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Clinical manifestations and diagnosis of pandemic H1N1 influenza ('swine influenza')
Author
Anna R Thorner, MD
Section Editor
Martin S Hirsch, MD
Deputy Editor
Barbara H McGovern, MD
Disclosures
All topics are updated as new evidence becomes available and our peer review process is complete.
Literature review current through: Aug 2012. | This topic last updated: sep 19, 2012.INTRODUCTION — In late March and early April 2009, an outbreak of H1N1 influenza A virus infection was detected in Mexico, with subsequent cases observed in many other countries, including the United States [1,2]. In June 2009, the World Health Organization (WHO) raised its pandemic alert level to the highest level, phase 6, indicating widespread community transmission on at least two continents [3]. Thepandemic was declared to be over in August 2010 [4].
The clinical manifestations and diagnosis of pandemic H1N1 influenza A virus infection will be reviewed here. The epidemiology, treatment, and prevention of pandemic H1N1 influenza A virus infection are discussed separately. (See "Epidemiology of pandemic H1N1 influenza ('swine influenza')" and "Treatment and prevention of pandemic H1N1influenza ('swine influenza')".)
The clinical manifestations and diagnosis of seasonal and avian (H5N1) influenza viruses are discussed separately. (See "Clinical manifestations of seasonal influenza in adults" and "Clinical features and diagnosis of seasonal influenza in children" and "Clinical manifestations and diagnosis of avian influenza" and "Diagnosis of seasonal influenza in adults".)
CASEDEFINITIONS — The following case definitions have been provided by the United States Centers for Disease Control and Prevention (CDC) [5]:
* Influenza-like illness (ILI) is defined as fever (temperature of 100ºF [37.8ºC] or greater) with cough or sore throat in the absence of a known cause other than influenza.
* A confirmed case of pandemic H1N1 influenza A is defined as an individual with anILI with laboratory-confirmed H1N1 influenza A virus detection by real-time reverse transcriptase (rRT)-PCR or culture.
DEFINITIONS OF ILLNESS SEVERITY — The United States Centers for Disease Control and Prevention (CDC) categorized the severity of illness from pandemic H1N1 influenza A as follows [6]:
Mild or uncomplicated illness — Mild or uncomplicated illness is characterized by fever,cough, sore throat, rhinorrhea, muscle pain, headache, chills, malaise, and sometimes diarrhea and vomiting, but no shortness of breath and little change in chronic health conditions.
Progressive illness — Progressive illness is characterized by typical symptoms plus signs or symptoms, such as chest pain, poor oxygenation (eg, tachypnea, hypoxia, labored breathing in children), cardiopulmonaryinsufficiency (eg, hypotension), central nervous system (CNS) impairment (eg, confusion, altered mental status), severe dehydration, or exacerbations of chronic conditions (eg, asthma, chronic obstructive pulmonary disease, chronic renal failure, diabetes, or other cardiovascular conditions).
Severe or complicated illness — Severe or complicated illness is characterized by signs of lower respiratorytract disease (eg, hypoxia requiring supplemental oxygen, abnormal chest radiograph, mechanical ventilation), CNS findings (encephalitis, encephalopathy), complications of hypotension (shock, organ failure), myocarditis or rhabdomyolysis, or invasive secondary bacterial infection based on laboratory testing or clinical signs (eg, persistent high fever and other symptoms beyond three days).TRANSMISSION — The transmission, incubation period, and duration of shedding of pandemic H1N1 influenza A are discussed separately. (See "Epidemiology of pandemic H1N1 influenza ('swine influenza')", section on 'Transmission'.)
ASYMPTOMATIC INFECTION — In a study that characterized the household transmission of pandemic H1N1 influenza A, 5 of 53 (9.4 percent) individuals who became infected from...