QUESTIONS & ANSWERS
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. Swine flu viruses cause high levels of illness and low death rates among pigs.
Swine influenza viruses may circulate in swine, including swine in Illinois.
Biology of swine influenza virus
Influenza A viruses in swine are categorized into subtypes based on two viral surface
antigens called hemagglutinin (H) and neuraminidase (N). Like all influenza viruses,
swine flu viruses change constantly. Pigs can be infected by avian influenza and
human influenza viruses as well as swine flu 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 H3N2 and H1N1 viruses.
How common is swine flu among pigs?
Outbreaks among pigs normally occur in late fall and winter and sometimes with the introduction of new pigs into herds. In the United States, studies have shown that 30 percent to 51 percent of the pig population has antibody evidence of having had
H1N1 infection. Antibody can be due to vaccination of pigs for swine flu or from natural
infection with swine flu. There is currently no way to differentiate antibody produced in
response to vaccination in pigs from antibody made in response to natural infection with H1N1.
Can humans catch swine flu?
Swine flu viruses do not normally infect humans. However, rare human infections with
swine flu have occurred. In the past several years, CDC has received, on average,
about one influenza virus isolate from a human that tests positive for swine flu each
year. Most commonly, these cases occur in persons with direct exposure to pigs. In
addition, there have been rare 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 does swine flu spread?
Pigs infected with influenza virus have a runny nose, lethargy, cough and decreased
appetite. The virus likely spreads from pig to pig through contact with infected mucous
secretions. (When pigs are really sick, their mucous carries high levels of virus).
Strains of swine flu virus also can be directly transmissible to humans. Most human
infections occurred following direct contact with infected pigs. However, there has been
at least one documented case of human-to-human transmission of swine flu.
What examples of swine flu outbreaks are there?
Probably most well-known is an outbreak of swine flu among soldiers in Fort Dix, New
Jersey, in 1976. The virus caused disease with pneumonia in at least four soldiers and
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 an animal anomaly caused by introduction of an animal virus into a stressed
human population in close contact in crowded facilities during the winter.
Is the H1N1 swine flu virus the same as human H1N1 viruses?
No. The H1N1 swine flu viruses are antigenically very different from human
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.
What are the public health implications of human infections with swine influenza
Human infections with animal influenza A viruses against which the human population
has little immunity should be investigated to determine the source of infection, and the
extent of spread and evidence of human-to-human transmission. Influenza A viruses
new to the human population that are able to efficiently transmit from person to person
and cause illness may represent a pandemic threat.
Although immunity to swine H1N1 viruses is low in the human population, a
high proportion of persons occupationally exposed to pigs (such as pig farmers or pig
veterinarians) have been shown in several studies to have antibody evidence of prior
swine H1N1 flu infection. And, for swine H1N1 viruses, only rare person-to-person
transmission has been documented in the past. Thus, human infections with swine
H1N1 viruses should be investigated particularly when they are detected among nonoccupationally
exposed persons to ensure that human-to-human transmission is not
occurring and to monitor for changes in circulating viruses.
Because most persons have some antibody to influenza H3N2 viruses since H3N2
viruses occur commonly in humans and because the swine and human H3N2 viruses
are similar, swine H3N2 virus infections in humans would not represent a possible
Adapted from CDC’s “Key Facts about Swine Influenza (Swine Flu)”
of Public Health
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Springfield, Illinois 62761
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