Swine Influenza (swine flu) is a respiratory disease of pigs caused by type A influenza that regularly cause outbreaks of influenza among pigs. Swine flu viruses do not normally infect humans, however, human infections with swine flu do occur, and cases of human-to-human spread of swine flu viruses has been documented.
Swine influenza virus (referred to as Swine influenza viruses or SIV) refers to influenza cases that are caused by Orthomyxoviruses endemic to populations of pigs. SIV strains isolated to date have been classified either as Influenzavirus C or one of the various subtypes of the genus Influenzavirus A.
Swine flu infects people every year and is found typically in people who have been in contact with pigs, although there have been cases of person-to-person transmission. Symptoms include: fever, disorientation, stiffness of the joints, vomiting, and loss of conciousness ending in death. Influenza A virus subtypes H1N1, H1N2, H3N1 and H3N2 are all known to cause SIV infections. and H2N3.
In swine, three influenza A virus subtypes (H1N1, H3N2, and H1N2) are circulating throughout the world. In the United States, the H1N1 subtype was exclusively prevalent among swine populations before 1998; however, since late August 1998, H3N2 subtypes have been isolated from pigs. Most H3N2 virus isolates are triple reassortants, meaning that it contains genes from human (HA, NA, and PB1), swine (NS, NP, and M), and avian (PB2 and PA) lineages.
In March and April 2009, more than 1,000 cases of swine flu in humans were detected in Mexico, and more than 80 deaths are suspected to have a connection with the virus. As of April 25, 2009 19:30 EDT there are 11 laboratory confirmed cases in the southwestern United States, New York City metropolitan area, and in Kansas. Following a series of reports of isolated cases of swine flu, the first announcement of the outbreak in Mexico was documented on April 23, 2009. Some of the cases have been confirmed by the World Health Organization to be due to a new genetic strain of H1N1. The new strain has been confirmed in 16 of the deaths and 44 others are being tested as of April 24, 2009. The Mexican fatalities are said to be mainly young adults, a hallmark of pandemic flu.
The new strain appears to be a recombinant between two older strains. Preliminary genetic characterization found that the hemagglutinin (HA) gene was similar to that of swine flu viruses present in U.S. pigs since 1999, but the neuraminidase (NA) and matrix protein (M) genes resembled versions present in European swine flu isolates. Viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, but there is no formal national surveillance system to determine what viruses are circulating in pigs in the U.S.
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, 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 swine flu 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 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.
There are four different antiviral drugs that are licensed for use in the US 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 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.