Influenza H1N1 is a highly contagious respiratory disease in pigs that is caused by one of several swine influenza (flu) type A viruses. Transmission to humans occurs when a person comes in contact with infected pigs or environments contaminated with a swine influenza virus. Once a person becomes infected, the virus can be spread to other humans by coughing or sneezing.
Influenza H1N1 causes acute febrile illness and other systemic symptoms ranging from mild fatigue to respiratory failure and death. In the Northern and Southern hemispheres, the virus is more prevalent in the winter months. In April 2009, U.S. public health officials declared swine influenza A (H1N1) virus a nationwide public health emergency. By early January 2010, the World Health Organization announced that 208 countries and territories had reported laboratory-confirmed cases of influenza H1N1, including 12,799 deaths worldwide. However, this number of reported cases undervalues the true number because many countries are no longer required to test and report individual cases.
Influenza virus may be one of three basic types (A, B, or C), depending on the number of gene segments surrounding the RNA viral core. H1N1, commonly referred to as swine flu virus, is one of the most common prevailing subtypes of influenza A, a zoonotic infection with over 100 types infecting most species of birds, pigs, horses, dogs, and seals. In humans, it is spread by respiratory secretions from an infected person. The virus then invades and replicates in the airway and respiratory tract cells, causing systemic symptoms from inflammatory mediators. The incubation period of influenza ranges from 18 to 72 hours.
Complications from influenza H1N1 include acute respiratory distress, conjunctivitis, myocarditis, pericarditis, pneumonia, sepsis, organ failure, and death.
Patients infected with influenza H1N1 may present with mild symptoms or severe illness. Initial symptoms may include high fever (as high as 104° F [40° C]), mild to severe myalgia, rhinorrhea, and sore throat. Tachycardia may occur from hypoxia, fever, or both. The skin may be warm or hot, depending on temperature status. Nausea and vomiting may occur, and diarrhea is common in children. This may produce signs of dehydration. Frontal orbital headache is often severe and accompanied by photophobia, burning sensations, or pain upon motion. Weakness and severe fatigue may prevent patients from performing their activities of daily living and lead to the need for bed rest. A nonproductive cough, pleuritic chest pain, and dyspnea may occur. Acute encephalopathy may occur in patients with influenza A virus, producing altered mental status, coma, seizures, and ataxia.