Shoshana Gordon/Axios
Dozens of newly confirmed cases of avian influenza in wild birds and the first verified U.S. case of a new strain of the virus are raising concern the bird flu crisis may be entering a troubling new phase. Why it matters: While the developments don’t necessarily raise the risk of a pandemic, they could create more havoc for farmers, exacerbate egg shortages and expose more gaps in government disease surveillance……..Continue reading…..
By: Adriel Bettelheim,Tina Reed
Source: Axios
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Critics:
Avian influenza, also known as avian flu or bird flu, is a disease caused by the influenza A virus, which primarily affects birds but can sometimes affect mammals including humans. Wild aquatic birds are the primary host of the influenza A virus, which is enzootic (continually present) in many bird populations. Symptoms of avian influenza vary according to both the strain of virus underlying the infection, and on the species of bird or mammal affected.
Classification of a virus strain as either low pathogenic avian influenza (LPAI) or high pathogenic avian influenza (HPAI) is based on the severity of symptoms in domestic chickens and does not predict severity of symptoms in other species. Chickens infected with LPAI display mild symptoms or are asymptomatic, whereas HPAI causes serious breathing difficulties, significant drop in egg production, and sudden death. Domestic poultry may potentially be protected from specific strains of the virus by vaccination.
Humans and other mammals can only become infected with avian influenza after prolonged close contact with infected birds. In mammals including humans, infection with avian influenza (whether LPAI or HPAI) is rare. Symptoms of infection vary from mild to severe, including fever, diarrhea, and cough. Influenza A virus is shed in the saliva, mucus, and feces of infected birds; other infected animals may shed bird flu viruses in respiratory secretions and other body fluids (e.g., cow milk). The virus can spread rapidly through poultry flocks and among wild birds. A particularly virulent strain,
influenza A virus subtype H5N1 (A/H5N1) has the potential to decimate domesticated poultry stocks and an estimated half a billion farmed birds have been slaughtered in efforts to contain the virus. Because of the impact of avian influenza on economically important chicken farms, a classification system was devised in 1981 which divided avian virus strains as either highly pathogenic (and therefore potentially requiring vigorous control measures) or low pathogenic.
The test for this is based solely on the effect on chickens – a virus strain is highly pathogenic avian influenza (HPAI) if 75% or more of chickens die after being deliberately infected with it. The alternative classification is low pathogenic avian influenza (LPAI). This classification system has since been modified to take into account the structure of the virus’ haemagglutinin protein. Other species of birds, especially water birds, can become infected with HPAI virus without experiencing severe symptoms and can spread the infection over large distances; the exact symptoms depend on the species of bird and the strain of virus.
Classification of an avian virus strain as HPAI or LPAI does not predict how serious the disease might be if it infects humans or other mammals. Since 2006, the World Organization for Animal Health requires all LPAI H5 and H7 detections to be reported because of their potential to mutate into highly pathogenic strains. Avian influenza is caused by the influenza A virus which principally affects birds but can also infect humans and other mammals.
Influenza A is an RNA virus with a genome comprising a negative-sense, RNA segmented genome that encodes for 11 viral genes. The virus particle (also called the virion) is 80–120 nanometers in diameter and elliptical or filamentous in shape. There is evidence that the virus can survive for long periods in freshwater after being excreted in feces by its avian host, and can withstand prolonged freezing.
There are two proteins on the surface of the viral envelope; hemagglutinin and neuraminidase.[4] These are the major antigens of the virus against which neutralizing antibodies are produced. Influenza virus epidemics and epizootics are associated with changes in their antigenic structure. Hemagglutinin (H) is an antigenic glycoprotein which allows the virus to bind to and enter the host cell. Neuraminidase (N) is an antigenic glycosylated enzyme which facilitates the release of progeny viruses from infected cells.
There are 18 known types of hemagglutinin, of which H1 thru H16 have been found in birds, and 11 types of neuraminidase. Analysis of the virus’ genome enables researchers to determine the order of its nucleotides. Comparison of the genome of a virus with that of a different virus can reveal differences between the two viruses. Genetic variations are important because they can change amino acids that make up the influenza virus’ proteins, resulting in structural changes to the proteins, and thereby altering properties of the virus.
Some of these properties include the ability to evade immunity and the ability to cause severe disease. Genetic sequencing enables influenza strains to be further characterised by their clade or subclade, revealing links between different samples of virus and tracing the evolution of the virus over time. Humans can become infected by the avian flu if they are in close contact with infected birds. Symptoms vary from mild to severe (including death), but as of December 2024 there have been no observed instances of sustained human-human transmission.
There are a number of factors that generally prevent avian influenza viruses from causing epidemics in humans or other mammals.
- The viral HA protein of avian influenza binds to alpha-2,3 sialic acid receptors, which are present in the respiratory tract and intestines of avian species, while human influenza HA binds to alpha-2,6 sialic acid receptors, which are present in the human upper respiratory tract.
- The myxovirus resistance protein (Mx1) is an important antiviral restriction factor that inhibits the replication of avian influenza viruses in particular. Human-adapted strains of IAV display reduced sensitivity to human Mx1 compared with avian strains.
- Other factors include the ability to replicate the viral RNA genome within the host cell nucleus, and to transmit between individuals.
Influenza viruses are constantly changing as small genetic mutations accumulate, a process known as antigenic drift. Over time, mutation may lead to a change in antigenic properties such that host antibodies (acquired through vaccination or prior infection) do not provide effective protection, causing a fresh outbreak of disease. The segmented genome of influenza viruses facilitates genetic reassortment.
This can occur if a host is infected simultaneously with two different strains of influenza virus; then it is possible for the viruses to interchange genetic material as they reproduce in the host cells. Thus, an avian influenza virus can acquire characteristics, such as the ability to infect humans, from a different virus strain.
The presence of both alpha 2,3 and alpha 2,6 sialic acid receptors in pig tissues allows for co-infection by avian influenza and human influenza viruses. This susceptibility makes pigs a potential “melting pot” for the reassortment of influenza A viruses.
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