Avian
Influenza viruses are divided into three types (strains) of viruses — influenza A, B and C. Type A is responsible for the deadly influenza pandemics. Type B can lead to smaller, more localized outbreaks. Less common and more stable than other strains, type C has milder symptoms. Either types A or B can cause the flu that circulates almost every winter. Types B and C are usually found only in humans, whereas type A infects both people and animals, including birds, pigs, horses, whales and seals. There are many different subtypes of type A influenza viruses. These subtypes differ because of changes in certain proteins on the surface of the influenza A virus (hemagglutinin [HA] and neuraminidase [NA] proteins). There are 16 known HA subtypes and 9 known NA subtypes of influenza A viruses. Many different combinations of HA and NA proteins are possible. Each combination represents a different subtype.. At least 15 flu subtypes affect birds, the most virulent of which is H5N1. Until recently, avian subtypes have rarely been found in humans or in animals other than pigs. Type A influenza viruses are further divided into strains, which are constantly evolving. And it is exactly this — the ability of influenza viruses to change their genetic makeup and to swap genes — that makes them so unpredictable and potentially deadly.
Avian influenza is an infection caused by avian (bird) influenza (flu) viruses. These influenza viruses occur naturally among birds. Wild birds worldwide carry the viruses in their intestines, but usually do not get sick from them. However, avian influenza is very contagious among birds and can make some domesticated birds, including chickens, ducks, and turkeys, very sick and kill them.
Infected birds shed influenza virus in their saliva, nasal secretions, and feces. Susceptible birds become infected when they have contact with contaminated secretions or excretions or with surfaces that are contaminated with secretions or...
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