Note: This page contains background information about swine influenza infections in humans. For information about 2009 H1N1 influenza (initially referred to as “swine flu” when it was first detected), visit the archived CDC 2009 H1N1 Flu website. For information about seasonal influenza, visit the CDC seasonal flu website.
Swine Influenza (swine flu) is a respiratory disease of pigs caused by type A influenza viruses that regularly cause outbreaks of influenza in pigs. Swine flu viruses can cause high levels of illness in swine herds, but usually cause few deaths. Common symptoms include fever, depression, coughing (barking), discharge from the nose or eyes, sneezing, breathing difficulties, eye redness or inflammation, and going off feed. However, influenza-infected pigs may also not appear ill or be only mildly ill. Swine influenza viruses may circulate among swine throughout the year, but most outbreaks occur during the late fall and winter months similar to outbreaks in humans. The classical swine flu virus (an influenza type A H1N1 virus) was first isolated from a pig in 1930.
Swine flu viruses do not normally infect humans. However, sporadic human infections with swine flu have occurred. Most commonly, these cases occur in persons exposed to infected pigs (e.g. children near pigs at a fair or workers in the swine industry). In addition, there have been documented cases of multiple persons becoming sick after exposure to one or a group of sick pigs and also cases of limited human to human spread of swine influenza viruses. For example, an outbreak of apparent swine flu infection in pigs in Wisconsin in 1988 resulted in multiple human infections, and, although no community outbreak resulted, there was antibody evidence of virus transmission from the infected person to health care workers who had close contact with the patient.
In the past, CDC received reports of approximately one human infected with swine influenza virus every one to two years in the U.S., but from December 2005 through December 2010, 19 cases of human infection with swine influenza have been reported.
Influenza viruses that infect pigs are different from human influenza viruses. Thus, influenza vaccines made against human strains of influenza viruses are generally not expected to protect people from influenza strains circulating in pigs. In addition, because pigs are susceptible to avian, human and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus that can spread easily from person-to-person. This type of major change in the influenza A viruses is known as antigenic shift. Antigenic shift results when a new influenza A subtype to which most people have little or no immune protection infects humans. If this new virus causes illness in people and can be transmitted easily from person to person, an influenza pandemic can occur. This is what occurred in 2009 when an influenza A H1N1 virus with swine, avian and human genes emerged in the Spring of 2009 and caused the first global pandemic in more than 40 years.
The symptoms of swine flu in people are expected to be similar to the symptoms of regular human seasonal influenza and include fever, lethargy, lack of appetite and coughing. Some people with swine flu also have reported runny nose, sore throat, eye irritation, nausea, vomiting and diarrhea.
No. Swine influenza viruses are not transmitted by food. You cannot get swine influenza from eating pork or pork products. Eating properly handled and cooked pork and pork products is safe.
Influenza viruses can be directly transmitted from pigs to people and from people to pigs. Human infection with flu viruses from pigs are most likely to occur when people are in close proximity to infected pigs, such as in pig barns and livestock exhibits housing pigs at fairs. Human-to-human transmission of swine flu also can occur. This is thought to occur in the same way as seasonal flu occurs in people, which is mainly person-to-person transmission through coughing or sneezing of people infected with the influenza virus. People may become infected by touching something with flu viruses on it and then touching their mouth or nose.
In September 1988, a previously healthy 32-year-old pregnant woman was hospitalized for pneumonia and died 8 days later. A swine H1N1 flu virus was detected. Four days before getting sick, the patient visited a county fair swine exhibition where there was widespread influenza-like illness among the swine.
In follow-up studies, 76% of swine exhibitors tested had antibody evidence of swine flu infection but no serious illnesses were detected among this group. Additional studies suggest that one to three health care personnel who had contact with the patient developed mild influenza-like illnesses with antibody evidence of swine flu infection.
To diagnose swine influenza A infection, a respiratory specimen would generally need to be collected within the first 4 to 5 days of illness (when an infected person is most likely to be shedding virus). However, some persons, especially children, may shed virus for 10 days or longer. Identification as a swine influenza A virus and testing for changes in virus genes requires sending the specimen to CDC for laboratory testing.
There are four different antiviral drugs that are licensed for use in the United States for the treatment of influenza: amantadine, rimantadine, oseltamivir and zanamivir. While most swine influenza viruses have been susceptible to all four drugs, the most recent seven swine influenza viruses isolated from humans are resistant to amantadine and rimantadine. At this time, CDC recommends the use of oseltamivir or zanamivir for the treatment and/or prevention of infection with swine influenza viruses. More information on treatment recommendations.
Probably the most well known is an outbreak of swine flu among soldiers in Fort Dix, New Jersey in 1976. The virus caused disease with x-ray evidence of pneumonia in at least four soldiers with one death; all of these patients had previously been healthy. The virus was transmitted to close contacts in a basic training environment, with limited transmission outside the basic training group. The virus is thought to have circulated for a month and disappeared. The source of the virus, the exact time of its introduction into Fort Dix, and factors limiting its spread and duration are unknown. The Fort Dix outbreak may have been caused by introduction of an animal virus into a stressed human population in close contact in crowded facilities during the winter. The swine influenza A virus collected from a Fort Dix soldier was named A/New Jersey/76 (Hsw1N1).
No. The 2009 H1N1 viruses were antigenically and genetically very different from previously circulating human H1N1 viruses and, therefore, vaccines for human seasonal flu did not provide protection from 2009 H1N1 viruses. More information about 2009 H1N1 flu.