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Although there is a wide range of severity of influenza symptoms, patients typically complain of a severe, febrile, respiratory illness that begins abruptly. The onset is often abrupt (“like being hit by a train”), associated with severe myalgias (even their eyes hurt when they look around), diffuse pain (they may complain that their hair or skin hurts), respiratory symptoms (cough, rhinitis, pharyngitis), and high fever (occasionally as high as 40–41°C) that peaks within 12 hours. Patients may have rigors (frankly shaking chills) and headache (Figure 10-3).

Figure 10-3.

Timetable of symptoms and signs of influenza. (Reproduced with permission from Montalto NJ: An office-based approach to influenza: Clinical diagnoses and laboratory testing, Am Fam Physician. 2003 Jan 1;67(1):111–118.)


  1. Pathogenesis

    1. Antigenic change in the virus surface glycoprotein (hemagglutinin or neuraminidase) renders populations susceptible to the virus. Antigenic shifts are most common with influenza virus A and are associated with epidemics.

    2. Influenza infects respiratory epithelium.

    3. Adults are infectious from the day prior to the onset of symptoms until about 5–7 days later (10 days in children).

    4. The incubation period is 1–4 days.

  2. Epidemiology

    1. Results in 55,000–431,000 hospitalizations per year in the United States and 17,000–51,000 deaths

    2. Influenza typically occurs during the winter months (between December and March in the Northern Hemisphere versus April and September in the Southern Hemisphere).

    3. Influenza occurs throughout the year in the tropics.

      image Influenza is an unlikely diagnosis in the late spring, summer, or early fall.

    4. Current prevalence of influenza helps determine likelihood and is updated frequently by the CDC:

    5. Spread is primarily airborne (inhalation of virus-containing large droplets aerosolized during coughing and sneezing).

  3. Manifestations

    1. History

      1. Onset is sudden in 75% of cases.

      2. Fever

        1. Present in 51% of cases

        2. Peaks within 12–24 hours of onset of illness

        3. Typically, 38.0–40.0°C, occasionally 41.0°C

        4. Typical duration is 3 days but may last 1–5 days

      3. image High fever within 12–24 hours of symptom onset is typical of influenza but not other viral respiratory pathogens. Fever that increases over several days is not typical of influenza. When accompanied by cough, such a fever suggests bacterial pneumonia.

      4. Prevalence of other symptoms in influenza

        1. Headache, 58–81%

        2. Cough, 48–94%

        3. Sore throat, 46–70%

        4. Gastrointestinal symptoms are not characteristic of influenza.

      5. image Patients with significant diarrhea or vomiting should be evaluated for an alternative diagnosis.

      6. Symptoms help distinguish influenza from acute bronchitis or pneumonia (Table 10-3).

      7. Influenza may also present as a COPD or heart failure exacerbation (with or without fever) and severe febrile illnesses.

    2. Crackles are heard in < 25% of patients.

  4. Complications

    1. Pneumonia

      1. Influenza may cause pneumonia. This should be suspected in patients with dyspnea, tachypnea, hypoxia, abnormal lung findings or sepsis and is confirmed radiographically. Dyspnea is seen in 82% of influenza patients with pneumonia vs 17% without.

        image Obtain a chest film in patients with influenza and shortness of ...

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