Infectious Diseases I

Institutions should begin implementing influenza screening and infection control measures when

influenza viruses are confirmed to be in the local community.

The CDC recommends the implementation of droplet precautions for hospitalized patients with

suspected or confirmed influenza; further, these precautions are recommended for 7 days after illness

onset or until 24 hours after the resolution of fever and respiratory symptoms, whichever is longer.

Patients with influenza may present with fever, myalgias, headache, malaise, dry cough, pharyngitis, and

rhinorrhea. Fever and myalgias generally last 3–5 days, whereas malaise and respiratory symptoms may

last 2 weeks or more. Patients with severe influenza may present with hypoxemic respiratory failure and

sepsis.

sampling of the upper respiratory tract using nasal washing or nasopharyngeal swab or lower respiratory

tract within 5 days of illness is preferred. Diagnostic tests obtained beyond 5 days may have false-

negative results.

Diagnostic tests available for influenza include viral culture, serology, rapid antigen testing, polymerase

chain reaction (PCR), and immunofluorescence assays.

Viral culture and reverse transcription polymerase chain reaction (RT-PCR) are the most sensitive

and are the only tests able to identify individual influenza subtypes. These tests can be performed

using nasopharyngeal swab or respiratory tract culture (e.g., sputum, BAL).

Rapid diagnostic tests of nasopharyngeal swab or nasal wash have high specificity (90%–95%)

(i.e., low false-positive rate and rapid turnaround), promoting these tests as first line for general

diagnosis. Depending on the rapid diagnostic assay used, differentiation between influenza A and

influenza B is possible, although further subtype identification is not available with these tests.

Because of poor sensitivity, negative rapid diagnostic tests should be followed up with viral culture

or RT-PCR.

Case definitions for influenza surveillance are suspected, probable, or confirmed based on patient

presentation and laboratory assessment.

Suspected – Mild to severe influenzalike illness within reasonable seasonal threshold.

Probable – Mild to severe influenzalike illness within reasonable seasonal threshold or after recent

contact with person with probable or confirmed influenza infection and not otherwise explained.

Confirmed – Mild to severe influenzalike illness with confirmatory laboratory tests indicating

presence of influenza A or B.

Management of critically ill patients with influenza includes treatment of primary influenza infection,

secondary bacterial infection(s), and related noninfectious complications (e.g., respiratory failure,

ARDS, prolonged mechanical ventilation, multiple organ failure). Management of common bacterial

infections and noninfectious complications is described in other chapters.

Diagnostic tests for influenza should be obtained and empiric antiviral treatment of influenza

initiated during seasonal outbreaks in critically ill patients presenting with acute febrile and

respiratory illness consistent with influenza. Severely ill patients with influenza may present with

hypothermia similar to other populations with sepsis. Accompanying severe hypoxemic respiratory

failure should heighten the concern for influenza.

Antiviral treatment (Table 4) should be initiated within 48 hours from onset of symptoms.

Hospitalized patients receiving antiviral therapy after 48 hours from symptom onset may benefit.

The CDC recommends that treatment be initiated even 5 days after symptoms in critically ill patients.