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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. Avian influenza viruses compose the Influenzavirus A genus of the Orthomyxoviridae family and are negative sense, single-stranded, segmented RNA viruses. InfectionStrains of avian influenzavirus may infect various type of animals, including birds, pigs, horses, seals, whales and humans. However, wild fowl act as natural asymptomatic carriers, spreading it to more susceptible domestic stocks. Avian influenzavirus spreads in the air and in manure. 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. This was first founded in the 1900s. "Influenza A viruses normally seen in one species sometimes can cross over and cause illness in another species. For example, until 1998, only H1N1 viruses circulated widely in the U.S. pig population. However, in 1998, 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." In humans, avian flu viruses cause similar symptoms to other types of flu 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. Avian influenza infections in humansThe 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. All avian influenza (AI) viruses are type A influenzavirus 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. H1N1 A variant of H1N1 was responsible for the Spanish flu pandemic of 1918 that killed between 25 and 50 million people worldwide. A different variant exists in pig populations. Controversy arose in October, 2005, after the H1N1 genome was published in the journal, Science. Many fear that this information could be used for bioterrorism. "When he compared the 1918 virus with today's human flu viruses, Dr. Taubenberger noticed that it had alterations in just 25 to 30 of the virus's 4,400 amino acids. Those few changes turned a bird virus into a killer that could spread from person to person." H2N2 The Asian Flu was a pandemic outbreak of H2N2 avian influenza that originated in China in 1957, spread worldwide that same year during which a flu vaccine was developed, lasted until 1958 and caused between one and four million deaths. H3N2 H3N2 evolved from H2N2 by antigenic shift and caused the Hong Kong Flu pandemic of 1968 and 1969 that killed up to 750,000. (Detailed chart of its evolution here.) H5N1 H5N1 is a highly pathogenic form of avian influenzavirus. Since 1997, outbreaks of H5N1 flu have caused the death or culling of tens of millions of birds. Over 100 people have been infected by H5N1, with a mortality rate of over 50%. H5N1 has been the focus of much concern amid warnings that the H5N1 strain will likely evolve into a form that causes a global human pandemic with a very high mortality rate. As of November 1, 2005, 122 cases of infections in humans, resulting in 62 deaths, have been confirmed outside of China. H7N7 In 2003 in Netherlands 89 people were confirmed to have H7N7 influenza virus infection following an outbreak in poultry on several farms. One death was recorded. H9N2
Low pathogenic avian influenza A (H9N2) infection was confirmed in 1999, in China and Hong Kong in two children, and in 2003 in Hong Kong in one child. All three fully recovered. CDC H7N2 One person in New York in 2003 and one person in Virginia in 2002 were found to have serologic evidence of infection with H7N2. Both fully recovered. H7N3 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. Two cases of humans with avian influenza have been confirmed in that region. "Symptoms included conjunctivitis and mild influenzalike illness." CDC detailed analysis Both fully recovered. Prevention and treatmentAlthough 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. Currently (6/05) 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. 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. 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. There is some evidence 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. Potential pandemicThe 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. (WHO) announced on November, 16, 2005 that an outbreak is most likely to hit the Hong Kong Special Administrative issue by mid-December of this year. "If it were to hit in a highly residential area like Tin Hau, it would be sure to spread like wildfire." Dr. N Column, Head of Epidemic Prevention announced. The WHO divides a pandemic into six phases, ranging from minimal risk of an outbreak to full scale pandemic. Most health authorities categorize the situation as of 2005 at Phase 3, by which is meant that human infections of a new sub-type has occurred but there is little evidence of sustained human-to-human transmission.An Oseltamivir resistant H5N1 avian influenza virus was isolated from a Vietnamese girl in Feb. 2005. |
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Avian influenza
