Influenza

Background

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

Clinical Features

• Constitutional
  • Fever
  • Headache
  • Myalgias
  • Malaise
• Respiratory
  • Non-productive cough
  • Sore throat
  • Rhinorrhea

Diagnosis

• 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)

Clinical Predictors

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

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

Treatment

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 is symptoms <48 hours

Risk Factors

• age <2 years or >65 years
• pregnancy
• 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

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)

Medications

• Antiviral agents (neuraminidase inhibitors)
  • Despite questions on efficacy and safety, CDC still recommends treatment for all hospitalized patients and outpatients at risk for complications
  • Oseltamivir 75mg PO BID x5d
    • 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
  • Zanamivir 10mg (2 inhalations) BID x5d
    • Relatively contraindicated in pts w/ asthma, COPD, or pregnancy
    • Shorten duration of illness by 14.4 hrs with no reduction in flu-related complications

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

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

See Also

• H1N1 (Swine) Flu

Source

UpToDate