What is the Bird Flu (Avian Influenza)?
Bird Flu (also known as avian influenza or avian flu) is 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. Avian influenza viruses compose the Influenzavirus A genus of the Orthomyxoviridae family and are negative-stranded, segmented RNA viruses. A strain of the H5N1-type of avian influenza that emerged in 1997 has been identified as the most likely source of a future influenza pandemic.
Subtypes pathogenic to humans
All avian influenza (AI) viruses are type A influenza in the virus family of Orthomyxoviridae and all known subtypes of influenza A virus can infect birds. Influenza 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, all AI viruses fall into one of 2 pathotypes: low (LPAI) and high (HPAI) pathogenicity, based on their virulence in poultry populations.
It is feared that if the avian influenza virus undergoes antigenic shift to the point where it can cross the species barrier (e.g., from birds to humans), the new subtype created could be both highly contagious and highly lethal in humans. Such a subtype could cause a global pandemic similar to the Spanish Flu that killed up to 50 million people in 1918.
However, there are substantial genetic differences between the subtypes that typically infect both people and birds. Within subtypes of avian influenza viruses there also are different strains. Avian influenza H5 and H7 viruses can be found in both "low pathogenic” or “high pathogenic” forms depending on genetic features and the severity of the illness they cause in poultry; influenza H9 virus has been identified only in a “low pathogenic” form.
H1N1
H1N1 was the first identified strain of Type A influenza. In early October 2005, researchers announced that they had successfully reconstructed the Spanish Flu virus. The gene sequence indicates that the 1918 epidemic was caused by H1N1, normally considered a strain of swine flu (which itself falls under avian influenza) that in this case passed directly from birds to humans. The reconstructed virus is very different from normal human viruses in that it infects lung cells which would normally be impervious to the virus.
H5N1
H5N1 is a highly pathogenic form of avian influenza. Since 1997, outbreaks of H5N1 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.
Other strains
Since 1997 the H5N1, H7N2, H7N3, H7N7, and H9N2 have been confirmed in humans.
- H2N2
- Responsible for the Asian Flu pandemic of 1957 and 1958 that killed at least a million people worldwide.
- H3N2
- Evolved from H2N2 by antigenic shift and caused the Hong Kong Flu pandemic of 1968 and 1969 that killed up to 750,000. This was the least deadly pandemic of the twentieth century.
- H7N2
- Following an outbreak of H7N2 among poultry in 2002, 44 people were found infected in Virginia, United States.
- 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.
- 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 has been recorded.
- H9N2
- The virus type has been documented only in low pathogenic form. Three infections in humans (China and Hong Kong) have been confirmed, all three patients recovered. In October 2005 an outbreak has been detected in the province of Tolima, central Colombia. No human infections have been reported.
Potential pandemic
The World Health Organization (WHO) has warned of a substantial risk of an influenza epidemic in the near future, most probably of the H5N1 type of avian influenza. 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.
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.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Bird Flu".