Key Facts about Swine Influenza (Swine Flu)

What is Swine Influenza?
Swine Influenza (swine flu) is a respiratory disease of pigs caused by type A influenza virus that regularly causes outbreaks of influenza in pigs. Swine flu viruses cause high levels of illness and low death rates in pigs.

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.

How many swine flu viruses are there?
Like all influenza viruses, swine flu viruses change constantly. Pigs can be infected by avian influenza and human influenza viruses as well as swine influenza viruses. When influenza viruses from different species infect pigs, the viruses can reassort (i.e. swap genes) and new viruses that are a mix of swine, human and/or avian influenza viruses can emerge.

Over the years, different variations of swine flu viruses have emerged. At this time, there are four main influenza type A virus subtypes that have been isolated in pigs: H1N1, H1N2, H3N2, and H3N1. However, most of the recently isolated influenza viruses from pigs have been H1N1 viruses.

Swine Flu in Humans
Can humans catch swine flu?
Swine flu viruses do not normally infect humans. However, sporadic human infections with swine flu have occurred. Most commonly, these cases occur in persons with direct exposure to pigs (e.g. children near pigs at a fair or workers in the swine industry).

In addition, there have been documented cases of one person spreading swine flu to others. 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 patient to health care workers who had close contact with the patient.

How common is swine flu infection in humans?
In the past, CDC received reports of approximately one human swine influenza virus infection every one to two years in the U.S., but from December 2005 through February 2009, 12 cases of human infection with swine influenza have been reported.

What are the symptoms of swine flu in humans?
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.

Can people catch swine flu from eating pork?
No. Swine influenza viruses are not transmitted by food. You can not get swine influenza from eating pork or pork products. Eating properly handled and cooked pork and pork products is safe. Cooking pork to an internal temperature of 160°F kills the swine flu virus as it does other bacteria and viruses.

How does swine flu spread?
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.

What do we know about human-to-human spread of swine flu?
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.

How can human infections with swine influenza be diagnosed?
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 flu influenza A virus requires sending the specimen to CDC for laboratory testing.

What medications are available to treat swine flu infections in humans?
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.

What other examples of swine flu outbreaks are there?
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 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 swine influenza A virus collected from a Fort Dix soldier was named A/New Jersey/76 (Hsw1N1).

Is the H1N1 swine flu virus the same as human H1N1 viruses?
No. The H1N1 swine flu viruses are antigenically very different from human H1N1 viruses and, therefore, vaccines for human seasonal flu would not provide protection from H1N1 swine flu viruses.

Swine Flu in Pigs
How does swine flu spread among pigs?
Swine flu viruses are thought to be spread mostly through close contact among pigs and possibly from contaminated objects moving between infected and uninfected pigs. Herds with continuous swine flu infections and herds that are vaccinated against swine flu may have sporadic disease, or may show only mild or no symptoms of infection.

What are signs of swine flu in pigs?
Signs of swine flu in pigs can include sudden onset of fever, depression, coughing (barking), discharge from the nose or eyes, sneezing, breathing difficulties, eye redness or inflammation, and going off feed.

How common is swine flu among pigs?
H1N1 and H3N2 swine flu viruses are endemic among pig populations in the United States and something that the industry deals with routinely. Outbreaks among pigs normally occur in colder weather months (late fall and winter) and sometimes with the introduction of new pigs into susceptible herds. Studies have shown that the swine flu H1N1 is common throughout pig populations worldwide, with 25 percent of animals showing antibody evidence of infection.

In the U.S. studies have shown that 30 percent of the pig population has antibody evidence of having had H1N1 infection. More specifically, 51 percent of pigs in the north-central U.S. have been shown to have antibody evidence of infection with swine H1N1. Human infections with swine flu H1N1 viruses are rare. There is currently no way to differentiate antibody produced in response to flu vaccination in pigs from antibody made in response to pig infections with swine H1N1 influenza.

While H1N1 swine viruses have been known to circulate among pig populations since at least 1930, H3N2 influenza viruses did not begin circulating among US pigs until 1998. The H3N2 viruses initially were introduced into the pig population from humans. The current swine flu H3N2 viruses are closely related to human H3N2 viruses.

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

INFLUENZA: Pigs, People and Public Health (Fact Sheet)External Web Site Policy
http://www.pork.org/PorkScience/Documents/PUBLICHEALTH%20influenza.pdf

Summary: Swine influenza viruses were first isolated in the United States in 1930. Since that time, they have become an economically important cause of respiratory disease in pigs throughout the world, and a human public health risk. The clinical signs/symptoms of influenza in pigs and people are remarkably similar, with fever, lethargy, lack of appetite and coughing prominent in both species.

Furthermore, influenza viruses can be directly transmitted from pigs to people as "zoonotic" disease agents, and vice versa, from people to pigs. These interspecies infections are most likely to occur when people are in close proximity to pigs, such as in swine production barns, livestock exhibits at fairs, and slaughterhouses. Finally, because of their unique susceptibility to infection with influenza viruses of both mammalian and avian species, pigs can serve as intermediaries in the transmission of influenza viruses from birds to people.

The birds of greatest concern are wild waterfowl, because these species provide an immense natural reservoir of influenza viruses. Replication of avian influenza viruses in pigs may allow them to adapt to and be able to efficiently infect mammals, and ultimately be transmitted to people.

In addition, pigs can serve as hosts in which two (or more) influenza viruses can undergo "genetic reassortment." This is a process in which influenza viruses exchange genes during replication. The influenza viruses responsible for the worldwide 1957 and 1968 "pandemics" of human influenza were reassortant viruses with genes from both human and avian influenza viruses. Veterinarians can help pig producers design farms and develop management and personnel policies to minimize interspecies transmission of influenza viruses, thereby contributing to the health of both the swine and human populations.

Background: Influenza viruses exist in three "types," designated A, B and C. Of these, only influenza A viruses are significant concerns for the health of pigs. However, there are a large number of different "subtypes" of influenza A viruses. These subtypes are defined by the hemagglutinin (H or HA) and neuraminidase (N or NA) proteins of the virus. The HA is also the protein against which the host directs antibodies that can neutralize the virus. Of practical significance, there is no cross-protective immunity mediated by antibodies from one HA subtype to another.

There are 15 different subtypes of hemagglutinin and 9 different subtypes of neuraminidase among influenza A viruses. Subtypes are distinguished by differences in their genetic sequences, which translate into differences in their antigenic structure.

The combination of HA and NA subtypes present in a virus are depicted by H and N designations, such as H1N1, H3N2, and so on. In the course of history, relatively few hemagglutinin and neuraminidase combinations have consistently circulated among pigs or people (predominantly H1N1, H1N2 and H3N2 in pigs, and H1N1, H1N2, H2N2 and H3N2 in people).

In contrast, virtually all of the possible influenza A virus subtypes exist among wild waterfowl. In these birds, influenza viruses infect the gastrointestinal tract rather than the respiratory tract, which is the target organ in pigs, people, horses and other mammalian hosts of influenza viruses.

The infections generally do not make the birds sick. In waterfowl, the viruses are shed in the bird’s feces, and ultimately into the water of lakes and ponds that the birds visit during migrations, but also potentially onto the ground of barnyards and farm fields.

Influenza viruses carry their genes on 8 separate pieces ("segments") of nucleic acid (RNA), rather than on one long single molecule. This structural feature has very important implications for virus evolution, because if two (or more) influenza viruses simultaneously infect cells in the same individual, then during replication, these viruses can exchange RNA segments with one another, thereby creating viruses with entirely new combinations of genes. This process of reassortment was the basis for the appearance of the pandemic viruses of 1957 (the "Asian" flu) and 1968 (the "Hong Kong" flu) in the human population.

These pandemic viruses were responsible for millions of cases of human illness and tens of thousands of human deaths. In both cases, influenza viruses from waterfowl reassorted with the previously circulating human influenza viruses to create viruses with different hemagglutinin subtypes (from H1 to H2 in 1957 and from H2 to H3 in 1968). It is the change to a hemagglutinin subtype against which the population has no immunity ("antigenic shift") that causes these periodic global disease outbreaks of human disease.

How does this process of reassortment occur? In general, there is a functional barrier to infection of people with avian influenza viruses, and vice versa (the H5N1 infections of people in Hong Kong and China in 1997 and 2003 and the H7N7 infections of poultry workers and veterinarians in The Netherlands in 2003 being exceptions).

This barrier is based, in part, on the fact that avian influenza viruses preferentially use receptors expressed on bird cells, and human viruses preferentially use receptors expressed in the human respiratory tract. Pigs, however, express both avian- and human-type receptors and can be infected with avian, human and swine influenza viruses. As such, they can serve as hosts in which avian viruses adapt to replication in mammals.

For example, in 1979, an avian H1N1 virus of waterfowl-origin entered the pig population of northern Europe and soon became the dominant cause of influenza among European pigs. Subsequently, these avian H1N1 viruses were also isolated from people in Europe.

Additionally, pigs are hypothesized to serve as the "mixing vessels" in which reassortment between avian and human influenza viruses can take place. The focus of such reassortment has historically been in Southeast Asia, the proposed "influenza epicenter," because agricultural practices in this region brought pigs, people and ducks into close contact with one another.

However, it is now clear that influenza virus reassortment in pigs can occur anywhere in the world, as evidenced by reassortant viruses isolated from pigs in Europe and, most recently, in the United States. The later include human/swine/avian virus reassortant H3N2 viruses that have spread widely within the American pig population since their emergence in 1998, as well as "second generation" reassortant H1N2 and H1N1 viruses derived by genetic mixing between the reassortant H3N2 and classical swine H1N1 viruses. The H1N2 viruses have also been isolated subsequently from wild waterfowl and domestic turkeys.

The isolation of these viruses from wild ducks was somewhat unexpected, but interspecies transmission of influenza viruses from pigs to domestic turkeys has been recognized previously on numerous occasions. In fact, turkey producers sometimes vaccinate their birds against swine virus infections. In contrast, transmission of influenza viruses between pigs and domestic chickens and other fowl, and vice versa, is very rarely reported.

Reducing interspecies transmission of influenza viruses: It is in the best interest of both human public health and animal health that transmission of influenza viruses from pigs to people, from people to pigs, from birds to pigs and from pigs to birds be minimized.

Interspecies transmission among pigs and people: About two dozen examples of zoonotic transmission of swine influenza viruses from pigs to people have been documented in the medical literature. Many more cases are likely to occur among swine workers. However, these will generally go unrecognized as anything but typical human influenza because the seasonal patterns of human and swine influenza largely overlap. A recent study by the author and colleagues from the Centers for Disease Control and Prevention sought to better understand the risks of zoonotic swine flu infections in the United States.

In studying swine farmers, employees and their family members compared to an urban population from Milwaukee, Wisconsin, the factors most strongly associated with seropositivity to swine viruses were being a swine farm owner and/or a member of a farm owner’s family, living on a swine farm, or entering a swine barn at least 4 days per week. ( See suggested reading: C.W. Olsen, et al., Serologic evidence of H1 swine influenza virus infection in swine farm residents and employees, Emerg. Infect. Dis. 8 (2002) 814-819).

Conversely, the impact of transmission of influenza viruses from people to pigs should not be under-estimated. The reassortant H3N2, H1N2 and H1N1 viruses currently circulating widely and causing disease throughout the swine population of the United States all contain human influenza virus genes.

The following steps are potentially useful to reduce transmission of influenza viruses between pigs and people:
■ Influenza virus vaccination of pigs - While the swine influenza virus vaccines used today may not induce sterilizing immunity nor completely eliminate clinical signs of infection, vaccination of pigs can reduce the levels of virus shed by infected animals, and thus reduce the potential for human exposure and zoonotic infections.

■ Influenza virus vaccination of swine farm workers - The vaccines produced on a yearly basis for the human population contain only human, not swine, strains of influenza viruses. Nonetheless, these vaccines are likely to provide some level of protection against infection with swine viruses of the same hemagglutinin subtype.

Conversely, vaccination of farm workers will reduce the amounts of viruses they shed if infected during human influenza outbreaks, and thereby limit the potential for human influenza virus infection of their pigs.

■ Sick-leave policies - To further reduce the chances for infection of pigs with human influenza viruses, the farm owner should provide sick-leave policies for employees that encourage them to remain away from work when they are suffering from acute respiratory infections. People typically shed influenza viruses for approximately 3-7 days, with the period of peak shedding correlated with the time of most severe clinical illness.

■ Ventilation - Ventilation systems in containment production facilities should be designed to minimizere-circulation of air within animal housing rooms. This is important to reduce the exposure of pigs to viruses from other pigs, to reduce their exposure to human influenza viruses, and conversely, to reduce exposure of workers to swine influenza viruses.

■ Basic hygiene practices -Workers should change clothes prior to leaving swine barns for office facilities, food breaks or their homes. In addition, hand-to-face contact should be minimized and hand-washing stations should be available throughout the animal housing areas. Influenza viruses spread not just by inhalation of aerosolized virus, but also by eye and nose contact with droplets of respiratory secretions.

Interspecies transmission among pigs and birds: The global reservoir of influenza viruses in waterfowl, the examples of infection of pigs with waterfowl-origin influenza viruses, the risks for reassortment of avian viruses with swine and/or human influenza viruses in pigs, and the risk for transmission of influenza viruses from pigs to domestic turkeys all indicate that contact between pigs and both wild and domestic fowl should be minimized. The following factors are potentially useful to reduce transmission of influenza viruses between birds and pigs:

■ Bird-proofing - All doorways, windows and air-flow vents in swine housing units should be adequately sealed or screened to prevent entrance of birds. Although small birds such as sparrows, swallows, finches, wrens etc. are not thought to be important in the overall ecology of influenza viruses, they may carry influenza viruses from waterfowl feces into barns on their bodies.

■ Water treatment - Do not use untreated surface water (because of waterfowl fecal contamination with influenza viruses) as either drinking water or water for cleaning in swine facilities. Likewise, it may be prudent to attempt to minimize waterfowl use of farm lagoons.

■ Separation of pig and bird production - Do not raise pigs and domestic fowl on the same premises.

■ Feed security - Keep pig feed in closed containers to prevent contamination with feces from over-flying waterfowl.

■ Worker biosecurity - Provide boots for workers that are worn only within the pig housing units, thus eliminating the chance to carry bird feces into housing units from outdoors.

These recommendations clearly cannot apply to production units in which pigs are raised outdoors. Outdoor housing places pigs at increased risk for infection with avian influenza viruses.

Suggested reading:
B.C. Easterday, K. Van Reeth, Swine influenza, in: B.E. Straw, S. D’Allaire, W.L. Mengeling, D.J. Taylor (Eds.), Disease of Swine (8th Edition), Iowa State University Press, Ames, 1999, pp. 277-290.

V.S. Hinshaw, R.G. Webster, W.J. Bean, J. Dowdle, D.A. Senne, Swine influenza viruses in turkeys - a potential source of virus for humans?, Science 220 (1983) 206-208.

A.I. Karasin, I.H. Brown, S. Carman, C.W. Olsen, Isolation and characterization of H4N6 avian influenza viruses from pigs with pneumonia in Canada, J. Virol. 74 (2000) 9322-9327.

H. Kida, T. Ito, J. Yasuda, C. Shimuzi, C. Itakura, K.F. Shortridge, Y. Kawaoka, R.G. Webster, Potential for transmission of avian influenza viruses to pigs, J. Gen.Virol. 75 (1994) 2183-218.

C.W. Olsen, Emergence of novel strains of swine influenza virus in North America, in: A. Morilla, K.-J. Yoon, J.J. Zimmerman (Eds.), Trends in Emerging Viral Infections of Swine, Iowa State University Press, Ames, 2002, pp. 37-43.

C.W. Olsen, The emergence of novel swine influenza viruses in North America, Virus Res. 85 (2002) 199-210.

C.W. Olsen, L. Brammer, B.C. Easterday, N. Arden, E. Belay, I. Baker, N.J. Cox, Serologic evidence of H1 swine influenza virus infection in swine farm residents and employees, Emerg. Infect. Dis. 8 (2002) 814-819.

C.W. Olsen, A. Karasin, G. Erickson, Characterization of a swine-like reassortant H1N2 influenza virus isolated from a wild duck in the United States, Virus Res. 93 (2003) 115-121.

P.R. Schnurrenberger, G.T. Woods, R.J. Martin, Serologic evidence of human infection with swine influenza virus, Am. Rev. Respir. Dis. 102 (1970) 356-361.

C. Scholtissek, V.S. Hinshaw, C.W. Olsen, Influenza in pigs and their role as the intermediate host, in: K.G. Nicholson, R.G. Webster, A. Hay (Eds.), Textbook of Influenza, Blackwell Healthcare Communications,  Oxford, 1998, 137-145.

D.L. Suarez, P.R. Woolcock, A.J. Bermudez, D.A. Senne, Isolation from turkey breeder hens of a reassortant H1N2 influenza virus with swine, human, and avian lineage genes, Avian Dis. 46 (2002) 111-121.

R.G. Webster, W.J. Bean, O.T. Gorman, T.M. Chambers, Y. Kawaoka, Evolution and ecology of influenza A viruses, Microbiol. Rev. 56 (1992) 152-179.

G.T. Woods, L.E. Hanson, R.D. Hatch, Investigation of four outbreaks of acute respiratory disease in swine and isolation of swine influenza virus, Health Lab. Sci. 5 (1968) 218-224.

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