Infectious Disease Book

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Influenza

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  1. See Also
    1. Upper Respiratory Infection
    2. Avian Influenza
  2. Epidemiology
    1. Annual Periodicity
      1. Temperate Climate
        1. Onset as early as October
        2. Peaks in late December to March
      2. Tropical Climate: Occurs year round
    2. Transmission
      1. Small-large particle aerosol from cough and sneeze
    3. Course
      1. Incubation: 2-3 days (may be as long as 7 days)
      2. Infectivity (Viral shedding)
        1. Begins within 1 day of symptom onset
        2. Peaks with illness severity
        3. Declines over 4-5 days
      3. Acute symptoms resolve in 4-5 days
      4. Persistent symptoms may not clear for 3 or more weeks
        1. Fatigue or malaise
        2. Persistent non-productive cough
    4. Attack rate:
      1. Epidemics (antigenic drift): 20-30%
      2. Pandemics (antigenic shift): 50%
    5. Ages affected
      1. Children
        1. Highest attack rate
      2. Elderly (over age 65 years)
        1. Lowest attack rate
        2. Highest risk of complication
          1. Relative risk of hospitalization: 5-10
          2. Relative risk of mortality: 5
        3. Highest mortality (80% of deaths are in elderly)
  3. Pathophysiology
    1. Vaccines directed at critical viral surface antigens
      1. Hemagglutinin
      2. Neuraminidase
    2. Antigenic drift
      1. Minor genetic mutations result in epidemics
      2. Influenza A most commonly involved
    3. Antigenic shift
      1. Major genetic changes result in pandemic
      2. Major Pandemics
        1. 1918: "Spanish flu" 20 Million deaths worldwide
        2. 1957: Asian Flu
        3. 1968: Hong Kong flu 34,000 deaths
      3. Recent Antigenic Shifts
        1. 1976: Swine flu isolated
        2. 1997: Hong Kong H5N1 (avian) influenza
  4. Types
    1. Influenza A
      1. Major outbreaks result from antigenic shifts
      2. See Avian Influenza
      3. Re-assortment of genomic expression
        1. Neuraminidase and Hemagglutinin
    2. Influenza B
      1. Less variation than Influenza A
      2. Outbreaks in Schools and Military camps
    3. Influenza C
  5. Symptoms
    1. Abrupt illness onset
    2. Viral prodrome
      1. High fever to 104 F (fever lasts 4-5 days)
      2. Chills
      3. Severe myalgias (lasts for first 3 days)
      4. Severe Headache (most severe in first 2 days)
    3. Eye
      1. Photophobia
      2. Red, Burning eyes
    4. Nose
      1. Coryza or profuse Nasal discharge (lasts 6-7 days)
        1. Often onset with fever and no other symptoms
      2. Rhinitis
      3. Nasal congestion or "stuffiness"
    5. Throat
      1. Sore Throat or dry throat (lasts for first 3 days)
    6. Chest
      1. Severe dry cough (lasts for first 3 days)
      2. Chest discomfort
    7. Other Constitutional symptoms
      1. Anorexia (may persist for first week)
      2. Fatigue persists weeks
      3. Severe Malaise (may persist for more than a week)
    8. Less common symptoms (20-40%)
      1. Nausea or Vomiting
      2. Dizziness
  6. Signs
    1. Fever up to 104 F (40 C)
    2. Non-exudative Pharyngitis
    3. Muscle tenderness
    4. Less Common Influenza signs
      1. Conjunctivitis
      2. Cervical adenopathy
  7. Diagnosis
    1. Findings most suggestive of Influenza
      1. Rigors
      2. Fever
      3. Sweating
      4. Onset of symptoms within 3 days of office visit
    2. Findings most suggestive of other diagnosis
      1. Systemic symptoms absent
      2. Cough absent
      3. Not confined to bed
      4. Able to perform daily activities without difficulty
    3. References
      1. Ebell (2004) J Am Board Fam Pract 17:1
  8. Differential Diagnosis
    1. Common Cold viruses
      1. Respiratory Syncytial Virus (RSV)
      2. Parainfluenza
      3. Adenovirus
    2. Factors suggesting Influenza
      1. High fever to 104 F with chills
      2. Severe malaise, Fatigue, and anorexia
      3. Severe myalgias
      4. Moderate to severe Headache
    3. Factors suggesting Common Cold
      1. Gradual onset of more mild symptoms
      2. Upper respiratory symptoms predominate
  9. Complications
    1. Primary Influenza Pneumonia (1% of adults)
      1. Increased risk with cardiac disease (Mitral Stenosis)
      2. Occurs 1 week after influenza symptom onset
      3. Occasionally fatal even in young adults
    2. Bacterial tracheobronchitis (occurs in 30% of adults)
      1. Increased risk in Tobacco smoking
    3. Acute Sinusitis (5-10%)
    4. Secondary Bacterial Pneumonia
      1. Occurs one week after influenza symptom onset
      2. Etiologies
        1. Streptococcal Pneumonia
        2. Staphylococcal Pneumonia
        3. Haemophilus Influenzae
      3. Risk factors
        1. Older than 65 years old
        2. Chronic renal disease
        3. Diabetes Mellitus and other endocrine disease
        4. Hematologic disease or Immunodeficiency
        5. Cardiopulmonary disease
    5. Rare Neurologic Complications
      1. Meningoencephalitis
      2. Transverse myelitis
      3. Reye's Syndrome
      4. Guillain-Barre Syndrome
      5. Myositis or Rhabdomyolysis
    6. Other rare complications
      1. Myoglobinuric Renal Failure
      2. Myocarditis
      3. Pericarditis
      4. Glomerulonephritis
      5. Parotitis
  10. Labs
    1. Complete Blood Count
      1. Leukopenia or slight Leukocytosis (up to 15,000)
      2. Relative Lymphopenia
    2. Virus Isolation (48-72 hours required for isolation)
      1. Nasopharyngeal swab
      2. Throat swab
      3. Sputum
    3. Rapid Influenza Test (Influenza Immunoassay)
      1. Sample site varies between products
    4. Serology (four fold rise over 10-14 days)
      1. Hemagglutination inhibition
      2. Complement fixation titers
  11. Management
    1. Symptomatic treatment
      1. Acetaminophen
      2. Pharyngitis Symptomatic Treatment
      3. Cough Symptomatic Treatment
      4. Consider antiviral agent below if ill <48 hours
        1. Shorten course of illness (~1 day)
        2. No evidence that antivirals prevent complications
    2. Influenza A
      1. See Amantadine or Rimantadine
      2. Course: 3-5 days or 48 hours after symptoms resolve
      3. CDC recommends Amantadine/Rimantadine for prevention
        1. Use Neuraminidase inhibitors for treatment
        2. See prevention below
        3. Prevents Amantadine resistance
    3. Influenza A or B: Neuraminidase inhibitors
      1. See Zanamivir (Relenza)
      2. See Oseltamivir (Tamiflu)
    4. Avoid Salicylates in patients younger than 16 years
      1. Risk of Reye's Syndrome
  12. Prevention
    1. Influenza Vaccine yearly
      1. Immunize all high risk groups
        1. See Influenza Vaccine for indications
        2. Nursing home residents and staff
        3. Comorbid illness
        4. Pregnant women after first trimester
      2. Efficacy (depends on vaccine components)
        1. Healthy younger patients: 70-90%
        2. Elderly: 30-40%
    2. FluMist
      1. Live virus intranasal vaccine
      2. May be used in healthy patients aged 5 to 49 years
    3. Postexposure prophylaxis in high risk groups
      1. Start within 48 hours of exposure
      2. Amantadine Or Rimantadine prophylaxis (Influenza A)
      3. See Zanamivir (Relenza)
      4. See Oseltamivir (Tamiflu)
    4. Other measures
      1. Respiratory isolate hospitalized influenza patients
      2. Isolate Nursing home residents with influenza
      3. Isolate Nursing home residents on prophylaxis
        1. Risk of virus shedding
  13. Pandemic Preparedness
    1. Federal, State and Local Planning
    2. Influenza Surveillance via WHO worldwide (CDC in US)
      1. Local Vital Statistics offices report deaths weekly
    3. Maximize Vaccine development and delivery
    4. Develop limited antiviral (Amantadine) indications
    5. Emergency medical, hospital and backup preparedness
    6. Ensure communication networks are in place
      1. Internet, Health Alert Network, Telephone
  14. Resources
    1. Is it the cold or the flu
      1. http://www.naid.nih.gov/publications/cold/sick.htm
    2. CDC Influenza Surveillance
      1. http://www.cdc.gov/ncidod/diseases/flu/weekly.htm
    3. CDC Influenza Information
      1. http://www.cdc.gov/ncidod/diseases/flu
    4. American Lung Association Influenza Information
      1. http://www.lungusa.org/diseases/luninfluenz.html
    5. National Foundation Infectious Disease on Influenza
      1. http://www.nfid.org/factsheets/inflfacts.html
  15. References
    1. (1999) Preparing Next Influenza Pandemic Teleconf, CDC
    2. Hayden (2000) N Engl J Med 343:1282
    3. Welliver (2001) JAMA 285:748

Influenza (C0021400)

Definition (CSP)acute viral infection involving the respiratory tract; marked by inflammation of the nasal mucosa, the pharynx, and conjunctiva, and by headache and severe, often generalized, myalgia.
Definition (NCI)An acute viral infection of the respiratory tract, occurring in isolated cases, in epidemics, or in pandemics; it is caused by serologically different strains of viruses (influenzaviruses) designated A, B, and C, has a 3-day incubation period, and usually lasts for 3 to 10 days. It is marked by inflammation of the nasal mucosa, pharynx, and conjunctiva; headache; myalgia; often fever, chills, and prostration; and occasionally involvement of the myocardium or central nervous system.
Definition (MSH)An acute viral infection in humans involving the respiratory tract. It is marked by inflammation of the NASAL MUCOSA; the PHARYNX; and conjunctiva, and by headache and severe, often generalized, myalgia.
ConceptsDisease or Syndrome (T047)
ICD9487, 487.1
EnglishFlu, FLU SYNDROME, GRIP, Grippe, Human Flu, Human Influenza, Human Influenzas, Influenza, Influenza in Human, Influenza in Humans, Influenzas, SYNDROME FLU
Spanishgripe, influenza
CreditsDerived from the NIH UMLS (Unified Medical Language System)



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