Influenza

Introduction

Influenza is a viral illness associated with high mortality and high hospitalization rates among persons younger than age 65 years. Seasonal influenza epidemics result in 25 million to 50 million influenza cases, ~200,000 hospitalizations, and more than 30,000 deaths each year in the United States. Overall, more people die of influenza than of any other vaccine-preventable illness.

• The route of influenza transmission is person-to-person via inhalation of respiratory droplets, which can occur when an infected person coughs or sneezes. The incubation period for influenza ranges between 1 and 7 days, with an average incubation of 2 days. Adults are considered infectious from the day before their symptoms begin through 7 days after the onset of illness, whereas children can be infectious for longer than 10 days after the onset of illness. Viral shedding can persist for weeks to months in severely immunocompromised people.

Clinical Presentation

• The presentation of influenza is similar to a number of other respiratory illnesses.
• The clinical course and outcome are affected by age, immunocompetence, viral characteristics, smoking, comorbidities, pregnancy, and the degree of preexisting immunity.
• Complications of influenza may include exacerbation of underlying comorbidities, primary viral pneumonia, secondary bacterial pneumonia or other respiratory illnesses (eg, sinusitis, bronchitis, and otitis), encephalopathy, transverse myelitis, myositis, myocarditis, pericarditis, and Reye syndrome.

SIGNS AND SYMPTOMS

• Classic signs and symptoms of influenza include rapid onset of fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis.
• Nausea, vomiting, and otitis media are also commonly reported in children.
• Signs and symptoms typically resolve in 3 to 7 days, although cough and malaise may persist for more than 2 weeks.

LABORATORY TESTS

• The gold standard for diagnosis of influenza is viral culture.
• Rapid antigen and point-of-care tests, direct fluorescence antibody test, and the reverse transcription polymerase chain reaction assay may be used for rapid detection of virus.
• Chest radiograph should be obtained if pneumonia is suspected.
• Rapid tests have allowed for prompt diagnosis and initiation of antiviral therapy and decreased inappropriate use of antibiotics.

Treatment

• Goals of Therapy:

The four primary goals of therapy of influenza are as follows:

1) control symptoms,

2) prevent complications,

3) decrease work and/or school absenteeism,

4) prevent the spread of infection.

• Antiviral drugs are most effective if started within 48 hours of the onset of illness. Adjunct agents, such as acetaminophen for fever or an antihistamine for rhinitis, may be used concomitantly with the antiviral drugs.

POSTEXPOSURE PROPHYLAXIS

• Amantadine and rimantadine are currently not recommended for prophylaxis or treatment in the United States because of the rapid emergence of resistance.
• The neuraminidase inhibitors oseltamivir and zanamivir are effective prophylactic agents against influenza in terms of preventing laboratory-confirmed influenza when used for seasonal prophylaxis and preventing influenza illness among persons exposed to a household contact who were diagnosed with influenza.

PHARMACOLOGIC THERAPY: The neuraminidase inhibitors are the only antiviral drugs available for treatment and prophylaxis of influenza and are oseltamivir and zanamivir. IV peramivir is another NA inhibitor under investigation for treatment of influenza. The adamantanes (amantadine and rimantadine) are no longer recommended due to high resistance among influenza viruses.