Avian influenza

Avian influenza is a disease caused by the influenza A virus. This virus primarily affects birds, but it can also infect mammals, including humans. Wild aquatic birds serve as the primary hosts for the virus. In many bird populations, the virus is enzootic, meaning it is continually present. This allows the virus to persist in nature over long periods.

To understand how the virus works, we must look at its microscopic structure. The virus particle, or virion, is an elliptical or filamentous shape. It measures between 80 and 120 nanometers in diameter. The virus is an RNA virus with a segmented genome. This genome contains 11 different viral genes. On its surface, the virus has two major proteins called antigens. These are hemagglutinin and neuraminidase.

These two surface proteins act like tools for the virus. Hemagglutinin, often called H, is a glycoprotein that binds to sialic acid receptors on a host cell. This binding allows the virus to enter the cell. Neuraminidase, or N, is an enzyme that helps new progeny viruses release from the infected cell. There are 18 known types of hemagglutinin, with H1 through H16 found in birds. There are also 11 known types of neuraminidase. Scientists name subtypes by combining these proteins, such as H5N1.

Scientists classify avian influenza into two main categories based on severity. This classification system was devised in 1981 to protect economically important chicken farms. The categories are low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI). A strain is labeled HPAI if 75% or more of chickens die after being deliberately infected. LPAI causes only mild symptoms or no symptoms at all in chickens. However, HPAI causes serious breathing difficulties and a significant drop in egg production.

It is important to note that these labels only describe the effect on chickens. They do not predict how severe the disease will be in humans or other mammals. For example, some water birds can carry HPAI without showing severe symptoms. These birds can then spread the infection over very large distances. Because of the risk of mutation, the World Organization for Animal Health has required reporting for LPAI H5 and H7 strains since 2006. This helps track viruses that might become more dangerous.

History shows how our understanding of the virus has grown. Edoardo Perroncito first identified the disease in 1878. He was able to distinguish it from other bird diseases. In 1955, researchers established that the fowl plague virus was related to human influenza. By 1972, it was evident that many subtypes were endemic in wild birds. Recent years have seen significant impact, such as the H5N1 strain. This strain has the potential to devastate poultry, and an estimated half a billion farmed birds have been slaughtered to contain it.

The virus can spread through several different pathways. Infected birds shed the virus in their saliva, mucus, and feces. The virus can survive for long periods in freshwater and can withstand freezing. Humans typically become infected through prolonged close contact with infected birds or mammals. Infection can also happen through contaminated surfaces or improperly prepared animal byproducts. While human-to-human spread is rare, scientists watch for it closely.

There is a biological barrier that often prevents the virus from spreading easily to humans. The H protein of avian influenza binds to alpha-2,3 sialic acid receptors found in bird intestines and respiratory tracts. In contrast, human influenza H proteins bind to alpha-2,6 receptors in the human upper respiratory tract. Additionally, a human protein called Mx1 can inhibit the replication of avian viruses. However, the virus is constantly changing through a process called antigenic drift. Through genetic reassortment, an avian virus could acquire the ability to infect humans. Pigs are often seen as a "melting pot" for this because they possess both types of receptors.