Signs and symptoms — The signs and symptoms of influenza caused by swine H1N1 influenza A virus are similar to those of seasonal influenza . Typical clinical manifestations include fever, headache, cough, sore throat, myalgias, chills, and fatigue. Diarrhea and vomiting may also occur.
During the 2009 outbreak, rapidly progressive pneumonia, respiratory failure, and acute respiratory distress syndrome have been reported in some cases in Mexico . In the United States, most described cases to date have been mild as of April 29, 2009 in the United States. (See “Clinical manifestations and diagnosis of influenza in adults”).
Children — Young children are less likely to have the usual influenza signs and symptoms, such as fever and cough . Infants may present with fever and lethargy, and may not have cough or other respiratory symptoms. Symptoms of severe disease in infants and young children may include apnea, tachypnea, dyspnea, cyanosis, dehydration, altered mental status, and extreme irritability. (See “Clinical features and diagnosis of influenza in children”).
Young children (eg, <5 years of age), pregnant women, and those with comorbidities are at increased risk of influenza complications . Increased rates of spontaneous abortion and preterm birth have also been reported among pregnant women, especially those with pneumonia .
The complications of infection with swine H1N1 influenza A are likely to be similar to those of seasonal influenza, such as :
- Exacerbation of underlying chronic medical conditions
- Upper respiratory tract disease (sinusitis, otitis media, croup)
- Lower respiratory tract disease (pneumonia, bronchiolitis, status asthmaticus)
- Cardiac (myocarditis, pericarditis)
- Neurologic (Acute and post-infectious encephalopathy, encephalitis, febrile seizures, status epilepticus)
- Toxic shock syndrome
- Secondary bacterial pneumonia with or without sepsis
The complications of seasonal influenza are discussed in greater detail separately. (See “Clinical manifestations and diagnosis of influenza in adults” and see “Clinical features and diagnosis of influenza in children”).
Mortality — Seasonal influenza results in higher mortality rates among patients with certain chronic medical conditions, as well as in pregnant women, and those at the extremes of age. (See “Antiviral drugs for the prevention of influenza in adults”, section on Definition of high risk and see “Antiviral drugs for the prevention and treatment of influenza in children”, section on Target groups).
As of April 30, 2009, over 150 deaths in Mexico have been ascribed to swine H1N1 influenza A, although only 20 of these deaths have been laboratory-confirmed . Most of these deaths were related to respiratory failure. Among more than 90 cases in the United States, there has been one death in a 23 month old child.
Variable severity of human infections with swine influenza viruses has been reported in several studies, as illustrated by the following findings [:
- In a review of 37 cases of human infections with swine influenza virus reported between 1958 and 2005, six cases (17 percent) resulted in death, all of which were due to pneumonia. Influenza virus was the only pathogen identified from the lungs in four patients; in one individual, Streptococcus viridans, Neisseria spp, and Klebsiella spp were also identified in addition to influenza virus .
- Between 2005 and January 2009, 12 cases of human infection with swine influenza virus were detected in the United States with no fatalities .
- An outbreak occurred among soldiers in Fort Dix, New Jersey in 1976, which involved up to 230 individuals but resulted in only one death .
A fatal human infection with swine influenza virus complicated by pneumonia was also reported in a previously healthy pregnant woman in 1988 .
- United States Centers for Disease Control and Prevention. Interim guidelines on antiviral recommendations for patients with confirmed or suspected swine influenza A (H1N1) virus infection and close contacts. http://www.cdc.gov/swineflu (Accessed April 29, 2009).
- United States Centers for Disease Control and Prevention. Interim Guidance for Clinicians on Identifying and Caring for Patients with Swine-origin Influenza A (H1N1) Virus Infection http://www.cdc.gov/swineflu/identifyingpatients.htm (Accessed April 29, 2009).
- United States Centers for Disease Control and Prevention. Interim Guidance on Case Definitions to be Used For Investigations of Swine Influenza A (H1N1) Cases. http://www.cdc.gov/swineflu/casedef_swineflu.htm (Accessed April 27, 2009).
- United States Centers for Disease Control and Prevention. Interim Guidance for Clinicians on the Prevention and Treatment of Swine-Origin Influenza Virus Infection in Young Children. http://www.cdc.gov/swineflu/childrentreatment.htm (Accessed April 29, 2009).
- United States Centers for Disease Control and Prevention. Interim Guidance — Pregnant Women and Swine Influenza: Considerations for Clinicians. http://www.cdc.gov/swineflu/clinician_pregnant.htm (Accessed April 29, 2009).
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