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BASICS

  • Description: Acute febrile infection caused by influenza virus types A and B; affects nasal mucosa, pharynx, conjunctiva, respiratory tract.
  • Virus: Orthomyxovirus family; Influenza A subtyped by hemagglutinin (H) and neuraminidase (N).

EPIDEMIOLOGY

  • Seasonal outbreaks in fall-winter (Northern and Southern Hemispheres).
  • Antigenic shift causes pandemics; antigenic drift causes seasonal epidemics.
  • Risk groups: <2 years, >65 years, pregnant women (up to 2 weeks postpartum), immunocompromised, chronic diseases.
  • 2022-2023 US flu season: 27-54 million cases, 300k-650k hospitalizations, 19k-58k deaths.
  • COVID-19 pandemic influenced flu rates and diagnosis.

ETIOLOGY AND PATHOPHYSIOLOGY

  • Influenza types A (majority) and B.
  • Incubation: 1-4 days; peak contagiousness during symptoms.
  • Transmission: aerosol droplets and contact with secretions.
  • Hemagglutinin binds to respiratory epithelium; neuraminidase facilitates spread.

RISK FACTORS

  • Close contact in crowded environments (nursing homes, schools).
  • High-risk populations: neonates, elderly, pregnant women, chronic pulmonary/cardiac/metabolic diseases, immunosuppression.

PREVENTION

  • Annual vaccination recommended for everyone ≥6 months (inactivated or live attenuated).
  • Vaccine types: trivalent/quadrivalent, high-dose, adjuvanted, recombinant.
  • Egg allergy is not a contraindication.
  • Antiviral prophylaxis in outbreaks for unvaccinated or high-risk individuals.

DIAGNOSIS

History

  • Sudden onset fever, chills, malaise, myalgia, headache, sore throat, cough, rhinorrhea.
  • GI symptoms (nausea, vomiting, diarrhea) common in children.
  • Altered mental status in elderly/immunocompromised.

Physical Exam

  • Nonspecific; rule out complications like pneumonia, sinusitis, otitis media.

Differential Diagnosis

  • Other viral respiratory infections, mononucleosis, atypical pneumonia, HIV seroconversion, bacterial infections.

Diagnostic Tests

  • Rapid antigen detection tests (RIDTs): 10-15 min, moderate sensitivity (~80%), high specificity.
  • RT-PCR: high sensitivity (90-95%) and specificity; gold standard.
  • Immunofluorescence and ELISA tests available.
  • Labs: CBC (normal/mild leukopenia), CMP (liver enzymes).
  • Imaging if pneumonia suspected.

TREATMENT

General

  • Symptomatic care: antipyretics, analgesics, saline sprays, humidification.
  • Counsel on droplet precautions; viral shedding ~5 days in immunocompetent.
  • Tobacco cessation advised.

Antiviral Medications

  • Most effective within 48 hours of symptom onset.
  • Agents: baloxavir, oseltamivir, zanamivir, peramivir.
  • Amantadine and rimantadine not recommended due to resistance.
  • Dosages vary by age and weight; dose adjustments for renal impairment.

Precautions and Side Effects

  • Zanamivir: risk of bronchospasm in COPD/asthma.
  • Oseltamivir: nausea/vomiting; take with food.
  • Baloxavir: diarrhea, hypersensitivity.
  • Avoid aspirin in children <16 years due to Reye syndrome risk.

INPATIENT CARE

  • Droplet precautions.
  • Prompt antiviral therapy regardless of symptom duration.
  • Supportive care including oxygen and ventilatory support if needed.

ONGOING CARE

  • Follow-up not routinely required in mild cases.
  • Monitor moderate/severe cases until recovery.

PROGNOSIS

  • Generally good.
  • Higher risk of complications and hospitalization in elderly and those with comorbidities.

COMPLICATIONS

  • Secondary bacterial pneumonia.
  • Myocarditis, encephalitis.
  • Rhabdomyolysis, multisystem organ failure.
  • Otitis media, sinusitis.
  • Apnea in neonates.
  • Reye syndrome in children with aspirin use.

REFERENCES

  1. Centers for Disease Control and Prevention. 2022-2023 U.S. flu season: preliminary in-season burden estimates. https://www.cdc.gov/flu/about/burden/preliminary-in-season-estimates.htm. Accessed October 19, 2023.
  2. Gaitonde DY, Moore FC, Morgan MK. Influenza: diagnosis and treatment. Am Fam Physician. 2019;100(12):751-758.

ICD10

  • J10.08 Influenza due to other identified influenza virus with other specified pneumonia
  • J11.89 Influenza due to unidentified influenza virus with other manifestations
  • J10.81 Influenza due to other identified influenza virus with encephalopathy

CLINICAL PEARLS

  • Annual influenza vaccination recommended for all ≥6 months except rare contraindications.
  • Antiviral treatment most effective if started within 48 hours of symptom onset.
  • Hand hygiene and respiratory etiquette reduce transmission.
  • Influenza complications highest in extremes of age and comorbidities.