Updated: 10/15/2018

Influenza Viruses

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  • A 26-year-old woman presents to the emergency room for 3 days of high fever, chills, muscle aches, a severe headache, and sore throat. She works in a nursing home but has not had her flu shot yet this year. She has a past medical history of systemic lupus erythematosus and hypothyroidism. A rapid strep test is negative, and a rapid antigen detection test for flu is positive. She is started on oseltamivir for this infection.
Introduction
  • Classification
    • influenza virus 
      • an enveloped, segmented, negative-sense, single-stranded RNA virus with a helical capsid
      • an orthomyxovirus
      • transmission via respiratory droplets
  • Epidemiology
    • incidence
      • annual epidemics, often occurring in the winter
    • demographics
      • affects both adults and children, though with higher frequency in children
    • risk factors
      • advanced or young age
      • pregnancy
      • immunosuppression
      • morbid obesity
      • pulmonary disease
  • Etiology
    • influenza A and B are causes of seasonal epidemics
    • can originate from animal strains
      • avian flu A
      • swine flu A (H1N1)
  • Pathogenesis
    • surface protein hemagglutinin (H) and neuraminidase (N)
    • hemagglutinin binds to sialic acid and allows for viral entry into cells
    • neuraminidase allows for progeny virion release from cells
  • Genetics
    • influenza viruses have 8 segments in the genome
    • genetic/antigenic shift
      • reassortment of viral genome segments, such as between human and swine strains, can cause pandemics
    • genetic/antigenic drift
      • random mutations in the H or N gene can cause epidemics
  • Prevention
    • annual flu vaccine for those 6 months or older
      • contains multiple killed viral strains that are thought to be likely to appear during flu season
        • intramuscular
      • live-attenuated vaccine
        • intranasal
  • Prognosis
    • may lead to severe bacterial superinfections
      • most commonly S. aureus, S. pneumoniae, and H. influenzae
    • may be fatal
Presentation
  • Symptoms
    • systemic symptoms predominate
      • high fevers and chills
      • myalgias and malaise
      • headaches
      • nausea and vomiting
    • may have cough or sore throat
  • Physical exam
    • may have cervical lymphadenopathy
Imaging
  • Chest radiography
    • indication
      • if pneumonia is suspected
    • findings
      • consolidation or infiltrate
      • ground-glass opacities
Studies
  • Labs
    • rapid antigen detection tests (RADTS)
      • most commonly used
    • detection of RNA on reverse transcriptase polymerase chain reaction
  • Making the diagnosis
    • based on clinical presentation and confirmed with laboratory studies
Differential
  • Upper respiratory infection
    • distinguishing factor
      • upper respiratory symptoms (e.g., cough, congestion, etc.) often predominates rather than systemic symptoms (e.g., high fevers, chills, malaise, etc.)
Treatment
  • Management approach
    • mainstay of treatment is supportive care
    • avoid aspirin in children
      • can cause Reye disease, leading to hepatorenal failure
  • Conservative
    • supportive care
      • indication
        • all patients
      • modalities
        • hydration
        • antipyretics
  • Medical
    • oseltamivir/zanamivir 
      • indication
        • patients with severe disease or at high risk of severe disease
Complications
  • Secondary bacterial infection
  • Myositis or myocarditis
  • Central nervous system involvement
 

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Questions (2)
Lab Values
Blood, Plasma, Serum Reference Range
ALT 8-20 U/L
Amylase, serum 25-125 U/L
AST 8-20 U/L
Bilirubin, serum (adult) Total // Direct 0.1-1.0 mg/dL // 0.0-0.3 mg/dL
Calcium, serum (Ca2+) 8.4-10.2 mg/dL
Cholesterol, serum Rec: < 200 mg/dL
Cortisol, serum 0800 h: 5-23 μg/dL //1600 h:
3-15 μg/dL
2000 h: ≤ 50% of 0800 h
Creatine kinase, serum Male: 25-90 U/L
Female: 10-70 U/L
Creatinine, serum 0.6-1.2 mg/dL
Electrolytes, serum  
Sodium (Na+) 136-145 mEq/L
Chloride (Cl-) 95-105 mEq/L
Potassium (K+) 3.5-5.0 mEq/L
Bicarbonate (HCO3-) 22-28 mEq/L
Magnesium (Mg2+) 1.5-2.0 mEq/L
Estriol, total, serum (in pregnancy)  
24-28 wks // 32-36 wks 30-170 ng/mL // 60-280 ng/mL
28-32 wk // 36-40 wks 40-220 ng/mL // 80-350 ng/mL
Ferritin, serum Male: 15-200 ng/mL
Female: 12-150 ng/mL
Follicle-stimulating hormone, serum/plasma Male: 4-25 mIU/mL
Female: premenopause: 4-30 mIU/mL
midcycle peak: 10-90 mIU/mL
postmenopause: 40-250
pH 7.35-7.45
PCO2 33-45 mmHg
PO2 75-105 mmHg
Glucose, serum Fasting: 70-110 mg/dL
2-h postprandial:<120 mg/dL
Growth hormone - arginine stimulation Fasting: <5 ng/mL
Provocative stimuli: > 7ng/mL
Immunoglobulins, serum  
IgA 76-390 mg/dL
IgE 0-380 IU/mL
IgG 650-1500 mg/dL
IgM 40-345 mg/dL
Iron 50-170 μg/dL
Lactate dehydrogenase, serum 45-90 U/L
Luteinizing hormone, serum/plasma Male: 6-23 mIU/mL
Female: follicular phase: 5-30 mIU/mL
midcycle: 75-150 mIU/mL
postmenopause 30-200 mIU/mL
Osmolality, serum 275-295 mOsmol/kd H2O
Parathyroid hormone, serume, N-terminal 230-630 pg/mL
Phosphatase (alkaline), serum (p-NPP at 30° C) 20-70 U/L
Phosphorus (inorganic), serum 3.0-4.5 mg/dL
Prolactin, serum (hPRL) < 20 ng/mL
Proteins, serum  
Total (recumbent) 6.0-7.8 g/dL
Albumin 3.5-5.5 g/dL
Globulin 2.3-3.5 g/dL
Thyroid-stimulating hormone, serum or plasma .5-5.0 μU/mL
Thyroidal iodine (123I) uptake 8%-30% of administered dose/24h
Thyroxine (T4), serum 5-12 μg/dL
Triglycerides, serum 35-160 mg/dL
Triiodothyronine (T3), serum (RIA) 115-190 ng/dL
Triiodothyronine (T3) resin uptake 25%-35%
Urea nitrogen, serum 7-18 mg/dL
Uric acid, serum 3.0-8.2 mg/dL
Hematologic Reference Range
Bleeding time 2-7 minutes
Erythrocyte count Male: 4.3-5.9 million/mm3
Female: 3.5-5.5 million mm3
Erythrocyte sedimentation rate (Westergren) Male: 0-15 mm/h
Female: 0-20 mm/h
Hematocrit Male: 41%-53%
Female: 36%-46%
Hemoglobin A1c ≤ 6 %
Hemoglobin, blood Male: 13.5-17.5 g/dL
Female: 12.0-16.0 g/dL
Hemoglobin, plasma 1-4 mg/dL
Leukocyte count and differential  
Leukocyte count 4,500-11,000/mm3
Segmented neutrophils 54%-62%
Bands 3%-5%
Eosinophils 1%-3%
Basophils 0%-0.75%
Lymphocytes 25%-33%
Monocytes 3%-7%
Mean corpuscular hemoglobin 25.4-34.6 pg/cell
Mean corpuscular hemoglobin concentration 31%-36% Hb/cell
Mean corpuscular volume 80-100 μm3
Partial thromboplastin time (activated) 25-40 seconds
Platelet count 150,000-400,000/mm3
Prothrombin time 11-15 seconds
Reticulocyte count 0.5%-1.5% of red cells
Thrombin time < 2 seconds deviation from control
Volume  
Plasma Male: 25-43 mL/kg
Female: 28-45 mL/kg
Red cell Male: 20-36 mL/kg
Female: 19-31 mL/kg
Cerebrospinal Fluid Reference Range
Cell count 0-5/mm3
Chloride 118-132 mEq/L
Gamma globulin 3%-12% total proteins
Glucose 40-70 mg/dL
Pressure 70-180 mm H2O
Proteins, total < 40 mg/dL
Sweat Reference Range
Chloride 0-35 mmol/L
Urine  
Calcium 100-300 mg/24 h
Chloride Varies with intake
Creatinine clearance Male: 97-137 mL/min
Female: 88-128 mL/min
Estriol, total (in pregnancy)  
30 wks 6-18 mg/24 h
35 wks 9-28 mg/24 h
40 wks 13-42 mg/24 h
17-Hydroxycorticosteroids Male: 3.0-10.0 mg/24 h
Female: 2.0-8.0 mg/24 h
17-Ketosteroids, total Male: 8-20 mg/24 h
Female: 6-15 mg/24 h
Osmolality 50-1400 mOsmol/kg H2O
Oxalate 8-40 μg/mL
Potassium Varies with diet
Proteins, total < 150 mg/24 h
Sodium Varies with diet
Uric acid Varies with diet
Body Mass Index (BMI) Adult: 19-25 kg/m2
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