InfluenzaVirus Infection
GENERAL PRINCIPLES
Influenza is an acute febrile respiratory illness, readily transmissible and associated with outbreaks of varying severity during the winter months.
DIAGNOSIS
Clinical Presentation
In immunocompetent patients, influenza virus infection causes an acute, self-limited febrile illness associated with headache, myalgias, cough, coryza, and malaise.
These symptoms may last up to 2 weeks.Diagnostic Testing
Diagnosis is usually made clinically during influenza season, with confirmation by nasopharyngeal swab for rapid antigen testing, PCR (higher sensitivity), or direct fluorescent antibody test and culture.
TREATMENT
• Treatment is usually symptomatic.
• Antiviral medications may shorten the duration of illness but must be initiated within 24-48 hours of the onset of symptoms to be effective in immunocompetent patients.18 Antiviral therapy should not be withheld from patients presenting >48 hours after symptom onset requiring hospitalization or at high risk for complications (see “Complications”).
î The neuraminidase inhibitors (oseltamivir 75 mg PO q12h or zanamivir 10 mg inhaled q12h, each for 5 days, or peramivir, 600 mg single dose IV) are approved by the US Food and Drug Administration for the treatment of influenza A and B.
o M2 inhibitors (amantadine and rimantadine, each 100 mg PO q12h) are not recommended due to high rates of resistance.
î Circulating strains change annually with varying resistance patterns to both classes of antivirals. Treatment decisions must be based on annual resistance data, available from the Centers for Disease Control and Prevention (CDC) (http://www.cdc.gov).
• Vaccination is the most reliable prevention strategy. Annual vaccination is recommended for all individuals 6 months of age and older. Efficacy of vaccination varies annually from 50% to 90% depending on prevailing outbreak and circulating influenza strains.
Complications
• Adults older than 65 years, residents of nursing homes and other long-term care facilities, pregnant women (and those up to 2 weeks postpartum), and patients with chronic medical conditions (e.g., pulmonary disease, cardiovascular disease, active malignancy, diabetes mellitus, chronic renal insufficiency, chronic liver disease, immunosuppression including HIV and transplantation, morbid obesity) are at greater risk of complications.
• Influenza pneumonia and secondary bacterial pneumonia, typically due to S. aureus, are the most common complications of influenza infection.
• Viral antigenic drift and shift can cause emergence of strains with enhanced virulence or the potential for pandemic spread, requiring modified therapy or heightened infection control measures.