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INFLUENZA A(H1N1) VIRUS, SWINE, HUMAN - USA (TEXAS)
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A ProMED-mail post
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ProMED-mail is a program of the
International Society for Infectious Diseases
<http://www.isid.org>
Date: Mon 24 Nov 2008
Source: CIDRAP News online [edited]
<http://www.cidrap.umn.edu/cidrap/content/influenza/panflu/news/nov2408swine-br.html>
CDC reports swine flu virus in Texas patient
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The Centers for Disease Control and Prevention (CDC), in its latest
update on the nation's seasonal influenza activity, reported on a
person who was infected with a swine influenza virus following several
exposures to pigs, including a sick one. The CDC said the patient was
infected with a swine influenza A/H1N1 virus. Although human
infections with swine flu viruses are uncommon, many years bring
reports of isolated cases, the report said.
The Texas Department of State Health Services, in a flu surveillance
activity report for the week ending 15 Nov 2008, said the patient got
sick in mid-October. The patient's specimen was collected and the
virus identified during routine influenza surveillance. Texas
officials, who gave no details about [the patient's] illness, said
their investigation found no illnesses in [the patient's] household or
close contacts.
According to the CDC's background information on
swine flu, the agency receives about one human
influenza isolate each year that tests positive
for a swine influenza virus. H1N1 and H3N2 swine
flu viruses are endemic in US pig populations. In
September 2008, researchers from the CDC and
public health officials from Wisconsin published
a case report in Emerging Infectious Diseases on
a healthy 17-year-old boy who had mild
respiratory symptoms in December 2005, 3 days
after helping his brother-in-law butcher pigs. At
an outpatient clinic a few days later, health
care workers collected nasal wash specimens,
which tested positive for influenza A and were
forwarded to the Wisconsin State Laboratory of
Hygiene. Though further testing isolated
influenza A, the virus didn't match human H3 or
H1 subtypes or the H5 avian subtype. CDC
investigators sequenced the virus, identifying it
as a swine influenza A (H1N1) triple reassortant
virus, A/Wisconsin/87/2005 H1N1.
The report said that triple reassortant H1N1
subtypes are the predominant genotype in North
American pigs and that human swine flu illnesses
often mimic seasonal flu infections. The authors
recommended that clinicians ask patients with
unexplained influenza-like illnesses about
exposure to animals, including pigs, and visits
to petting zoos and county fairs.
Human infections with novel influenza A subtypes
now are nationally notifiable diseases in the
United States, the group reported. Though
human-to-human swine flu transmission is rare,
the CDC said human infections with swine H1N1
viruses should be investigated to ensure that
they are not spreading among humans -- as spread
could represent a pandemic threat -- and to
monitor changes in circulating viruses.
In 1988, an H1N1 swine flu virus was found in a
previously healthy 32-year-old pregnant woman who
died 8 days after she was hospitalized for
pneumonia, according to the CDC. 4 days before
she got sick she had visited a swine exhibit at a
county fair where a flu-like illness was
widespread among the pigs. Follow-up studies
showed that 76 percent of swine exhibitors had
antibodies to the swine flu virus, though no
illnesses were reported. However, researchers
found that one to 3 health care workers who had
contact with the woman experienced mild flu
symptoms with antibody evidence of swine flu exposure.
In December 2007, researchers reported that a new
swine flu subtype found recently in Missouri
pigs [H2N3] combined genes from avian and swine flu
viruses, could cause experimentally induced
infections in mice, and was transmissible in pigs
and ferrets. The findings, which appeared in
Proceedings of the National Academy of Sciences
(PNAS), bolstered the theory that pigs can serve
as a mixing vessel for flu viruses and a possible
source for a human pandemic strain.
The CDC said swine flu outbreaks in pigs
typically occur in late fall and winter months.
The agency said seasonal influenza vaccines are
likely to partially protect against swine H3N2
viruses, but not the H1N1 subtype.
[Byline: Lisa Schnirring]
--
Communicated by:
ProMED-mail Rapporteur Brent Barrett
[The following is reproduced from the CDC Web site
<http://www.cdc.gov/flu/swine/>
to put these observations in perspective.
"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 [another] 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 4 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.
"The H1N1 swine flu viruses are antigenically
very different from human H1N1 viruses. 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.
"Public health implications: 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 non-occupationally exposed persons
to ensure that human to human transmission is not
occurring and to monitor for changes in
circulating viruses and the emergence of novel 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 pandemic threat."
The original case report of the Wisconsin case referred to in the
above CIDRAP News report is the following:
Alexandra P. Newman et al., Human Case of Swine Influenza A (H1N1)
Triple Reassortant Virus Infection, Wisconsin, Emerging Infectious
Diseases, Vol.14, No.9, September 2008
<http://www.cdc.gov/eid/content/14/9/1470.htm>.
See also the CIDRAP News story: "New swine flu virus supports 'mixing
vessel' theory"
<http://www.cidrap.umn.edu/cidrap/content/biosecurity/ag-biosec/news/dec2007swine.html>.
- Mod.CP]
[see also:
2007
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Influenza A (H2N3) virus, swine - USA 20071219.4079
Influenza, swine, human - USA (IA): November 2006 20070108.0077]
...............................cp/ejp/jw
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