Abstract: Bird f lu or aviation influenza is a contagious disease caused by avian (bird) influenza (flu) viruses. These flu viruses occur naturally among birds but usually birds do not get disease from them. Worldwide wild birds carry these viruses in their intestines. Since bird flu is very contagious, sometimes infection is passed on to the domestic animals including chickens, ducks, and turkeys and kill them. Avian influenza viruses normally infect only birds and, less commonly, pigs. Thus, they are highly species-specific, but have, on rare occasions, crossed the species barrier to infect humans.
Types of Influenza Viruses
Influenza viruses are grouped into three types, A, B, and C. Influenza A and B viruses are of concern for human health and only type A can cause pandemics. The H subtypes of type A viruses are epidemiologically most important. The H subtypes have the ability to bind to and enter cells, where multiplication of the virus occurs. The N subtypes govern the release of newly formed virus from the cells.
In domestic poultry, two types of diseases can occur with avian influenza viruses distinguished by low and high extremes of virulence. Low virulence or so-called "low pathogenic" form causes mild symptoms such as ruffled feathers, a drop in egg production that goes usually undetected. High virulence or high pathogenic form, on the other hand, is more fatal that spreads very rapidly through poultry flocks, causes disease affecting multiple internal organs, and has a mortality that can approach 100%, often within 48 hours.
All type A influenza viruses are genetically labile and well adapted to elude host defenses. They lack mechanisms for the "proofreading" and repair of errors that occur during replication. Due to this, the genetic composition of the viruses changes as they replicate in humans and animals, and the existing strain is replaced with a new antigenic variant. These constant, permanent and usually small changes in the antigenic composition of influenza A viruses are known as antigenic "drift".
Influenza A viruses have 16 H subtypes and 9 N subtypes. Only viruses of the H5 and H7 subtypes cause the highly pathogenic form of disease. However, not all viruses of the H5 and H7 subtypes are highly pathogenic and not all will cause severe disease in poultry. These viruses can mutate, usually within a few months, into highly pathogenic form, which is a cause of concern even when the initial signs of infection are mild. Recent events make it likely that some migratory birds are now directly spreading the H5N1 virus in its highly pathogenic form. Further spread to new areas is expected. |
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