H1N1 is a respiratory disease of pigs caused by type A influenza virus that regularly causes outbreaks of influenza in pigs. H1N1 viruses cause high levels of illness and low death rates in pigs. H1N1 viruses may circulate among swine throughout the year, but most outbreaks occur during the late fall and winter months similar to outbreaks in humans.

In the past, CDC received reports of approximately one H1N1 virus infection every one to two years in the U.S., but from December 2005 through February 2009, 12 cases of human infection with H1N1 have been reported.

The symptoms of H1N1 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 H1N1 also have reported runny nose, sore throat, nausea, vomiting, and diarrhea.

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 H1N1 can also 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 on 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. An H1N1 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 H1N1 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 H1N1 infection.

To diagnose H1N1 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 H1N1 virus requires sending the specimen to CDC for laboratory testing.

There are four different antiviral drugs that are licensed for use in the U.S. for the treatment of influenza: amantadine, rimantadine, oseltamivir, and zanamivir. While most similar viruses have been susceptible to all four drugs, the most recent H1N1 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 H1N1 viruses.

Probably the most well known is an outbreak among soldiers in Fort Dix, New Jersey in 1976. The virus caused disease with x-ray evidence of pneumonia in at least 4 soldiers and 1 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 virus collected from a Fort Dix soldier was named A/New Jersey/76 (Hsw1N1).

Vaccines are available to be given to pigs to prevent H1N1. There is no vaccine to protect humans from H1N1. The seasonal influenza vaccine will likely help provide partial protection against swine H3N2, but not swine H1N1 viruses.