Influenza A: Symptoms, Contagious Period, and Treatment
Symptoms
Influenza A presents with abrupt onset of fever (>38°C), accompanied by respiratory symptoms (cough, nasal congestion, sore throat) and systemic symptoms (myalgia, chills/sweats, malaise, fatigue, headache). 1, 2
- The hallmark of infection is the sudden onset of fever, cough, chills or sweats, myalgias, and malaise 2
- In children, fever may be defined as higher than 37.8°C, while in adults it is typically >38°C 1
- Gastrointestinal symptoms such as diarrhea, vomiting, and abdominal pain can occur, particularly in children 1
- Elderly patients may present with new or worsening respiratory symptoms without prominent fever 1
- Infants and young children may present with fever alone without other obvious symptoms 1
Contagious Period
Patients with influenza are contagious from approximately 1 day before symptom onset through 5-7 days after illness begins, with immunocompromised patients potentially shedding virus for prolonged periods. 1
- Prolonged viral shedding has been documented in solid organ transplant recipients and other immunocompromised patients 1
- The contagious period may extend beyond 7-10 days in severely immunosuppressed individuals 1
- Transmission occurs primarily through respiratory droplets and contact with contaminated surfaces 2
Treatment
Antiviral Therapy
Oseltamivir 75 mg orally twice daily for 5 days is the first-line treatment for Influenza A and should be initiated as soon as possible, ideally within 48 hours of symptom onset. 3, 4
Standard Dosing:
- Adults and children >24 kg: 75 mg twice daily for 5 days 1, 4, 5
- Children 15-23 kg: 45 mg twice daily for 5 days 1, 4
- Children ≤15 kg: 30 mg twice daily for 5 days 1, 4
- Renal impairment (CrCl <30 mL/min): Reduce dose by 50% to 75 mg once daily 1, 6
Who Should Receive Treatment:
All hospitalized patients with confirmed or suspected influenza should receive immediate antiviral treatment regardless of illness duration or time since symptom onset. 3, 4
High-risk outpatients should be treated even if presenting beyond 48 hours of symptom onset, as mortality benefit may still occur. 4
High-risk groups requiring treatment include: 1, 3, 4
- Children <2 years of age
- Adults ≥65 years
- Pregnant and postpartum women
- Immunocompromised patients (including transplant recipients)
- Patients with chronic cardiac or pulmonary disease
- Patients with chronic metabolic, renal, or hepatic disease
- Patients on long-term aspirin therapy
- Nursing home residents
Clinical Benefits:
- Reduces illness duration by approximately 24 hours (1-1.5 days) 3, 4, 2
- Decreases illness severity by up to 38% 4
- May reduce hospitalization rates and need for subsequent antibiotics 3, 4
- Greatest benefit occurs when treatment is initiated within 12-36 hours of symptom onset 4
Alternative Antivirals:
- Zanamivir (inhaled): 10 mg (two 5-mg inhalations) twice daily for 5 days, approved for ages ≥7 years 4
- Peramivir (IV): Consider for severely ill patients with concerns about oral absorption 4
- Adamantanes (amantadine, rimantadine): NOT recommended due to high resistance rates among current Influenza A strains 4, 5
Extended Treatment Duration:
- Longer courses (>5 days) may be needed for immunocompromised patients, including solid organ transplant recipients 1
- Some experts recommend continuing therapy until viral replication has ceased (documented by negative PCR) in transplant patients 1
- Consider extended treatment for patients with persistent fever after 6 days or critically ill patients 4
Management of Complications
Previously healthy adults with acute bronchitis complicating influenza do NOT routinely require antibiotics in the absence of pneumonia. 1, 6
When to Add Antibiotics:
Consider antibiotics only if patients develop worsening symptoms such as recrudescent fever or increasing dyspnea, or if bacterial pneumonia is suspected. 1, 6
For non-severe influenza-related pneumonia: 1, 3
- Oral co-amoxiclav or tetracycline (e.g., doxycycline) is preferred
- Macrolides (clarithromycin or erythromycin) or fluoroquinolones (levofloxacin, moxifloxacin) are alternatives for penicillin-allergic patients
- Antibiotics should be administered within 4 hours of admission 1
For severe influenza-related pneumonia: 1
- IV combination therapy with broad-spectrum β-lactam (co-amoxiclav or 2nd/3rd generation cephalosporin) plus macrolide
- Immediate parenteral antibiotic administration after diagnosis
Supportive Care
Monitor temperature, respiratory rate, pulse, blood pressure, oxygen saturation, and mental status at least twice daily, more frequently in severe illness. 1
- Provide supplemental oxygen to maintain saturation >90% 1
- Assess for volume depletion and provide IV fluids as needed 1
- Nutritional support should be given in severe or prolonged illness 1
- Fever-reducing medications are allowed 5
Common Pitfalls and Caveats
Taking oseltamivir with food reduces gastrointestinal side effects, which occur in approximately 10-15% of patients. 4
- Do NOT delay antiviral treatment while awaiting laboratory confirmation in high-risk patients 4
- Patients unable to mount adequate febrile response (immunocompromised, very elderly) may still require treatment despite lack of documented fever 1
- In immunocompromised patients, consider higher doses (double the approved dose) as some experts recommend this approach 1
- Monitor for antiviral resistance if viral replication persists beyond 7-10 days despite therapy 1
- Neuropsychiatric events have not been definitively linked to oseltamivir 4
Discharge Criteria
Patients should be reviewed 24 hours prior to discharge and should NOT be discharged if they have two or more of the following unstable clinical 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
Prevention
Annual influenza vaccination remains the primary preventive measure. 3