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The Meaning of 'Bird Flu'

Scientifically referred to as avian influenza, the bird flu is a disease triggered by the contraction of an influenza virus predominantly existent amongst bird species. 'Avian' is the term relating to birds and 'influenza' is the virus itself. These bird flu viruses are generally infectious among birds only, but have proven to be capable of human infection during an avian influenza outbreak in 1997 in Hong Kong. More recently, human contraction of bird flu has occurred consistently throughout Southeast Asia, raising a renewed surge of alarm worldwide.

General Forms of the Bird Flu

Avian influenza can generally be categorised into one of two common forms based on its level of pathogenicity – these are respectively the low pathogenic and highly pathogenic forms of bird flu. Corresponding to their names, the low pathogenic form is rarely fatal and produces only minor symptoms among flock while the highly pathogenic form tends to be extremely contagious in nature and potentially lethal to both humans and birds.

Bird flu viruses responsible for low pathogenic forms of the disease are also capable of mutating whilst infecting poultry, to transform into the highly pathogenic forms of bird flu that reemerge and produce considerable damage to poultry populations.

Classification

Bird flu is classified as a type A influenza virus and belongs to the family orthomyxoviridae. They are single stranded RNA viruses.

Outbreaks in Domestic Poultry

Domestic bird species in general lack resistance against bird flu viruses and are particularly vulnerable to contracting avian influenza from wild bird species, which have higher resistance against the disease. There are 15 recognised subtypes of bird flu that circulate bird populations, with a tendency for highly pathogenic forms to arise from a contraction of H5 or H7 type viruses. (Find out more under Influenza)

Varying species of birds also possess different levels of resistance toward avian influenza, (many domestic ducks for example, carry viruses without infection) but outbreaks amongst domestic flock do not eliminate any bird species from the possibility of contraction, often resulting in the need for mass cullings within the infected region.

Highly pathogenic bird flu poses the greatest threat to poultry, being extremely infectious and known to produce a 90-100% death rate in infected birds. Outbreaks in poultry typically begin with a circulation of a low pathogenic disease, which if left undetected, develops into the highly pathogenic form to cause a large-scale epidemic over a matter of months.

Human Contraction of Bird Flu

In August 1997, the first cases of human bird flu infection emerged in Hong Kong – an unprecedented incident that demanded the culling of all poultry in the city. One third of the 18 infected victims (6) died as a result of the disease, a highly pathogenic form of avian influenza caused by the H5N1 bird flu virus. This particular subtype has subsequently been identified as the greatest concern of the disease, not only capable of infecting humans, but also producing critical conditions, if not death, in victims.

To date, over 300 different cases of bird flu contraction in humans have occurred worldwide. Various virus strains, both low and highly pathogenic have been identified as the causes of contraction but no stable human-to-human virus transmissions have yet arisen and human infection of the disease is comparatively scarce in contrast to the total number of bird flu incidences (in humans and animals) overall.

Types That Have Been Contracted By Humans

Currently, there have been six subtypes of bird flu that have caused human infection. These include:

• H5N1 (258 cases documented as at November 13 2006)
• H7N2
• H7N3
• H7N7 (Over 80 cases)
• H9N2
• H10N7

The listed subtypes vary in pathogenicity and hence severity. Human cases are generally confirmed in areas where the poultry population has also been infected by avian influenza, with H5N1 causing the greatest number of infections as well as the greatest number of deaths.