Infectious Disease Book















Aka: Influenza, Influenza Virus
  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. Attack rate:
      1. Epidemics (Antigenic drift): 20-30%
      2. Pandemics (Antigenic shift): 50%
    3. 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)
    4. Incidence (worldwide)
      1. Severe Influenza: 3 to 5 million people annually
      2. Influenza-related deaths: Up to 500,000 annually
  3. Course
    1. Incubation: 2-3 days (may be as long as 7 days)
    2. Infectivity (Viral load and shedding correlates with symptom severity)
      1. Begins 1 day prior to symptom onset
      2. Peaks with illness severity
      3. Declines over 4-5 days
      4. Ceases with fever resolution
      5. Absent after 10 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. Pathophysiology
    1. Classification
      1. Single-stranded DNA virus in the Orthomyxovirus Family
    2. Transmission
      1. Small-large particle aerosol from cough and sneeze
      2. Receptors are primarily in nose (and to lesser extent in the lung)
    3. Vaccines directed at critical viral surface Antigens
      1. Surface Proteins typically change over successive Influenza generations, rendering prior Vaccines ineffective
        1. See Antigenic drift and Antigenic shift below
      2. Antigens (Influenza surface Proteins)
        1. Hemagglutinin
        2. Neuraminidase
    4. Influenza A hosted in multiple species
      1. Migratory birds (main host)
        1. Typically carry Influenza asymptomatically
        2. Transmit Influenza to other species (especially pigs)
        3. Direct transmission of Avian Influenza to humans is uncommon
          1. Exception: H5N1 Avian flu is contracted by humans in sustained close contact with birds
      2. Horse
      3. Pigs
        1. Key to transmission to humans
        2. Receptors for both human and Avian Influenza
        3. Co-infection with avian and human Influenza can allow exchange of segmented genome components
          1. Allows for Antigenic shift in human Influenza
          2. Swine flu (e.g. H3N2) is then transmitted to humans and can lead to pandemic
    5. Antigenic drift
      1. Minor genetic mutations result in epidemics
      2. Influenza A most commonly involved
    6. Antigenic shift
      1. Major genetic changes (surface Protein changes) result in pandemic
        1. Typically results from co-infection in pigs (see above)
      2. Major Pandemics
        1. 1918: "Spanish flu" (H1N1) 50 Million deaths worldwide (500,000 in United States)
          1. Young, previously healthy adults were more likely to succumb in this pandemic (likely ARDS related)
        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
        3. 2009: H1N1 Novel Influenza
          1. Reported April 12, 2009 in Veracruz, Mexico and WHO declared pandemic by April 27, 2009
          2. Chimera of swine flu, avian flu, and human flu
          3. (2009) N Engl J Med 361:674-9 [PubMed]
  5. 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. Less virulent than Influenza A in most cases (although children have a higher rate of complications)
    3. Influenza C
  6. Symptoms
    1. Abrupt illness onset
    2. Viral prodrome (Cytokine response leads to primary symptoms)
      1. High fever to 104 F (fever lasts 4-5 days)
      2. Severe myalgias (lasts for first 3 days)
      3. Severe Headache (most severe in first 2 days)
      4. Chills
    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. Gastrointestinal Symptoms (present in 30% of children, uncommon in adults)
      1. Nausea or Vomiting
      2. Abdominal Pain
    8. Other Constitutional symptoms
      1. Anorexia (may persist for first week)
      2. Fatigue persists weeks
      3. Severe Malaise (may persist for more than a week)
      4. Dizziness
  7. 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
  8. Diagnosis
    1. Findings most suggestive of Influenza
      1. Sudden onset of classic Influenza symptoms
      2. High fever to 104 F with chills, sweats, rigors
      3. Severe malaise, Fatigue, and Anorexia
      4. Severe myalgias
      5. Moderate to severe Headache
      6. Onset of symptoms within 3 days of office visit
    2. Classic triad (Test Sensitivity 80-85% in adults, 60% in children; Test Specificity >75% in adults)
      1. Fever
      2. Cough
      3. Pharyngitis
    3. 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
    4. References
      1. Ebell (2004) J Am Board Fam Pract 17:1-5 [PubMed]
  9. Differential Diagnosis
    1. Common Cold viruses
      1. Respiratory Syncytial Virus (RSV)
      2. Parainfluenza
      3. Adenovirus
    2. Factors suggesting Common Cold
      1. Findings suggestive of Influenza (see diagnosis above) are absent
      2. Gradual onset of more mild symptoms
      3. Upper respiratory symptoms predominate
  10. 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
  11. Labs: Diagnosis
    1. General
      1. Influenza diagnosis should be made clinically (lab testing is only needed in certain groups)
      2. Rapid Influenza Testing has poor Test Sensitivity (50%) and does not exclude Influenza if negative
      3. High risk groups should still be treated without delay if high clinical suspicion despite negative testing
    2. Indications for testing
      1. Influenza-like illness in patients or workers in the hospital, Nursing Home or daycare (limit spread)
      2. Alternative diagnosis evaluation subjects patient to extensive testing (e.g. Sepsis work-up)
      3. Serious underlying comorbidity (e.g. oxygen dependent COPD) for which diagnosis might alter disposition
    3. Initial testing at point of care
      1. Do not rely on Influenza testing to determine management (see above)
      2. Rapid Influenza Test (Influenza Immunoassay)
        1. Sample site varies between products
        2. Test Sensitivity 10-70% (very high False Negative Rate)
        3. Test Specificity >95%
    4. Confirmatory Testing
      1. Real Time Reverse Transcriptase PCR (RT-PCR) for RNA detection (preferred)
        1. Test Sensitivity: 86 to 100%
        2. Requires 1 hour to run test (but often delayed 1 day if sent to outside lab)
        3. If Rapid Influenza Test negative despite high suspicion, consider PCR (especially in Nursing Home)
      2. Influenza Culture (48-72 hours required for isolation)
        1. Nasopharyngeal swab
        2. Throat swab
        3. Sputum
      3. Serology (diagnostic if four fold rise over 10-14 days)
        1. Hemagglutination inhibition
        2. Complement fixation titers
  12. Labs: Other
    1. Complete Blood Count
      1. Leukopenia or slight Leukocytosis (up to 15,000)
      2. Relative Lymphopenia
  13. 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. Anti-viral agent indications
      1. Treat hospitalized or seriously ill patients with suspected Influenza regardless of time since onset (even >48 hours)
      2. Treat high risk populations who can start treatment within 48 hours
        1. Children under age 2 years old (some guidelines use under age 5 years)
        2. Elderly (over 65 years old)
        3. Chronic medical conditions (e.g. COPD, Asthma, hematologic disorders)
        4. Immunosuppressed patients
        5. Obese patients with BMI>40
        6. Alaskan natives and native americans
        7. Pregnancy (despite Pregnancy category C due to higher risk of Influenza related morbidity)
    3. Influenza A
      1. Neuraminidase Inhibitors
        1. Oseltamivir (Tamiflu)
          1. First-line agent for high risk patients (e.g. hospitalized or severe illness, immunosuppressed)
        2. Zanamivir (Relenza)
        3. Peramivir (Rapivab)
          1. IV antiviral with no better efficacy than Oseltamivir (Tamiflu), at 10 times the cost
          2. Indicated in hospitalized Influenza patients unable to take oral Oseltamivir (Tamiflu)
          3. Dose 600 mg IV as single dose in adults >18 years old (Category C in pregnancy, adjust for CKD)
          4. May cause Diarrhea (common), Anaphylaxis, skin reactions, transient neuropsychiatric events
        4. Baloxavir marboxil (Xofluza)
          1. One single dose, but no evidence of benefit in high risk patients
          2. Consider in non-severe, outpatient Influenza with moderate risks (e.g. diabetes, coronary disease)
      2. Resistance to Adamantanes (Amantadine, Rimantadine) is common (esp. H1N1)
        1. CDC no longer recommends Amantadine or Rimantadine for Influenza management
          1. Due to resistance, not used for chemoprophylaxis or treatment
        2. Consider combination therapy in the Nursing Home
          1. Rimantadine 100 mg daily for 5 days AND
          2. Neuraminidase Inhibitors
      3. Course: 5 days or 48 hours after symptoms resolve
    4. Influenza A or B: Neuraminidase Inhibitors
      1. See Zanamivir (Relenza)
      2. See Oseltamivir (Tamiflu)
    5. Avoid Salicylates in patients younger than 16 years
      1. Risk of Reye's Syndrome
    6. Avoid herbal preparations
      1. Elderberry and Oscillococcinum (unlikely to be helpful)
        1. Oscillococcinum is homeopathic and unlikely to contain any active ingredient (but unlikely to be harmful)
        2. Elderberry (e.g. Sambucol) may be helpful in first 48 hours, but available doses are likely too low
        3. (2018) Presc Lett 25(3)
  14. Management: Hospitalization Indications (findings suggestive of severe case)
    1. Chest Pain
    2. Altered Level of Consciousness
    3. Seizures
    4. Severe weakness
    5. Hemoptysis
    6. Hypoxia, Cyanosis, labored breathing or Shortness of Breath
    7. Decreased Urine Output, Hypotension or Dehydration
    8. High fever or progressive worsening after first 72 hours
  15. Complications
    1. StreptococcusPneumonia
    2. Staphylococcal Pneumonia (and empyema risk)
    3. Acute Respiratory Distress Syndrome (ARDS)
    4. Myocarditis
      1. ECMO has been required in some cases
    5. Acute Exacerbation of Chronic Bronchitis (AECB)
    6. Asthma Exacerbation
    7. Acute Sinusitis
    8. Acute Bronchitis
    9. Acute Otitis Media
    10. Seizures
      1. Most common neurologic complication
  16. Prevention
    1. Influenza Vaccine yearly
      1. Immunize everyone over 6 months of age (and especially high risk groups)
        1. CDC recommends immunizing everyone over age 6 months (as of 2012)
        2. See Influenza Vaccine for indications
        3. Nursing Home residents and staff
        4. Comorbid illness
        5. Pregnant women after first trimester
      2. Efficacy
        1. Varies by year, selected Vaccine components, Antigenic drifts and shifts
          1. Predominant strain in 2014/15 was H3N2
          2. Influenza Vaccine was 55% effective in 2013/14, but only 23% effective in 2014/15
        2. Healthy younger patients: 70-90%
        3. Elderly: 30-40%
    2. Flumist
      1. Was not recommended in U.S. in 2016 due to lower efficacy, but offered again in 2018 as alternative
      2. Alternative to standard injectable Influenza Vaccine who otherwise refuse Influenza Vaccine
      3. Live virus intranasal Vaccine
      4. May be used in healthy, non-pregnant patients aged 2 to 49 years
    3. Postexposure Prophylaxis
      1. Indications
        1. Influenza exposure from 1 day prior to symptom onset to resolution of fever
        2. High risk groups (for serious Influenza related complication)
        3. Nursing Home or other high risk institutional outbreaks
      2. Start within 48 hours of exposure
      3. Nursing Home: Treat for at least 2 weeks and for at least 7 days after the last infected case
      4. Amantadine Or Rimantadine prophylaxis is no longer recommended for Influenza A due to resistance (use Neuraminidase Inhibitors)
      5. Neuraminidase Inhibitors
        1. See Zanamivir (Relenza)
        2. See Oseltamivir (Tamiflu)
      6. May consider single dose Baloxavir (not FDA approved)
        1. May prevent up to 1 case in 9 contacts
        2. Ikematsu (2020) N Engl J Med 383:309-20 [PubMed]
    4. Other measures
      1. Respiratory isolate hospitalized Influenza patients
      2. Isolate Nursing Home residents with Influenza to room
      3. Isolate Nursing Home residents on prophylaxis to room
        1. Risk of virus shedding
  17. Prevention: 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
  18. Resources
    1. Is it the cold or the flu
    2. CDC Influenza Surveillance
    3. CDC Influenza Information
    4. CDC MMWR - ACIP Guidelines on antivirals in Influenza (2011)
    5. American Lung Association Influenza Information
  19. References
    1. (2020) Presc Lett 27(10): 55-6
    2. (1999) Preparing Next Influenza Pandemic Teleconf, CDC
    3. Claudius and Zangwill in Herbert (2018) EM:Rap 18(12): 17-8
    4. Takhar in Herbert (2012) EM:Rap 12(12): 11-12
    5. Hayden (2000) N Engl J Med 343:1282-9 [PubMed]
    6. Welliver (2001) JAMA 285:748-54 [PubMed]
    7. Erlikh (2010) Am Fam Physician 82(9):1087-95 [PubMed]

Influenza (C0021400)

Definition (MEDLINEPLUS)

Flu is a respiratory infection caused by a number of viruses. The viruses pass through the air and enter your body through your nose or mouth. Between 5% and 20% of people in the U.S. get the flu each year. The flu can be serious or even deadly for elderly people, newborn babies, and people with certain chronic illnesses.

Symptoms of the flu come on suddenly and are worse than those of the common cold. They may include

  • Body or muscle aches
  • Chills
  • Cough
  • Fever
  • Headache
  • Sore throat

Is it a cold or the flu? Colds rarely cause a fever or headaches. Flu almost never causes an upset stomach. And "stomach flu" isn't really flu at all, but gastroenteritis.

Most people with the flu recover on their own without medical care. People with mild cases of the flu should stay home and avoid contact with others, except to get medical care. If you get the flu, your health care provider may prescribe medicine to help your body fight the infection and lessen symptoms.

The main way to keep from getting the flu is to get a yearly flu vaccine. Good hygiene, including hand washing, can also help.

NIH: National Institute of Allergy and Infectious Diseases

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.(from Dorland)
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 (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.
Concepts Disease or Syndrome (T047)
MSH D007251
ICD9 487
ICD10 J11.1
SnomedCT 155562009, 195931008, 155559006, 266353003, 266393000, 6142004
LNC LP28598-8
English Grippe, Influenza, Influenzas, INFLUENZA, influenza (diagnosis), influenza, Flu syndrome, Syndrome flu, Influenza, Human [Disease/Finding], Influenzae, flu syndrome, flus, flu, influenzas, grippe, Influenza NOS, Influenza NOS (disorder), Flu, Influenza (disorder), Influenza, NOS, Flu, Human, Human Flu, Human Influenzas, Human Influenza, Influenza in Humans, Influenza in Human, Influenza, Human, Influenzas, Human
Spanish gripe, influenza, gripe (trastorno), Síndrome gripal, Influenza en Humanos, influenza (trastorno), gripe, SAI (trastorno), gripe, SAI, Influenza, Gripe, Gripe Humana, Influenza Humana
Dutch syndroom griep, griepsyndroom, Influenza, griep
French Syndrome de grippe, Grippe, Grippe humaine, Grippe chez l'homme
German Grippesyndrom, Syndrom Grippe, Grippe, Influenza
Italian Sindrome di influenza, Sindrome influenzale, Influenza
Portuguese Síndrome gripal, Influenza em Humanos, Influenza Humana, Gripe, Gripe Humana
Japanese インフルエンザ症候群, インフルエンザショウコウグン, インフルエンザ, インフルエンザ-ヒト, インフルエンザ, ヒトインフルエンザ, 流感, 流行性感冒
Swedish Influensa
Czech chřipka, chřipka lidská, Chřipkový syndrom, Chřipka
Finnish Ihmisen influenssa
Polish Grypa azjatycka, Grypa ludzka
Hungarian influenza, Influenza, nátha syndroma
Norwegian Influensa
Derived from the NIH UMLS (Unified Medical Language System)

You are currently viewing the original '\legacy' version of this website. Internet Explorer 8.0 and older will automatically be redirected to this legacy version.

If you are using a modern web browser, you may instead navigate to the newer desktop version of fpnotebook. Another, mobile version is also available which should function on both newer and older web browsers.

Please Contact Me as you run across problems with any of these versions on the website.

Navigation Tree