Influenza

Background

  • A history of influenza vaccination does not rule out influenza in an ill patient with clinical signs and symptoms compatible with influenza. Therefore, vaccination status should not impede the initiation of prompt antiviral treatment.

Transmission

  • Occurs in 6ft radius around infected pt who is sneezing and/or coughing
  • Viral shedding lasts ~5d (starts 24-48hr before onset of symptoms)
    • Longer duration of shedding occurs in children, elderly, pts w/ chronic illnesses
    • Shedding from asymptomatic individuals doesn't contribute significantly to transmission

Risk Factors (for complicated course)

  • Age <2 years or >65 years
  • Pregnancy through 2 weeks after delivery
  • Chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, hematological (including sickle cell disease), neurologic, neuromuscular, or metabolic disorders (including diabetes mellitus)
  • Immunosuppression, including that caused by medications or by HIV
  • Persons younger than 19 years of age who are receiving long-term aspirin therapy
  • American Indians and Alaska Natives
  • Morbid obesity
  • Patients in nursing homes or chronic-care facilities

Clinical Features

Cough and Fever

Odds that acute cough and fever are due to flu during flu season:[1]

  • Adolescents ≥ 12 yo - 79-88%
  • Children 5-12 yo - 71-83%
  • Children < 5 yo - 64%
  • Adults - unreliable predictors especially when older than 60 yo

Convalescence

  • Most gradually improve over 2-5d, although may last for one week or more
  • Some have persistent weakness lasting several weeks (postinfluenza asthenia)

Differential Diagnosis

Fever

Infectious

Non-infectious

Diagnosis

Workup

  • Influenza PCR preferred for inpatients (sensitivity >95%)
  • RSV/Flu/metapneumovirus test low sensitivity for adults (48-60%) and children (62-72%), with turnaround time <24 hours
  • The Viral Respiratory Panel (influenza, RSV, adenovirus, parainfluenzavirus) discouraged (sensitive 70-90%) with 3-5 days turnaround
  • Rapid tests specific but not sensitive (cannot be used to rule-out)

Evaluation

Outpatients

  • Risk factors:
    • Yes
      • Do NOT send Point of Care influenza test
      • Do NOT send diagnostic test for influenza
      • Empirically treat for influenza using antivirals if symptoms for <48 hours
    • No
      • Do NOT send Point of Care influenza test.
      • Do NOT send diagnostic test for influenza.
      • May consider treating with anti-influenza antivirals if symptoms <48 hours

Admitted Patients

  • Do not send Point of Care influenza test
  • Send diagnostic test for influenza
    • Influenza PCR preferred for inpatients (see Diagnosis section)
  • Empirically treat for influenza using antivirals
    • Most effective when administered when symptoms of influenza have occurred for < 48 hours
    • May be benefit when initiated in severely ill inpatients with 48 hours to 5 days of symptoms
    • No evidence of benefit after 5 days of symptoms
  • Treat empirically promptly with oseltamavir unless there is an alternative diagnosis
  • Droplet precautions (see below)

Management

Despite questions on efficacy and safety, CDC still recommends treatment for all hospitalized patients and outpatients at risk for complications[2]

Oseltamivir (Tamiflu)

Shorten duration of illness by 16.8 hrs while NNTH (number needed to harm) was 28 in regards to causing n/v, HA, and renal and psych syndromes[3]

  • Greatest benefit if within 48hrs of symptom onset[4]
    • However, may be beneficial up to 4-5 days, including in pregnant patients
    • Early treatment of hospitalized pts can reduce death

Dosing

  • Age <1 year: 3mg/kg PO BID x 5 days
  • <15kg: 30mg PO BID x 5 days
  • 15-23kg: 45mg PO BID x 5 days
  • 24-40kg: 60mg PO BID x5d
  • Adult: 75mg PO BID x 5 days

Zanamivir (Relenza)

Relatively contraindicated in patients with asthma, COPD, or pregnancy. Shorten duration of illness by 14.4 hrs with no reduction in flu-related complications[5]

Dosing

  • Age >7yo: 10mg (2 inhalations) BID x 5d
  • Prophylaxis: 10mg (2 inhalations) once daily x 7 days
    • Not for age < 5yo
Antiviral Agent Recommended For Not Recommended With Adverse Events
Oseltamivir (Tamiflu®)
  • Treatment: any age
  • Prophylaxis: >3 months
 N/A
  • Adverse events: nausea, vomiting. Sporadic, transient neuropsychiatric events (self-injury or delirium) mainly reported among Japanese adolescents and adults.
Zanamivir (Relenza®)
  • Treatment: >7 yrs
  • Prophylaxis >5 years
 Underlying respiratory disease (e.g., asthma, COPD)
  • Allergic reactions: oropharyngeal or facial edema.
  • Adverse events: diarrhea, nausea, sinusitis, nasal signs and symptoms, bronchitis, cough, headache, dizziness, and ear, nose and throat infections.

Isolation Precautions

  • Droplet precautions
    • If the patient is in an area in which they are in contact with other patients or need to be transported and thus may come in close contact (<3 feet) with staff, visitors, or other patients, the patient needs to wear a surgical mask (or N-95 respirator, if not available).

Complications

  • Pneumonia
    • Primary influenza PNA
      • Most severe and least common type of PNA
      • Rare in otherwise healthy adults
      • Consider in pts w/ persistent and worsening symptoms (esp high fever, SOB, cyanosis)
      • CXR shows b/l opacities w/ or w/o superimposed consolidation
    • Secondary bacterial PNA
      • Exacerbation of fever and respiratory symptoms after initial improvement
        • Higher fever, productive cough, radiographic evidence of infiltrates
      • Microbiology
  • Otitis Media
    • More common in children
  • Myositis and rhabdo
    • More common in children
    • Extreme tenderness of affected muscles (most commonly in the legs)
  • Pericarditis/myocarditis
    • Rare complication

See Also

References

  1. ↑ CDC Clinical Flu
  2. ↑ Fiore AE, et al. Antiviral Agents for the Treatment and Chemoprophylaxis of Influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP). CDC. Recommendations and Reports. January 21, 2011. 60(RR01);1-24.
  3. ↑ Jefferson T, et al. Oseltamivir for influenza in adults and children: systematic review of clinical study reports and summary of regulatory comments. BMJ. 2014; 348:g2545.
  4. ↑ CDC Guidelines. 2015-2016. http://www.cdc.gov/flu/professionals/antivirals/summary-clinicians.htm
  5. ↑ Heneghan CJ, et al. Zanamivir for influenza in adults and children: systematic review of clinical study reports. BMJ. 2014; 348:g2547.
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