Signs and symptoms

In swine

In pigs influenza infection produces fever, lethargy, sneezing, coughing, difficulty breathing and decreased appetite.^[11] In some cases the infection can cause abortion. Although mortality is usually low (around 1–4%),^[2] the virus can produce weight loss and poor growth, causing economic loss to farmers.^[11] Infected pigs can lose up to 12 pounds of body weight over a 3 to 4 week period.^[11]

In humans

Main symptoms of swine flu in humans^[73]

Direct transmission of a swine flu virus from pigs to humans is occasionally possible (called zoonotic swine flu). In all, 50 cases are known to have occurred since the first report in medical literature in 1958, which have resulted in a total of six deaths.^[74] Of these six people, one was pregnant, one had leukemia, one had Hodgkin disease and two were known to be previously healthy.^[74] Despite these apparently low numbers of infections, the true rate of infection may be higher, since most cases only cause a very mild disease, and will probably never be reported or diagnosed.

According to the Centers for Disease Control and Prevention (CDC), in humans the symptoms of the 2009 "swine flu" H1N1 virus are similar to those of influenza and of influenza-like illness in general. Symptoms include fever, cough, sore throat, body aches, headache, chills and fatigue. The 2009 outbreak has shown an increased percentage of patients reporting diarrhea and vomiting.^[75] The 2009 H1N1 virus is not zoonotic swine flu, as it is not transmitted from pigs to humans, but from person to person.

Because these symptoms are not specific to swine flu, a differential diagnosis of probable swine flu requires not only symptoms but also a high likelihood of swine flu due to the person's recent history. For example, during the 2009 swine flu outbreak in the United States, CDC advised physicians to "consider swine influenza infection in the differential diagnosis of patients with acute febrile respiratory illness who have either been in contact with persons with confirmed swine flu, or who were in one of the five U.S. states that have reported swine flu cases or in Mexico during the 7 days preceding their illness onset."^[76] A diagnosis of confirmed swine flu requires laboratory testing of a respiratory sample (a simple nose and throat swab).^[76]

The most common cause of death is respiratory failure. Other causes of death are pneumonia (leading to sepsis)^[77], high fever (leading to neurological problems), dehydration (from excessive vomiting and diarrhea) and electrolyte imbalance. Fatalities are more likely in young children and the elderly.