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
  • Myalgia
  • Malaise
• Respiratory
  • Non-productive cough
  • Sore throat
  • Rhinorrhea

Diagnosis

• During outbreaks clinical criteria alone is sufficient for otherwise healthy pts
• Rapid tests
  • Specific but not sensitive (cannot be used to rule-out)
  • Do not wait for results before initiating treatment
  • Consider in:
    • Inpatients w/ acute febrile respiratory illness
    • All immunocompromised pts

Treatment

• Antiviral therapy
  • Efficacy
    • Shortens symptoms by approximately 1 day in those recieving neuraminidase inhibitor in <48 hours
      • Little to no benefit when tx is started 2d or more after symptom onset
      • Do not wait for test results (if obtained) before beginning treatment
    • Controversial whether tx significantly prevents influenza-associated complications
  • Indications:
    • Illness requiring hospitalization
    • Severe, complicated, or progressive illness
    • High risk for complications:
      • Age <2yr OR age >65yr
      • Chronic illnesses:
        • Pulmonary (including asthma), CV (except HTN), renal, hepatic, hematological, DM
      • Immunosuppression (meds and HIV)
      • Pregnant or within 2wk after delivery
      • Morbidly obese (BMI >40)
      • Pts age <19yr who are receiving long-term aspirin therapy
      • American Indians/Alaska Natives
      • Residents of nursing homes / chronic care facilities
    • Can consider in pts w/ mild illness who are otherwise healthy to reduce length of sx
  • Meds
    • Neuraminidase inhibitors
      • Oseltamivir 75mg PO BID x5d
      • Zanamivir 10mg (2 inhalations) BID x5d
        • Relatively contraindicated in pts w/ asthma, COPD, or pregnancy
    • Adamantanes (amantadine, rimantadine) NOT recommended (high resistance rates)
• Antibiotics
  • Only recommended if secondary bacterial pneumonia

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

Source

UpToDate