Influenza A virus subtype H3N2

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Influenza

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Influenza A virus subtype H3N2 (also H3N2) is a subtype of viruses that cause influenza (flu). H3N2 viruses can infect birds and mammals. In birds, humans, and pigs, the virus has mutated into many strains. H3N2 is increasingly abundant in seasonal influenza, which kills an estimated 36,000 people in the United States each year.

[edit] Classification

H3N2 is a subtype of the viral genus Influenzavirus A, which is an important cause of human influenza. Its name derives from the forms of the two kinds of proteins on the surface of its coat, hemagglutinin (H) and neuraminidase (N). By reassortment, H3N2 exchanges genes for internal proteins with other influenza subtypes.

[edit] Seasonal H3N2 flu

Seasonal influenza kills an estimated 36,000 people in the United States each year. Flu vaccines are based on predicting which mutants of H1N1, H3N2, H1N2, and influenza Bwill proliferate in the next season. Separate vaccines are developedfor the northern and southern hemispheres in preparation for theirannual epidemics. In the tropics, influenza shows no clear seasonality.In the past ten years, H3N2 has tended to dominate in prevalence overH1N1, H1N2, and influenza B. Measured resistance to the standardantiviral drugs amantadine and rimantadine in H3N2 has increased from 1% in 1994 to 12% in 2003 to 91% in 2005.^[1]

Seasonal H3N2 flu is a human flu from H3N2 that is slightly different from one of last year's flu season H3N2 variants. Seasonal influenza viruses flow out of overlapping epidemics in East and Southeast Asia,then trickle around the globe before dying off. Identifying the sourceof the viruses allows global health officials to better predict whichviruses are most likely to cause the most disease over the next year.An analysis of 13,000 samples of influenza A/H3N2 virus that werecollected across six continents from 2002 to 2007 by the WHO's Global Influenza Surveillance Networkshowed that newly emerging strains of H3N2 appeared in East andSoutheast Asian countries about 6 to 9 months earlier than anywhereelse. The strains generally reached Australia and New Zealand next, followed by North America and Europe. The new variants typically reached South America after an additional 6 to 9 months, the group reported.^[2]

[edit] Swine flu

Pigs can harbor influenza viruses adapted to humans and others that areadapted to birds, allowing the viruses to exchange genes and create apandemic strain.

"In swine, 3 influenza A virus subtypes (H1N1, H3N2, and H1N2)are circulating throughout the world. In the United States, the classicH1N1 subtype was exclusively prevalent among swine populations before1998; however, since late August 1998, H3N2 subtypes have been isolatedfrom pigs. Most H3N2 virus isolates are triple reassortants, containinggenes from human (HA, NA, and PB1), swine (NS, NP, and M), and avian (PB2 and PA) lineages. [...] Present vaccination strategies for SIV control and prevention in swine farmstypically include the use of 1 of several bivalent SIV vaccinescommercially available in the United States. Of the 97 recent H3N2isolates examined, only 41 isolates had strong serologiccross-reactions with antiserum to 3 commercial SIV vaccines. Since theprotective ability of influenza vaccines depends primarily on thecloseness of the match between the vaccine virus and the epidemicvirus, the presence of nonreactive H3N2 SIV variants suggests thatcurrent commercial vaccines might not effectively protect pigs frominfection with a majority of H3N2 viruses."^[3]

Avian influenza virus H3N2 is endemic in pigs in Chinaand has been detected in pigs in Vietnam, increasing fears of theemergence of new variant strains. Health experts say pigs can carryhuman influenza viruses, which can combine (i.e. exchange homologous genome sub-units by genetic reassortment) with H5N1, passing genes and mutating into a form which can pass easily among humans. H3N2 evolved from H2N2 by antigenic shift and caused the Hong Kong Flu pandemic of 1968 and 1969 that killed up to 750,000 humans. The dominant strain of annual flu in humans in January 2006 is H3N2. Measured resistance to the standard antiviral drugs amantadine and rimantadine in H3N2 in humans has increased to 91% in 2005. In August 2004, researchers in China found H5N1 in pigs.^[4]

[edit] Hong Kong Flu (1968-1969)

It has been suggested that this section be split into a new article. (Discuss)

The influenza viruses that caused Hong Kong flu. (magnified approximately 100,000 times)

The Hong Kong Flu was a category 2 flu pandemic caused by a strain of H3N2 descended from H2N2 by antigenic shift,in which genes from multiple subtypes reassorted to form a new virus.This pandemic of 1968 and 1969 killed an estimated one million peopleworldwide.^[5][6][7] The pandemic infected an estimated 500,000 Hong Kong residents, 15% of the population, with a low death rate.^[8] In the United States, approximately 33,800 people died.^[9]

Both the H2N2 and H3N2 pandemic flu strains contained genes from avian influenzaviruses. The new subtypes arose in pigs coinfected with avian and humanviruses and were soon transferred to humans. Swine were considered theoriginal "intermediate host" for influenza, because they supportedreassortment of divergent subtypes. However, other hosts appear capableof similar coinfection (e.g., many poultry species), and directtransmission of avian viruses to humans is possible. H1N1 may have beentransmitted directly from birds to humans (Belshe 2005).^[10]

The Hong Kong flu strain shared internal genes and the neuraminidase with the 1957 Asian Flu (H2N2). Accumulated antibodies to the neuraminidase or internal proteins may have resulted in much fewer casualties than most pandemics. However, cross-immunity within and between subtypes of influenza is poorly understood.

The Hong Kong flu was the first known outbreak of the H3N2 strain,though there is serologic evidence of H3N? infections in the late 19thcentury. The first record of the outbreak in Hong Kong appeared on 13July 1968 in an area with a density of about 500 people per acre in anurban setting. The outbreak reached maximum intensity in 2 weeks,lasting 6 weeks in total. The virus was isolated in Queen Mary Hospital. Flu symptoms lasted 4 to 5 days.^[8]

By July 1968, extensive outbreaks were reported in Vietnam and Singapore. By September 1968, it would reach India, Philippines, northern Australia and Europe. That same month, the virus entered California from returning Vietnam War troops. It would reach Japan, Africa and South America by 1969.^[8]

"Three strains of Hong Kong influenza virus isolated from humanswere compared with a strain isolated from a calf for their ability tocause disease in calves. One of the human strains. A/Aichi/2/68, wasdetected for five days in a calf, but all three failed to cause signsof disease. Strain A/cal/Duschanbe/55/71 could be detected for sevendays and caused an influenza-like illness in calves."^[11]

[edit] Fujian flu (2003–2004)

Diagram of influenza virus nomenclature.

Fujian flu refers to flu caused by either a Fujian human flu strain of the H3N2 subtype of the Influenza A virus or a Fujian bird flu strain of the H5N1 subtype of the Influenza A virus. These strains are named after Fujian, a coastal province of the People's Republic of China that is across the Taiwan strait from Taiwan.^[12]

A/Fujian (H3N2) human flu (from A/Fujian/411/2002(H3N2) -like fluvirus strains) caused an unusually severe 2003–2004 flu season. Thiswas due to a reassortment event that caused a minor clade to provide a haemagglutiningene that later became part of the dominant strain in the 2002–2003 fluseason. A/Fujian (H3N2) was made part of the trivalent influenzavaccine for the 2004-2005 flu season and its descendants are still the most common human H3N2 strain.

[edit] 2004-2005 flu season

The 2004–05 trivalent influenza vaccine for the United States contained A/New Caledonia/20/99-like (H1N1), A/Fujian/411/2002-like (H3N2), and B/Shanghai/361/2002-like viruses.^[13]

[edit] 2005-2006 flu season

The vaccines produced for the 2005–2006 season use:

• an A/New Caledonia/20/1999-like(H1N1);
• an A/California/7/2004-like(H3N2) (or the antigenically equivalent strain A/New York/55/2004);
• a B/Jiangsu/10/2003-like viruses.

[edit] 2006-2007 flu season

The 2006–2007 influenza vaccine composition recommended by the WorldHealth Organization on 15 February 2006 and the U.S. FDA's Vaccines andRelated Biological Products Advisory Committee (VRBPAC) on 17 February2006 use:

• an A/New Caledonia/20/99 (H1N1)-like virus;
• an A/Wisconsin/67/2005 (H3N2)-like virus (A/Wisconsin/67/2005 and A/Hiroshima/52/2005 strains);
• a B/Malaysia/2506/2004-like virus from B/Malaysia/2506/2004 and B/Ohio/1/2005 strains which are of B/Victoria/2/87 lineage.^[14]

[edit] 2007-2008 flu season

The composition of influenza virus vaccines for use in the 2007–2008Northern Hemisphere influenza season recommended by the World HealthOrganization on 14 February 2007^[15] was:

• an A/Solomon Islands/3/2006 (H1N1)-like virus;
• an A/Wisconsin/67/2005 (H3N2)-like virus (A/Wisconsin/67/2005 (H3N2) and A/Hiroshima/52/2005 were used at the time);
• a B/Malaysia/2506/2004-like virus^[16][17]

"A/H3N2 has become the predominant flu subtype in the United States,and the record over the past 25 years shows that seasons dominated byH3N2 tend to be worse than those dominated by type A/H1N1 or type B."Many H3N2 viruses making people ill in this 2007-2008 flu season differfrom the strains in the vaccine and may not be well covered by thevaccine strains. "The CDC has analyzed 250 viruses this season todetermine how well they match up with the vaccine, the report says. Of65 H3N2 isolates, 53 (81%) were characterized asA/Brisbane/10/2007-like, a variant that has evolved [notably] from theH3N2 strain in the vaccine—A/Wisconsin/67/2005."^[18]

[edit] 2009 H1N1 influenza outbreak

See main page 2009 H1N1 influenza outbreak

The British Columbia Center for Disease Control reported on 5 May2009 that on 28 April 2009 it had isolated a new variant H3N2 virusfrom a person who recently visited Mexico. It has been suggested thatsome of the differences in the pathology of the influenza in Mexico maybe due to the presence of this variant H3N2 and not H1N1.^[19]

[edit] See also

• 2009 H1N1 influenza outbreak
• Bird flu
• Dog flu
• Horse flu
• Human flu
• Swine flu

[edit] Further reading

• Graphic showing H3N2 mutations, amino acid by amino acid, among 207 isolates completely sequenced by the Influenza Genome Sequencing Project.
• Influenza A (H3N2) Outbreak, Nepal
• Hot topic - Fujian-like strain A influenza
• Monitoring Antibodies for Swine Influenza H3N2 Virus
• Human influenza A(H3N2) activity remains widespread in many countries - update 7
• Human influenza A/H3N2 activity increases in many countries in central and eastern Europe - update 8
• Influenza A H3N2
• New Scientist: Bird Flu

[edit] External links

• BioHealthBase Bioinformatics Resource Center Database of influenza sequences and related information.

[edit] Sources