Avian influenza: Encyclopedia - Avian influenza

Avian influenza

Avian influenza (also known as bird flu, avian flu, influenzavirus A flu, type A flu, or genus A flu) is a flu due to a type of influenza virus that is hosted by birds, but may infect several species of mammals. It was first identified in Italy in the early 1900s and is now known to exist worldwide. [1] A strain of the H5N1-type of avian influenzavirus that emerged in 1997 has been identified as the most likely source of a future influenza pandemic and is known to have infected 137 people in Asia since 2003, killing 70. [2]

The avian influenzavirus subtypes that have been confirmed in humans, ordered by the number of known human deaths, are: H1N1 caused Spanish flu, H2N2 caused Asian Flu, H3N2 caused Hong Kong Flu, H5N1, H7N7, H9N2, H7N2, H7N3.

Avian influenza viruses compose the Influenzavirus A genus of the Orthomyxoviridae family and are negative sense, single-stranded, segmented RNA viruses. "There are 16 different HA antigens (H1 to H16) and nine different NA antigens (N1 to N9) for influenza A. Until recently, 15 HA types had been recognized, but a new type (H16) was isolated from black-headed gulls caught in Sweden and the Netherlands in 1999 and reported in the literature in 2005." [3]

Avian influenza - In nonhumans

Wild fowl act as natural asymptomatic carriers of avian flu virus. Prior to the current H5N1 epizootic, strains of avian influenzavirus had been demonstated to be transmitted from wild fowl to only birds, pigs, horses, seals, whales and humans; and only between humans and pigs and between humans and domestic fowl; and not other pathways such as domestic fowl to horse. [4] H5N1 has been shown to be also transmitted to tigers, leopards, and domestic cats who were fed uncooked domestic fowl (chickens) with the virus. H3N8 viruses from horses have crossed over and caused outbreaks in dogs. Laboratory mice have been successfully infected with a variety of avian flu genotypes. [5] [6]

Avian influenzavirus spreads in the air and in manure and survives longer in cold weather. It can also be transmitted by contaminated feed, water, equipment and clothing; however, there is no evidence that the virus can survive in well cooked meat. The incubation period is 3 to 5 days. Symptoms in animals vary, but virulent strains can cause death within a few days.

"Highly pathogenic avian influenza virus is on every top ten list available for potential agricultural bioweapon agents". [7]

1979: "More than 400 harbor seals, most of them immature, died along the New England coast between December 1979 and October 1980 of acute pneumonia associated with influenza virus, A/Seal/Mass/1/180 (H7N7)." [8]

1995: "[V]accinated birds can develop asymptomatic infections that allow virus to spread, mutate, and recombine (ProMED-mail, 2004j). Intensive surveillance is required to detect these “silent epidemics” in time to curtail them. In Mexico, for example, mass vaccination of chickens against epidemic H5N2 influenza in 1995 has had to continue in order to control a persistent and evolving virus (Lee et al., 2004)." [9]

1997: "Influenza A viruses normally seen in one species sometimes can cross over and cause illness in another species. For example, until 1997, only H1N1 viruses circulated widely in the U.S. pig population. However, in 1997, H3N2 viruses from humans were introduced into the pig population and caused widespread disease among pigs. Most recently, H3N8 viruses from horses have crossed over and caused outbreaks in dogs." [10]

2000: "In California, poultry producers kept their knowledge of a recent H6N2 avian influenza outbreak to themselves due to their fear of public rejection of poultry products; meanwhile, the disease spread across the western United States and has since become endemic." [11] [12]

2003: In Netherlands H7N7 influenza virus infection broke out in poultry on several farms. [13]

2004: In North America, the presence of avian influenza strain H7N3 was confirmed at several poultry farms in British Columbia in February 2004. As of April 2004, 18 farms had been quarantined to halt the spread of the virus. CDC detailed analysis

2005: Tens of millions of birds died of H5N1 influenza and hundreds of millions of birds were culled to protect humans from H5N1. H5N1 is endemic in birds in southeast Asia and represents a long term pandemic threat.

"[C]ontemporary human H3N2 influenza viruses are now endemic in pigs in southern China and can reassort with avian H5N1 viruses in this intermediate host." [14]

Avian influenza - In humans

Avian influenza - Infections

In humans, avian flu viruses cause similar symptoms to other types of flu. [15] These include fever, cough, sore throat, muscle aches, conjunctivitis and, in severe cases, severe breathing problems and pneumonia that may be fatal. The severity of the infection will depend to a large part on the state of the infected person's immune system and if the victim has been exposed to the strain before, and is therefore partially immune. In one case, a boy with H5N1 experienced diarrhea followed rapidly by a coma without developing respiratory or flu-like symptoms, suggesting non-standard symptoms. [16]

The avian influenza subtypes that have been confirmed in humans, ordered by the number of known human deaths, are: H1N1 caused Spanish flu, H2N2 caused Asian Flu, H3N2 caused Hong Kong Flu, H5N1, H7N7, H9N2, H7N2, H7N3, H10N7.

All avian influenza (AI) viruses are type A influenza virus in the virus family of Orthomyxoviridae and all known strains of influenza A virus infect birds. Influenzavirus type A is subdivided into subtypes based on hemagglutinin (H) and neuraminidase (N) protein spikes from the central virus core. There are 16 H types, each with up to 9 N subtypes, yielding a potential for 144 different H and N combinations.

In addition, avian influenza viruses may fall into one of 2 pathotypes: low (LPAI) and high (HPAI) pathogenicity, based on their virulence in poultry populations. Avian influenzavirus H5 and H7 strains are found in both "low pathogenic” or “high pathogenic” forms; influenza H9 virus has been identified only in a “low pathogenic” form.

It is feared that if a strain of avian influenza virus to which humans have not been previously exposed undergoes antigenic shift to the point where it can cross the species barrier from birds to humans, the new subtype created could be both highly contagious and highly lethal in humans. If a human infected with influenzavirus also acquires H5N1, a mutant strain of bird flu that can be transmitted from human to human could form. Such a subtype could cause a global pandemic similar to the Spanish Flu that killed up to 50 million people in 1918.

Avian influenza - Prevention and treatment

Although avian influenzavirus in humans can be detected with standard influenza virus tests, these tests have not always proved reliable. In March 2005, the World Health Organization announced that seven people from Vietnam who initially tested negative for bird flu were later found to have carried the virus. All seven have since recovered. As of June 2005, the most reliable test (microneutralization) requires use of the live virus to interact with antibodies from the patient's blood; because live virus is required, for safety reasons the test can only be done in a level three laboratory [23].

Antiviral drugs such as oseltamivir, zanamivir and amantadine are sometimes effective in both preventing and treating the infection. Countries have been stockpiling olestamivir, but may shift towards zanamivir due to a November 2005 issue of JAMA, which reported oseltamivir resistant strains of avian flu in Vietnam.

Vaccines, however, take at least four months to produce and must be prepared for each subtype.

Further, as a result of widespread use of the antiviral drug amantadine as a preventive or treatment for chickens in China starting in the late 1990s, some strains of the avian flu virus in Asia have developed drug resistance against amantadine [24]. The use of amantadine was approved by the Chinese Ministry of Agriculture. This use of amantadine for poultry goes against international livestock regulations, but China kept it secret until recently, in a manner reminiscent of the secrecy around the early spread of SARS.

Osteopathic physicians used the lymphatic pump method during the 1917 influenza pandemic to increase the immune system's response. According to (Knott 2005), "osteopathic manipulative treatment (OMT) decreased the mortality rate from 5% to 0.25% among 100,000 patients."

There is some evidence [25] that indicates that Kimchi and by extension Sauerkraut may be used to treat avian influenza in birds. There is currently no evidence of its effects on humans.

Avian influenza - Potential pandemic

The World Health Organization (WHO) has warned of a substantial risk of an influenza epidemic in the near future, most probably from the H5N1 type of avian influenzavirus. One of the primary concerns is that the virus could quickly spread across countries as various birds follow their migration routes. In response, countries have begun planning in anticipation of an outbreak. While short-term strategies to deal with an outbreak focus on limiting travel and culling and vaccinating poultry, long-term strategies require substantial changes in the lifestyles of the most at-risk populations.

See also

• The Spanish flu from 1918–19
• The Asian Flu from 1957–58
• The Hong Kong Flu from 1968–69

Adapted from the Wikipedia article "Avian influenza", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki