H1N1 Influenza (Swine Flu) Clinical Presentation

Updated: Aug 14, 2023
  • Author: Michael Stuart Bronze, MD; Chief Editor: Russell W Steele, MD  more...
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Manifestations of H1N1 influenza (swine flu) are similar to those of seasonal influenza. Patients present with symptoms of acute respiratory illness, including at least 2 of the following:

  • Fever
  • Cough
  • Sore throat
  • Body aches
  • Headache
  • Chills and fatigue
  • Diarrhea and vomiting (possible)

Persons with these symptoms should call their healthcare provider promptly. If an antiviral agent is warranted, it ideally should be initiated with 48 hours from the onset of symptoms (see Medication). The duration of illness is typically 4-6 days. The infectious period for a confirmed case is defined as 1 day prior to the onset of symptoms to 7 days after onset.

In children, signs of severe disease include apnea, tachypnea, dyspnea, cyanosis, dehydration, altered mental status, and extreme irritability. [4]

In children hospitalized for influenza, neurologic complications are common and sometimes life-threatening. In an effort to assess the extent and range of such complications in this population, Australian investigators in 6 tertiary pediatric referral centers carried out active hospital-based surveillance of 506 children younger than 15 years who had laboratory-confirmed pandemic influenza A (H1N1) 2009 infection (pH1N1'09). [28] Of the 506, 49 (9.7%) had neurologic complications.

Further study findings were as follows:

  • Patients with neurologic complications tended to be slightly older than those without (median age, 4.8 years versus 3.7 years)
  • Of patients with neurologic complications, 55.1% had preexisting medical conditions and 42.8% had preexisting neurologic conditions
  • On presentation, only 36.7% had cough, fever, and coryza or runny nose; 38.7% had only 1 respiratory symptom or none at all
  • Neurologic complications, in descending order of frequency, included seizure (7.5%), encephalitis or encephalopathy (1.4%), confusion or disorientation (1.0%), loss of consciousness (1.0%), and paralysis or Guillain-Barré syndrome (0.4%)
  • Intensive care unit (ICU) admission was required in 30.6% of the patients, mechanical ventilation in 24.5%
  • Mean hospital stay was 6.5 days, mean ICU stay 4.4 days
  • Two (4.1%) of the 49 patients died

Specific treatment for influenza-related neurologic complications generally is unavailable. Consequently, early diagnosis of influenza, appropriate use of antiviral therapy, and universal influenza vaccination in children are vital. Influenza should be considered as a diagnosis in children with neurologic symptoms, even when few or even no respiratory symptoms are noted.