{
 "topic": "Influenza A",
 "slug": "influenza-a",
 "category_id": 15765,
 "summary": "Seasonal influenza A infection in children: antigenic subtyping and drift, the age-dependent clinical picture from classic myalgic illness to infant sepsis-like presentation, and antiviral treatment indications.",
 "written_by": "claude-sonnet",
 "references": [
  {
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   "author": null,
   "pages": [
    272
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  {
   "title": "Red Book Atlas of Pediatric Infectious Diseases",
   "author": "American Academy of Pediatrics,Carol J. Baker, MD, FAAP",
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   ]
  },
  {
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    1227
   ]
  },
  {
   "title": "Kliegman R. Nelson Textbook of Pediatrics 2-Volume Set 22ed 2024",
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   ]
  },
  {
   "title": "Fever without a source in children 3 to 36 months of age: Evaluation and management - UpToDate",
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    2
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   "author": null,
   "pages": [
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  {
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   ]
  },
  {
   "title": "MedStudy Pediatrics Core 11th Edition 2024-2025",
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    800
   ]
  }
 ],
 "short": [
  {
   "title": "In short",
   "content": "- Influenza A and B viruses (Orthomyxoviridae) cause seasonal epidemics; influenza A is subtyped by its surface hemagglutinin (HA) and neuraminidase (NA) proteins (e.g., H1N1, H3N2), and small antigenic changes in HA/NA (\"antigenic drift\") continuously generate new strains, driving yearly epidemics and requiring an annually updated 3-4 antigen vaccine.\n- Transmission is via large-particle respiratory droplets (cough, sneeze) and contaminated surfaces; incubation is 1-4 days (also cited as 2-7 days), and outbreaks occur mainly in fall/winter, often starting in schools; hospitalization rates are highest in children under 2 and adults over 65.\n- Classic influenza in older children/adults: sudden high fever, severe myalgia, headache, and chills, overshadowing accompanying coryza, pharyngitis, and cough; rash, marked conjunctivitis, adenopathy, exudative pharyngitis, and dehydrating enteritis are usually absent.\n- Influenza is a less distinct illness in infants and young children: fever, diarrhea, vomiting, and abdominal pain are common, and infants can appear highly febrile and toxic, or develop a sepsis-like illness with apnea; diarrhea has been reported more often with A(H1N1)pdm09 than A(H3N2) or B.\n- Uncomplicated influenza typically resolves in 3-7 days (febrile illness itself 2-4 days), though cough can persist longer, with evidence of small airway dysfunction sometimes found weeks later.\n- Complications: primary viral pneumonia, secondary bacterial infection (S. aureus, S. pneumoniae), sinusitis, otitis media, encephalitis/encephalopathy, myocarditis, myositis (especially calf muscles), parotitis, croup (most severe with influenza A), and asthma exacerbation.\n- Children should never receive aspirin or salicylate-containing products during influenza illness because of Reye syndrome risk; acetaminophen or another nonsalicylate antipyretic is preferred for fever control.\n- Emerging avian influenza strains \u2014 A(H5N1) since 1997 and A(H7N9) since 2013 \u2014 have caused severe disease and high case-fatality rates in sporadic human infections; A(H7N9) has been considered the influenza virus with the highest pandemic potential since 2017, and infection with a novel influenza A virus is nationally notifiable to the CDC.\n- Diagnosis is usually clinical, confirmed when needed by antigen testing (rapid flu test) or, increasingly, PCR; treatment with antivirals is indicated for any hospitalized child with presumed influenza, confirmed/suspected influenza with severe or progressive illness, any child at high risk for complications regardless of vaccination status, and any child for whom shortening symptom duration is felt to be warranted."
  }
 ],
 "long": [
  {
   "title": "Definition",
   "content": "Influenza viruses are large, single-stranded RNA viruses of the family Orthomyxoviridae, comprising three types (genera): A, B, and C. Influenza A and B are the primary human pathogens causing seasonal epidemics, while type C causes only sporadic, mild, influenza-like illness and is not included in vaccines. Influenza A viruses are further divided into subtypes based on two surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA) \u2014 for example, H1N1 and H3N2."
  },
  {
   "title": "Epidemiology",
   "content": "Influenza causes annual epidemics, typically in fall and winter, with three or four types/subtypes co-circulating each season (commonly A/H3N2, A/H1N1, and two B lineages), though which subtype predominates in a given season is difficult to predict; a season dominated by A(H3N2) has had roughly 2.7 times higher average mortality than other seasons. Infection rates are higher in children than adults, and children are instrumental in initiating community outbreaks; hospitalization rates are highest in children under 2 and adults over 65. Influenza causes a substantial outpatient burden, contributing an estimated 600,000-2,500,000 outpatient visits annually in US children under 5, and is identified in 10-25% of outpatient visits for respiratory symptoms during flu season; it is likely underdiagnosed since many patients are not tested. The 2009 H1N1 pandemic combined unusual pediatric virulence with lack of prior immunity, causing nearly four times the usual number of pediatric deaths. Emerging avian influenza strains \u2014 A(H5N1), first identified in 1997, and A(H7N9), first detected in 2013 \u2014 have caused sporadic but severe human infections with high case-fatality rates; A(H7N9) has been considered the influenza virus with the highest pandemic potential since 2017. A swine-origin A(H3N2)v has also caused clusters of US infections, typically linked to pig exposure. Infection with a novel influenza A virus is a nationally notifiable disease reported to the CDC through state health departments."
  },
  {
   "title": "Etiology",
   "content": "Spread occurs via large-particle respiratory droplets (coughing, sneezing) and contact with contaminated surfaces followed by touching the face; incubation is generally cited as 1-4 days (also described as 2-7 days). Minor antigenic changes in the HA or NA surface proteins \u2014 antigenic drift \u2014 occur continuously and generate new influenza A and B strains, driving seasonal epidemics and requiring the influenza vaccine to be reformulated annually with three or four antigens matched to expected circulating strains. Virus-specific IgG and IgA antibodies against HA can block infection when present. Neuraminidase facilitates viral penetration through the respiratory mucous layer and later release of new virus particles from infected cells, while HA mediates viral attachment and membrane fusion with respiratory epithelial cells."
  },
  {
   "title": "Pathophysiology",
   "content": "Infection begins with viral inoculation in the upper or lower airway, followed by replication in respiratory epithelium, viral shedding into secretions, local spread, and epithelial desquamation; the entire airway from pharynx to alveoli can be involved, and diffuse alveolar pneumonia can be life-threatening. Infection is usually confined to the respiratory tract, though viremia has been described with certain strains."
  },
  {
   "title": "Clinical features",
   "content": "In older children and adults, influenza produces a characteristic syndrome of sudden-onset high fever, severe myalgia, headache, and chills that overshadow accompanying coryza, pharyngitis, and cough; rash, marked conjunctivitis, adenopathy, exudative pharyngitis, and dehydrating enteritis are usually absent, and chest examination is typically unremarkable. Fever, diarrhea, vomiting, and abdominal pain are common in young children, with diarrhea reported more often with influenza A(H1N1)pdm09 than with A(H3N2) or B in some studies. Influenza is a less distinct illness in infants and young children, who may appear highly febrile and toxic, prompting a full sepsis workup, or develop a sepsis-like illness with apnea. The febrile illness typically lasts 2-4 days, with uncomplicated disease generally resolving within 3-7 days overall; cough can persist longer, and evidence of small airway dysfunction is sometimes found weeks later. Because of high transmissibility, other household or close contacts often develop a similar illness around the same time."
  },
  {
   "title": "Diagnostics",
   "content": "Diagnosis is most often made clinically. When confirmation is needed \u2014 for atypical presentations or diagnosis outside flu season \u2014 testing options include viral isolation (rarely used), enzyme immunoassay-based rapid antigen testing, or, increasingly, PCR, which detects virus, viral antigen, or nucleic acid in respiratory secretions. For children under 5, and especially under 2, laboratory confirmation is recommended given the higher risk of complications in this age group."
  },
  {
   "title": "Differential diagnosis",
   "content": "Croup, encephalopathy distinct from Reye syndrome, and other complications can overlap in presentation with unrelated illnesses; parainfluenza viruses account for about 75% of croup cases, but influenza A and B, adenovirus, RSV, COVID-19, and measles are also associated with croup, with influenza A specifically associated with severe laryngotracheobronchitis. COVID-19 in infants and young children can also present with fever, pharyngitis, nasal congestion, cough, vomiting, and diarrhea, overlapping substantially with influenza's presentation and requiring testing to distinguish."
  },
  {
   "title": "Treatment",
   "content": "Control fever with acetaminophen or another nonsalicylate antipyretic, since fever and other influenza symptoms can exacerbate underlying chronic conditions in young children. Children and adolescents with influenza should never receive aspirin or salicylate-containing products because of the risk of Reye syndrome. The CDC and AAP recommend antiviral treatment for any child hospitalized with presumed influenza; children with confirmed or suspected influenza and severe, complicated, or progressive illness; children of any illness severity who are at high risk for complications, regardless of vaccination status; and any otherwise healthy child for whom a provider feels shortening symptom duration is warranted."
  },
  {
   "title": "Complications",
   "content": "Complications include primary viral pneumonia, secondary bacterial infection (commonly S. aureus and S. pneumoniae), sinusitis, otitis media, and encephalitis/encephalopathy. Less frequent findings include croup (most severe with influenza A), asthma exacerbation, myositis (especially of the calf muscles), myocarditis, and parotitis. Severe disease, including myocarditis and CNS disease/encephalitis, can occur even outside the classically recognized high-risk groups."
  }
 ],
 "clinical": [
  {
   "title": "Recognizing the age-dependent presentation",
   "content": "In an older child or adolescent, expect the classic syndrome \u2014 sudden high fever, severe myalgia, headache, and chills that overshadow coryza, pharyngitis, and cough \u2014 with a typically unremarkable chest exam and absence of rash, marked conjunctivitis, adenopathy, exudative pharyngitis, or dehydrating enteritis (features that would suggest a different diagnosis). In infants and young children, expect a less distinct picture: fever, diarrhea, vomiting, and abdominal pain are common, and the child may look highly febrile and toxic or present with a sepsis-like illness with apnea, prompting a full sepsis evaluation. In children under 5, and especially under 2, obtain laboratory confirmation (rapid antigen test or PCR) given their higher complication risk, rather than relying on clinical diagnosis alone."
  },
  {
   "title": "Treatment decisions and safety",
   "content": "Start antiviral treatment for any child hospitalized with presumed influenza, any child with confirmed or suspected influenza and severe/complicated/progressive illness, any child at high risk for complications regardless of vaccination status, and any otherwise healthy child for whom the provider feels shortened symptom duration is clinically warranted. Use acetaminophen or another nonsalicylate antipyretic for fever control \u2014 never aspirin or salicylate-containing products, given the risk of Reye syndrome. Watch for and counsel on the range of possible complications: secondary bacterial infection (particularly pneumonia), sinusitis, otitis media, encephalitis, myocarditis, myositis, and croup (which can be especially severe with influenza A) \u2014 and set expectations that while the febrile illness usually resolves in 2-4 days and overall illness in 3-7 days, cough and subtle airway dysfunction can linger for weeks."
  }
 ]
}