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Stat Consult

Influenza Management

Brian S. Alper, MD, MSPH, and Larissa Lucas, MD

October 07, 2009

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Artist's rendering of a flu virus as seen through an electron microscope

ICD-9 codes

• 487.0 influenza with pneumonia

• 487.1 influenza with other respiratory manifestations

• 487.8 influenza with other manifestations

• 488.0 influenza due to identified avian influenza virus

• 488.1 influenza due to identified pandemic (H1N1) 2009

Risk factors for complications

• Age <5 years or >65 years

• Pregnancy

• Chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, cognitive, neurologic/neuromuscular, hematologic, or metabolic disorders (including diabetes mellitus)

• Immunosuppression, including medication-induced or due to HIV infection

• Residents of nursing homes and other long-term care facilities

• Persons <19 years old on long-term aspirin

Diagnosis

• Diagnosis made clinically with confirmatory testing in selected cases

• Clinical diagnosis

— If influenza already documented in community or geographic area, clinical diagnosis can be made based on signs and symptoms.
— Signs and symptoms of influenza can vary by age and underlying medical conditions.
— Not all patients with influenza virus infection will have typical symptoms and signs.

• Diagnostic testing should be considered if

— No recent cases of influenza locally
— Results would change management, such as deciding whether to use influenza antiviral agents
— Institutional outbreak of influenza suspected (to allow prompt implementation of control measures)

• Rapid antigen detection tests

— Nasopharyngeal and nasal specimens have higher yields than throat swab specimens for rapid detection.
— Rapid antigen detection tests have moderate sensitivity and high specificity for influenza.
— Positive predictive value of rapid influenza tests varies with influenza prevalence.

• Chest x-ray if signs or symptoms of pneumonia

Antiviral treatment

• Antivirals include neuraminidase inhibitors (oseltamivir [Tamiflu], zanamivir [Relenza]), and adamantanes (amantadine [Symmetrel], rimantadine [Flumadine])

• May be indicated if influenza and symptom duration <48 hours

• Priority groups for treatment

— Hospital patients (even if >48 hours after symptom onset)
— Patients at higher risk for influenza complications
— Patients with lower respiratory tract symptoms

• Treatment should not wait for laboratory confirmation.

• For patients not hospitalized or in high-risk group, antivirals are generally not needed.

• Drug selection may vary based on antiviral resistance patterns.

— Pandemic (H1N1) 2009 mostly resistant to adamantanes, sensitive to neuraminidase inhibitors
— Seasonal influenza A (H1N1) mostly resistant to oseltamivir and mostly sensitive to zanamivir and adamantanes
— Seasonal influenza A (H3N2) and influenza B resistant to adamantanes, sensitive to neuraminidase inhibitors

• Specific drug recommendations may vary with local influenza activity; if co-circulating influenza A subtypes or viruses with varied antiviral susceptibilities, use zanamivir, or oseltamivir plus adamantane.

• Dosing in adults (treatment duration five days)

— Oseltamivir 75 mg orally twice daily
— Zanamivir 10 mg (two inhalations) twice daily; not recommended in patients with airways disease
— Amantadine 200 mg orally once daily or 100 mg twice daily
— Rimantadine 100 mg orally twice daily
— Dose may be reduced if renal impairment (for oseltamivir or adamantanes), age >65 years (for adamantanes) or adverse effects (for adamantanes).

• Efficacy of neuraminidase inhibitors in otherwise healthy adults with influenza (but results may vary with antiviral resistance patterns)

— Modest reduction in duration of influenza (by one-half to one day)
— Reduction in rates of secondary complications
— Possible reduction in antibiotic use and hospitalizations

• Antiviral agents may be associated with reduced mortality in adults hospitalized for influenza.

• Neuraminidase inhibitors have been associated with neuropsychiatric symptoms, including self-injury and delirium.

• Also consider antiviral prophylaxis for high-risk close contacts.

— Dosing similar to treatment except once daily dosing for neuraminidase inhibitors.

— Postexposure chemoprophylaxis recommended for high-risk persons with close contact during infectious period (one day before symptoms until 24 hours after fever ends)