Influenza A virus is the subtype of influenza A virus that was the most common cause of
human influenza in 2009. It is an Orthomyxovirus that contains the glycoproteins Haemagglutinin
and Neuraminidase. For this reason, they are described as H1N1, H1N2 etc. depending on
the type of H or N antigens they express. Haemagglutin causes red blood cells to clump
together and binds the virus to the infected cell. Neuraminidase are a type of glycoside
hydrolase enzyme which help to move the virus particles through the infected cell and assist
in budding from the host cells. Some strains of H1N1 are endemic in humans
and cause a small fraction of all influenza-like illness and a small fraction of all seasonal
influenza. H1N1 strains caused a small percentage of all human flu infections in 2004–2005.
Other strains of H1N1 are endemic in pigs and in birds.
In June 2009, the World Health Organization declared the new strain of swine-origin H1N1
as a pandemic. This strain is often called swine flu by the public media. This novel
virus spread worldwide and had caused about 17,000 deaths by the start of 2010. On August
10, 2010, the World Health Organization declared the H1N1 influenza pandemic over, saying worldwide
flu activity had returned to typical seasonal patterns.
Swine influenza Swine influenza is a respiratory disease that
occurs in pigs that is caused by the Influenza A virus. Influenza viruses that are normally
found in swine are known as swine influenza viruses. The known SIV strains include influenza
C and the subtypes of influenza A known as H1N1, H1N2, H3N1, H3N2 and H2N3. Pigs can
also become infected with the H4N6 and H9N2 subtypes.
Swine influenza virus is common throughout pig populations worldwide. Transmission of
the virus from pigs to humans is not common and does not always lead to human influenza,
often resulting only in the production of antibodies in the blood. If transmission does
cause human influenza, it is called zoonotic swine flu or a variant virus. People with
regular exposure to pigs are at increased risk of swine flu infection. The meat of an
infected animal poses no risk of infection when properly cooked.
Pigs experimentally infected with the strain of swine flu that caused the human pandemic
of 2009-10 showed clinical signs of flu within four days, and the virus spread to other uninfected
pigs housed with the infected ones. During the mid-20th century, identification
of influenza subtypes became possible, allowing accurate diagnosis of transmission to humans.
Since then, only 50 such transmissions have been confirmed. These strains of swine flu
rarely pass from human to human. Symptoms of zoonotic swine flu in humans are similar
to those of influenza and of influenza-like illness in general, namely chills, fever,
sore throat, muscle pains, severe headache, coughing, weakness, and general discomfort.
The recommended time of isolation is about five days.
Notable incidents Spanish flu
The Spanish flu, also known as la grippe, La Gripe Española, or La Pesadilla, was an
unusually severe and deadly strain of avian influenza, a viral infectious disease, that
killed some 50 to 100 million people worldwide over about a year in 1918 and 1919. It is
thought to be one of the deadliest pandemics in human history.
The 1918 flu caused an unusual number of deaths, possibly due to it causing a cytokine storm
in the body. The Spanish flu virus infected lung cells, leading to overstimulation of
the immune system via release of cytokines into the lung tissue. This leads to extensive
leukocyte migration towards the lungs, causing destruction of lung tissue and secretion of
liquid into the organ. This makes it difficult for the patient to breathe. In contrast to
other pandemics, which mostly kill the old and the very young, the 1918 pandemic killed
unusual numbers of young adults, which may have been due to their healthy immune systems
mounting a too-strong and damaging response to the infection.
The term "Spanish" flu was coined because Spain was at the time the only European country
where the press were printing reports of the outbreak, which had killed thousands in the
armies fighting World War I. Other countries suppressed the news in order to protect morale.
Fort Dix outbreak
In 1976, a novel swine influenza A caused severe respiratory illness in 13 soldiers
with 1 death at Fort Dix, New Jersey. The virus was detected only from January 19 to
February 9 and did not spread beyond Fort Dix. Retrospective serologic testing subsequently
demonstrated that up to 230 soldiers had been infected with the novel virus, which was an
H1N1 strain. The cause of the outbreak is still unknown and no exposure to pigs was
identified. Russian flu
The 1977–1978 Russian flu epidemic was caused by strain Influenza A90/77. It infected mostly
children and young adults under 23 because a similar strain was prevalent in 1947–57,
causing most adults to have substantial immunity. Because of a striking similarity in the viral
RNA of both strains – one which is unlikely to appear in nature due to antigenic drift
– it was speculated that the later outbreak was due to a laboratory incident in Russia
or Northern China, though this was denied by scientists in those countries. The virus
was included in the 1978–1979 influenza vaccine.
See also 1889–1890 flu pandemic for the earlier Russian flu pandemic caused either
by H3N8 or H2N2 2009 A(H1N1) pandemic
In the 2009 flu pandemic, the virus isolated from patients in the United States was found
to be made up of genetic elements from four different flu viruses – North American swine
influenza, North American avian influenza, human influenza, and swine influenza virus
typically found in Asia and Europe – "an unusually mongrelised mix of genetic sequences."
This new strain appears to be a result of reassortment of human influenza and swine
influenza viruses, in all four different strains of subtype H1N1.
Preliminary genetic characterization found that the hemagglutinin gene was similar to
that of swine flu viruses present in U.S. pigs since 1999, but the neuraminidase and
matrix protein genes resembled versions present in European swine flu isolates. The six genes
from American swine flu are themselves mixtures of swine flu, bird flu, and human flu viruses.
While viruses with this genetic makeup had not previously been found to be circulating
in humans or pigs, there is no formal national surveillance system to determine what viruses
are circulating in pigs in the U.S. In April 2009, an outbreak of influenza-like
illness occurred in the United States and then in Mexico; the CDC reported seven cases
of novel A/H1N1 influenza. By April 24 it became clear that the outbreak of ILI in Mexico
and the confirmed cases of novel influenza A in the southwest US were related and WHO
issued a health advisory on the outbreak of "influenza-like illness in the United States
and Mexico". The disease then spread very rapidly, with the number of confirmed cases
rising to 2,099 by May 7, despite aggressive measures taken by the Mexican government to
curb the spread of the disease. On June 11, 2009, the WHO declared an H1N1
pandemic, moving the alert level to phase 6, marking the first global pandemic since
the 1968 Hong Kong flu. On October 25, 2009, U.S. President Barack
Obama officially declared H1N1 a national emergency Despite President Obama's concern,
a Fairleigh Dickinson University PublicMind poll found in October 2009 that an overwhelming
majority of New Jerseyans were not very worried or not at all worried about contracting the
H1N1 flu virus. However, the President’s declaration caused many U.S. employers to
take actions to help stem the spread of the swine flu and to accommodate employees and
/ or workflow which may be impacted by an outbreak.
A study conducted in coordination with the University of Michigan Health Service is scheduled
for publication in the December 2009 American Journal of Roentgenology warning that H1N1
flu can cause pulmonary embolism, surmised as a leading cause of death in this current
pandemic. The study authors suggest physician evaluation via contrast enhanced CT scans
for the presence of pulmonary emboli when caring for patients diagnosed with respiratory
complications from a "severe" case of the H1N1 flu.
The March 21, 2010 worldwide update, by the U.N.'s World Health Organization, states that
"213 countries and overseas territories/communities have reported laboratory confirmed cases of
pandemic influenza H1N1 2009, including at least 16,931 deaths."
As of May 30, 2010, worldwide update by World Health Organization(WHO) more than 214 countries
and overseas territories or communities have reported laboratory confirmed cases of pandemic
influenza H1N1 2009, including over 18,138 deaths.
The research team of Andrew Miller MD showed pregnant patients are at increased risk. It
has been suggested that pregnant women and certain populations such as native North Americans
have a greater likelihood of developing a T helper type 2 response to H1N1 influenza
which may be responsible for the systemic inflammatory response syndrome that causes
pulmonary edema and death. Recent developments
On 26 April 2011, an H1N1 pandemic preparedness alert was issued by the World Health Organization
for the Americas. In May 2013, seventeen people died during
an H1N1 outbreak in Venezuela, and a further 250 were infected.
As of early January 2014, Texas health officials have confirmed at least thirty-three H1N1
deaths and widespread outbreak during the 2013/2014 flu season, while twenty-one more
deaths have been reported across the US. Nine people have been reported dead from an outbreak
in several Canadian cities, and Mexico reports outbreaks resulting in at least one death.
Spanish health authorities have confirmed 35 H1N1 cases in the Aragon region, 18 of
whom are in intensive care On March 17, 2014, three cases were confirmed
with a possible fourth awaiting results occurring at the Centre for Addiction and Mental Health
in Toronto, Ontario, Canada. According to the U.S. Geological Survey and
the CDC, northern sea otters, off the coast of Washington state, are infected with the
H1N1 flu virus and "may be a newly identified animal host of influenza viruses".
In pregnancy Pregnant women who contract the H1N1 infection
are at a greater risk of developing complications because of hormonal changes, physical changes
and changes to their immune system to accommodate the growing fetus. For this reason the Center
for Disease Control and Prevention recommends that those who are pregnant to get vaccinated
to prevent the influenza virus. The vaccination should not be taken if you have had a severe
allergic reaction to the influenza vaccination. Additionally those who are moderately to severely
ill, with or without a fever should wait until they recover before taking the vaccination.
Pregnant women become infected with the influenza are advised to contact their doctor immediately.
Influenza can be treated using antiviral medication, which are available by prescription. Oseltamivir
and Zanamivir are two neuraminidase inhibitors currently recommended. It has been shown that
they work best when taken within two days of becoming sick.
Since October 1, 2008, the CDC has tested 1,146 seasonal influenza A viruses for resistance
against oseltamivir and zanamivir. It was found that 99.6% of the samples were resistant
to oseltamivir while none were resistant to zanamivir. In 853 samples of 2009 Influenza
A virus only 4% showed resistance to oseltamivir, while none of 376 samples showed resistance
to zanamivir. A study conducted in Japan during the 2009 H1N1 pandemic concluded that infants
exposed to either oseltamivir or zanamivir had no short term adverse effects. Both amantadine
and rimantadine have been found to be teratogenic and embryotoxic when given at high doses in
animal studies. Additional images
Notes
External links European Commission – Public Health EU Coordination
on Pandemic 2009. Health-EU Portal EU work to prepare a global
response to influenza A(H1N1). Influenza Research Database Database of influenza
genomic sequences and related information. Centers For Disease Control and Prevention
H1N1 Flu. Consultant Magazine H1N1 Center
Pandemic Influenza: A Guide to Recent Institute of Medicine Studies and Workshops A collection
of research papers and summaries of workshops by the Institute of Medicine on major policy
issues related to pandemic influenza and other infectious disease threats.
H1N1 Flu, 2009: Hearings before the Committee on Homeland Security and Governmental Affairs,
United States Senate, of the One Hundred Eleventh Congress, First Session: April 29, 2009, Coordinating
the Federal Response; September 21, 2009, Protecting Our Community: Field Hearing in
Hartford, CT; October 21, 2009, Monitoring the Nation’s Response; November 17, 2009,
Getting the Vaccine to Where It is Most Needed. Nontechnical
Shreeve, J.. "Why Revive a Deadly Flu Virus?". New York Times. Six-page human-interest
story on the recreation of the deadly 1918 H1N1 flu virus
"1918 flu virus's secrets revealed". BBC News. 28 September 2006. Results from analyzing
a recreated strain. Noymer, A.. "Some information on TB and the
1918 flu". Data from Noymer A, Garenne M. "The 1918 influenza epidemic's effects on
sex differentials in mortality in the United States". Popul Dev Rev 26: 565–81. doi:10.1111/j.1728-4457.2000.00565.x.
PMC 2740912. PMID 19530360. Oral history by 1918 pandemic survivor
Technical Ludwig S, Haustein A, Kaleta EF, Scholtissek
C. "Recent influenza A infections of pigs and turkeys in northern Europe". Virology
202: 281–6. doi:10.1006/viro.1994.1344. PMID 8009840.
Centers for Disease Control. "Influenza A(H1N1) associated with mild illness
in a nursing home–Maine". MMWR Morb. Mortal. Wkly. Rep. 36: 57–9. PMID 3100930.
Lederberg J. "H1N1-influenza as Lazarus: Genomic resurrection from the tomb of an unknown".
Proc. Natl. Acad. Sci. U.S.A. 98: 2115–6. doi:10.1073/pnas.051000798. PMC 33382. PMID 11226198.
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