Background


  • respiratory infection caused by influenza viruses that often has systemic symptoms

  • influenza viruses (family Orthomyxoviridae)

    • groups A, B, and C based on ribonucleoprotein (internal capsid protein)

    • classified as type A or B, strain place/year, subtype H#N#

    • type C influenza virus relatively rare

  • usually occurs in epidemics

  • seasonal incidence varies with latitude

    • temperate countries of Northern and Southern hemispheres have peak activities during winter (in United States, annual epidemics typically occur late fall through early spring)

    • tropical regions have high background influenza activity throughout the year with epidemics in months between temperate region activity

  • nonseasonal influenza viruses

    • pandemic (H1N1) 2009
      • antigenically distinct from other human influenza A (H1N1) viruses circulating since 1977 so was associated with widespread transmission

    • highly pathogenic avian influenza A (H5N1)
      • has been transmitted from birds to humans in Asia, Africa, Europe, and Middle East with limited, nonsustained human-to-human transmission

    • swine influenza A (H1N1), A (H1N2), and A (H3N2) viruses
      • are endemic among pig populations in United States

      • sporadically have infected humans

  • pathogenesis

    • transmitted via large particle respiratory droplets 

    • typical incubation period 1 to 4 days (mean 2 days) 

    • children may be infectious for > 10 days

    • severely immunocompromised persons can shed virus for months

    • mean duration of influenza-like illness 
10.4 days


Complications 


  • more frequent in 

    • children < 5 years old (especially children 
< 2 years old) 

    • children with chronic medical conditions

  • common complications include

    • viral pneumonia

    • secondary bacterial pneumonia

    • otitis media

    • tracheobronchitis

    • acute sinusitis

    • exacerbations of chronic pulmonary or cardiac 
disease

    • febrile seizure

  • less common complications include 

    • pyocarditis

    • pericarditis

    • myositis

    • myoglobinuria

    • rhabdomyolysis

    • neurologic sequelae

    • death 


Clinical presentation and evaluation


  • pattern of illness may vary from mild respiratory illness (similar to common cold) to severe prostration with nonspecific signs and symptoms

  • initial presentation may be abrupt onset of fever (temperature 37.7° to 40° C [100° to 104° F]) and dry cough

  • symptoms may include 

    • Fever

    • Malaise

    • Headache

    • Fatigue

    • Weakness

    • Anorexia

    • Myalgia

    • Arthralgia

    • dry cough

    • chest discomfort

    • nausea

    • vomiting

  • physical exam

    • no specific signs rule in or rule out influenza

    • findings may include
      • conjunctival injection

      • nonexudative pharyngitis

      • rhonchi

      • scattered rales

  • diagnosis

    • make diagnosis of influenza on the basis of clinical presentation without testing, especially during periods of peak influenza activity

    • diagnostic accuracy may be limited due to overlap with symptoms of other illnesses

    • confirm influenza by diagnostic testing
      • testing recommended for patients with clinical signs and symptoms of influenza for whom results will change clinical care

      • methods include
        • rapid antigen testing (nasopharyngeal or nasal specimen)

        • serology

        • reverse transcriptase-polymerase chain reaction (RT-PCR)
          • reported to be most sensitive and specific test for influenza
          • 
recommended as test of choice by Infectious Diseases Society of America (IDSA)
        • viral culture

        • immunofluorescence assays

  • consider chest x-ray if respiratory compromise


From the January 01, 2016 Issue of Clinical Advisor

Topics:

Infectious Disease Influenza Respiratory Syncytial Virus