Influenza

The US Centers for Disease Control and Prevention (CDC) estimates that seasonal influenza is responsible for an average of more than 20,000 deaths annually. Mortality is highest in infants and the elderly. According to a CDC analysis, the 2015-2016 US influenza season was milder than the past three seasons and peaked later than usual. Influenza A(H1N1)pdm09 was the predominant circulating virus (~60%), followed by influenza A(H3N2), influenza B/Yamagata lineage, and influenza B/Victoria lineage.

Signs and symptoms

The presentation of influenza virus infection varies, but it usually includes many of the following signs and symptoms:

• – Fever
• – Sore throat
• – Myalgias
• – Frontal or retro-orbital headache
• – Nasal discharge
• – Weakness and severe fatigue
• – Cough and other respiratory symptoms
• – Tachycardia
• – Red, watery eyes

The incubation period of influenza is 2 days long on average but may range from 1 to 4 days in length. Aerosol transmission may occur 1 day before the onset of symptoms ; thus, it may be possible for transmission to occur via asymptomatic persons or persons with subclinical disease, who may be unaware that they have been exposed to the disease.

See Presentation for more detail.

Diagnosis

Influenza has traditionally been diagnosed on the basis of clinical criteria, but rapid diagnostic tests, which have a high degree of specificity but only moderate sensitivity, are becoming more widely used. The criterion standard for diagnosing influenza A and B is a viral culture of nasopharyngeal samples or throat samples. In elderly or high-risk patients with pulmonary symptoms, chest radiography should be performed to exclude pneumonia.

Avian influenza

Avian influenza (H5N1) is rare in humans in developed countries (see the image below). Unless advised by the CDC or regional health departments, clinicians do not routinely need to test for avian influenza.