• Myalgia affecting mainly the back and limbs 1
• Headache and severe malaise 1
• Nonproductive cough (dry in 60% of cases, productive in 40%) 1
• Sore throat and rhinitis 1

Physical Examination Findings:

• Toxic appearance in initial stages 1
• Hot and moist skin with flushed face 1
• Injected eyes and hyperemic mucous membranes around nose and pharynx 1
• Tender cervical lymphadenopathy (<10% of cases) 1
• Wheezing or lung crackles (<10%, more common with chronic lung disease) 1

Age-Specific Presentations:

• Children: Otitis media, nausea, vomiting, high fevers mimicking bacterial sepsis, and febrile seizures (6-20% of hospitalized children) 1, 2
• Infants: May present with non-specific signs resembling sepsis including lethargy, poor feeding, and apnea 2
• Adults: Gastrointestinal symptoms (vomiting, diarrhea) are uncommon (<10%) 1

Disease Course and Contagiousness

• Incubation period: 1-4 days (average 2 days) 1, 3
• Adults are contagious: From 1 day before symptom onset through 5-6 days after illness begins 1, 3
• Children are contagious: Up to 10 days after symptom onset, sometimes before symptoms appear 1, 3
• Immunocompromised patients: May shed virus for weeks to months 1, 3
• Uncomplicated illness: Typically resolves in 3-7 days, though cough and malaise can persist for >2 weeks 1

High-Risk Groups for Complications

The following patients face significantly elevated risks and require aggressive monitoring and early antiviral treatment: 1

• Adults aged ≥65 years (account for >90% of influenza-related deaths) 3
• Infants aged 0-1 years (hospitalization rates 200-1,000 per 100,000) 3
• Pregnant women 1
• Chronic respiratory disease (asthma, COPD) 1
• Chronic heart disease 1
• Chronic renal or liver disease 1
• Diabetes mellitus 1
• Immunosuppression due to disease or treatment 1
• Long-stay residential care residents 1

Complications

Respiratory Complications:

• Primary influenza viral pneumonia 1
• Secondary bacterial pneumonia (occurs in 20-38% of severe cases requiring ICU) with predominant pathogens being S. pneumoniae (most common), S. aureus (including MRSA), and H. influenzae 1, 2
• Acute bronchitis 1
• Sinusitis and otitis media (25% of children under 5) 1, 2

Non-Respiratory Complications:

• Exacerbation of underlying cardiac or pulmonary disease 1
• Encephalopathy or encephalitis 2
• Myositis, myocarditis, pericarditis 3
• Febrile seizures in children 1

Warning Signs of Bacterial Superinfection:

• Initial improvement followed by worsening (recrudescent fever or increasing dyspnea) 1, 4, 2
• Persistent symptoms ≥10 days without improvement 4
• High fever (≥39°C) with purulent nasal discharge during first 3-4 days 4

Treatment Recommendations

Antiviral Therapy:

Oseltamivir 75 mg every 12 hours for 5 days should be initiated in high-risk patients presenting within 48 hours of symptom onset, with greatest benefit when started within 24 hours. 1

Eligibility criteria for antivirals: 1

• Acute influenza-like illness with fever (>38°C)
• Symptomatic for ≤2 days (ideally)
• Dose reduction to 75 mg once daily if creatinine clearance <30 mL/min 1

Special considerations: 1

• Immunocompromised or very elderly patients may be eligible despite lack of documented fever
• Hospitalized severely ill patients may benefit from treatment started >48 hours from onset, though evidence is limited

Antibiotic Therapy:

For uncomplicated influenza without pneumonia: 1

• Previously well adults with acute bronchitis do NOT routinely require antibiotics
• Consider antibiotics if worsening symptoms develop (recrudescent fever or increasing dyspnea)
• High-risk patients should receive antibiotics with lower respiratory features

For non-severe influenza-related pneumonia: 1

• First-line oral therapy: Co-amoxiclav or tetracycline
• Alternative: Macrolide (clarithromycin/erythromycin) or fluoroquinolone (levofloxacin/moxifloxacin) for penicillin-intolerant patients
• Administer within 4 hours of admission 1

For severe influenza-related pneumonia: 1

• Immediate parenteral therapy: IV co-amoxiclav or 2nd/3rd generation cephalosporin (cefuroxime/cefotaxime) PLUS macrolide (clarithromycin/erythromycin)
• Alternative: Fluoroquinolone with enhanced pneumococcal activity plus broad-spectrum β-lactamase stable antibiotic
• Coverage must include S. pneumoniae, S. aureus (including MRSA), and H. influenzae 1

Pediatric-Specific Treatment:

For children under 12 years with suspected bacterial superinfection: 4, 2

• Co-amoxiclav is the antibiotic of choice
• Alternatives: Clarithromycin or cefuroxime for penicillin allergy
• Oseltamivir may be considered if symptomatic <6 days (ideally <48 hours)

Critical Clinical Pitfalls

• Do not delay antibiotic treatment while awaiting culture results in patients with suspected bacterial superinfection, as rapid deterioration can occur 4, 2
• Color of nasal discharge alone does not differentiate viral from bacterial infection 4
• Respiratory illness caused by influenza is difficult to distinguish from other respiratory pathogens based on symptoms alone (sensitivity 63-78%, specificity 55-71%) 3
• Secondary staphylococcal pneumonia carries significantly higher mortality (47% vs 16% for non-staphylococcal) and higher incidence of lung abscess formation (14% vs 2%) 1
• Children with recent influenza-like illness are 12 times more likely to develop severe pneumococcal complications 4, 2

Hospital Discharge Criteria

Patients should remain hospitalized if they have ≥2 of the following unstable factors: 1

• Temperature >37.8°C
• Heart rate >100/min
• Respiratory rate >24/min
• Systolic blood pressure <90 mmHg
• Oxygen saturation <90%
• Inability to maintain oral intake
• Abnormal mental status