Key Facts about Swine Influenza (Swine Flu)
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.
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).
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.