Current H1N1 influenza (formerly called swine influenza) outbreakHuman cases of influenza A (H1N1) have been reported worldwide. In 2009, cases of influenzalike illness were first reported in Mexico on March 18; the outbreak was subsequently confirmed as H1N1 influenza A.7 Investigation is continuing to clarify the spread and severity of H1N1 influenza (swine flu) in Mexico. Suspected clinical cases have been reported in 19 of the country's 32 states. Although only 97 of the Mexican cases have been laboratory-confirmed as Influenza A/H1N18 (12 of them genetically identical to Influenza A/H1N1 viruses from California7 ). As of May 5th, 2009, nearly 600 H1N1 influenza cases have been confirmed in Mexico, including 25 deaths.9
According to the WHO, as of May 4, 2009, H1N1 influenza had been laboratory-confirmed in 20 countries.8 For an updated tally of affected countries and counts, see WHO's Influenza A (H1N1) Web page. Cases are also suspected in Brazil.
On April 17, 2009, the CDC determined that two cases of febrile respiratory illness in children who resided in adjacent counties in southern California were caused by infection with a swine influenza A (H1N1) virus.10 By April 26, 2009, the US Department of Health and Human Services declared a national public health emergency involving H1N1 influenza A, citing its significant potential to affect national security.11 By May 5, 2009, more than 400 cases of H1N1 influenza had been confirmed in nearly all states within the United States and more than 1000 cases in 21 countries worldwide.8,12
For an updated tally and case counts in specific states, see the CDC's H1N1 Flu (Swine Flu) Web page. All affected patients have had mild influenzalike illness, with only two requiring brief hospitalization. As of May 1, 2009, only one death attributed to H1N1 flu had been reported in the United States, involving a 23-month-old child from Mexico who was visiting relatives in the United States.
Government and public health officials are monitoring this situation worldwide to assess the threat from H1N1 influenza and to provide guidance to health care professionals and the public. Because the situation is changing rapidly, it is important to check regularly for changes in recommendations as new information becomes available. Online resources for daily guidance include the Centers for Disease Control and Prevention (CDC), World Health Organization (WHO), and Medscape's H1N1 Influenza A (Swine Flu) Alert Center.
Morbidity and Mortality
H1N1 influenza (swine flu) tends to cause high morbidity but low mortality rates (1%-4%).
Symptoms
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 health care provider promptly. If an antiviral agent is warranted, it should ideally be initiated with 48 hours from the onset of symptoms (see Medications). 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