With devastating consequences for the poultry industry, farmer’s livelihoods, international trade, and the health of wild birds, avian influenza, also known as ‘bird flu’, has captured the attention of the international community over the years. Where outbreaks occur in domestic birds, it is often the policy to cull all poultry, whether infected or healthy, to contain the spread of avian influenza. This represents heavy economic losses for farmers and a long-lasting impact on their livelihoods.
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Avian Influenza
Migratory wild birds especially waterfowl, are natural reservoir of avian influenza viruses and they play a role in the spread the viruses across large geographical areas and also becomes victims of the disease. Avian influenza is also a major concern for public health. Whenever avian influenza viruses circulate in poultry, sporadic cases of avian influenza in humans are sometimes identified.
Signs and symptoms
What is avian influenza?
Avian influenza is a highly contagious viral disease that affects both domestic and wild birds. Avian influenza viruses have also been isolated, although less frequently, from mammalian species, including humans. This complex disease is caused by viruses divided into multiple subtypes (i.e. H5N1, H5N3, H5N8 etc.) whose genetic characteristics rapidly evolve. The disease occurs worldwide but different subtypes are more prevalent in certain regions than others.
Avian influenza or bird flu refers to the disease caused by infection with avian (bird) influenza (flu) Type A viruses. These viruses naturally spread among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species. The many strains of avian influenza viruses can generally be classified into two categories according to the severity of the disease in poultry. Low pathogenicity avian influenza (LPAI) that typically causes little or no clinical signs; high pathogenicity avian influenza (HPAI) that can cause severe clinical signs and possible high mortality rates.
What is bird flu?
In recent years, the changes in the ecology and epidemiology of specific avian influenza lineages led to infection of numerous wild bird species. Consequently, this facilitated the spread of the virus along established migratory routes, resulting in death of many wild birds, including endangered species, and serving as a source for transmission to poultry and wild mammals. Clinical signs of H5N1 in birds range from mild—decrease in egg production, nasal discharge, coughing and sneezing—to severe, including loss of coordination, energy, and appetite; soft-shelled or misshapen eggs; purple discoloration of the wattles, head, eyelids, combs, and hocks; and diarrhea. Sometimes the first noticeable sign is sudden death. In birds, avian influenza viruses are shed in the faeces and respiratory secretions. They can all be spread through direct contact with secretions from infected birds, especially through faeces or through contaminated feed and water. Because of the resistant nature of avian influenza viruses, including their ability to survive for long periods when temperatures are low, they can also be carried on farm equipment and spread easily from farm to farm.
Humans
In general, humans who catch a humanized influenza A virus (a human flu virus of type A) usually have symptoms that include fever, cough, sore throat, muscle aches, conjunctivitis, and, in severe cases, breathing problems and pneumonia that may be fatal. The severity of the infection depends in large part on the state of the infected persons’ immune systems and whether they had been exposed to the strain before (in which case they would be partially immune). No one knows if these or other symptoms will be the symptoms of a humanized H5N1 flu.
The avian influenza hemagglutinin prefers to bind to alpha-2,3 sialic acid receptors, while the human influenza hemagglutinin prefers to bind to alpha-2,6 sialic acid receptors. This means that when the H5N1 strain infects humans, it will replicate in the lower respiratory tract (where alpha-2,3 sialic acid receptors are more plentiful in humans) and consequently cause viral pneumonia. As of April 2024, there is no human-adapted form of H5N1 influenza, so all humans who have caught it so far have caught avian H5N1.
Between 2003 and March 2024, the World Health Organization has recorded 888 cases of confirmed H5N1 influenza, leading to 463 deaths. This yields a CFR estimate of 52%. The true fatality rate may be lower because some cases with mild symptoms may not have been identified as H5N1
Terminology
H5N1 isolates are identified like this actual HPAI A(H5N1) example, A/chicken/Nakhon-Pathum/Thailand/CU-K2/04(H5N1):
(1) A stands for the genus of influenza (A, B or C).
(2) chicken is the animal species the isolate was found in (note: human isolates lack this component term and are thus identified as human isolates by default)
(3) Nakhon-Pathum/Thailand is the place this specific virus was isolated
(4) CU-K2 is the laboratory reference number that identifies it from other influenza viruses isolated at the same place and year
(5) 04 represents the year of isolation 2004
(6) H5 stands for the fifth of several known types of the protein hemagglutinin.
(7) N1 stands for the first of several known types of the protein neuraminidase.
The N in H5N1 stands for “Neuraminidase”, the protein depicted in this ribbon diagram
H5N1 is a subtype of the species Influenza A virus of the genus Parainfluenza virus of the family Orthomyxoviridae. Like all other influenza A subtypes, the H5N1 subtype is an RNA virus. It has a segmented genome of eight negative sense, single-strands of RNA, abbreviated as PB2, PB1, PA, HA, NP, NA, MP and NS.
HA codes for hemagglutinin, an antigenic glycoprotein found on the surface of the influenza viruses and is responsible for binding the virus to the cell that is being infected. NA codes for neuraminidase, an antigenic glycosylated enzyme found on the surface of the influenza viruses. It facilitates the release of progeny viruses from infected cells.[36] The hemagglutinin (HA) and neuraminidase (NA) RNA strands specify the structure of proteins that are most medically relevant as targets for antiviral drugs and antibodies. HA and NA are also used as the basis for the naming of the different subtypes of influenza A viruses. This is where the H and N come from in H5N1.
Current H5N1 Bird Flu Situation
WHAT TO KNOW
A multistate outbreak of HPAI A(H5N1) bird flu in dairy cows was first reported on March 25, 2024. This is the first time that these bird flu viruses had been found in cows. In the United States, since 2022, USDA APHIS has reported HPAI A(H5N1) virus detections in more than 200 mammals.
Risk to Humans
The wide geographic spread of HPAI A(H5N1) viruses in wild birds, poultry, and some other mammals, including in cows, could create additional opportunities for people to be exposed to these viruses. Therefore, there could be an increase in sporadic human infections resulting from bird and animal exposures, even if the risk of these viruses spreading from birds to people has not increased. CDC believes the current risk to the general public from bird flu viruses is low. People who have job-related or recreational exposure to infected birds or animals, including cows, are at greater risk of contracting HPAI A(H5N1) virus. CDC has recommendations related to testing, treatment of HPAI A(H5N1) infection and prevention of exposure to these viruses
Treatment
There is no highly effective treatment for H5N1 flu, but oseltamivir (commercially marketed by Roche as Tamiflu) can sometimes inhibit the influenza virus from spreading inside the user’s body. This drug has become a focus for some governments and organizations trying to prepare for a possible H5N1 pandemic. On April 20, 2006, Roche AG announced that a stockpile of three million treatment courses of Tamiflu are waiting at the disposal of the World Health Organization to be used in case of a flu pandemic; separately Roche donated two million courses to the WHO for use in developing nations that may be affected by such a pandemic but lack the ability to purchase large quantities of the drug.Even now, we remain unsure about Tamiflu’s real effectiveness. As for a vaccine, work cannot start on it until the emergence of a new virus, and we predict it would take six to nine months to develop it. For the moment, we cannot by any means count on a potential vaccine to prevent the spread of a contagious influenza virus, whose various precedents in the past 90 years have been highly pathogenic.
Epidemiology
Further information: Transmission and infection of H5N1 and Global spread of H5N1
The earliest infections of humans by H5N1 coincided with an epizootic (an epidemic in nonhumans) of H5N1 influenza in Hong Kong’s poultry population in 1997. This panzootic (a disease affecting animals of many species, especially over a wide area) outbreak was stopped by the killing of the entire domestic poultry population within the territory. However, the disease has continued to spread; outbreaks were reported in Asia again in 2003. On December 21, 2009, the WHO announced a total of 447 cases which resulted in the deaths of 263.
Society and culture
H5N1 has had a significant effect on human society, especially the financial, political, social, and personal responses to both actual and predicted deaths in birds, humans, and other animals. Billions of dollars are being raised and spent to research H5N1 and prepare for a potential avian influenza pandemic. Over $10 billion have been spent and over 200 million birds have been killed to try to contain H5N1.
People have reacted by buying less chicken, causing poultry sales and prices to fall. Many individuals have stockpiled supplies for a possible flu pandemic. International health officials and other experts have pointed out that many unknown questions still hover around the disease.
Dr. David Nabarro, Chief Avian Flu Coordinator for the United Nations, and former Chief of Crisis Response for the World Health Organization has described himself as “quite scared” about H5N1’s potential impact on humans. Nabarro has been accused of being alarmist before, and on his first day in his role for the United Nations, he proclaimed the avian flu could kill 150 million people. In an interview with the International Herald Tribune, Nabarro compares avian flu to AIDS in Africa, warning that underestimations led to inappropriate focus for research and intervention.
What does H5N1 stand for?
Highly pathogenic avian influenza virus A(H5N1).HPAIV A(H5N1) is highly infectious for a number of bird species, including most species of domestic poultry .
What is the cause of avian flu?
Bird flu mostly spreads from bird to bird through direct or indirect contact with infected saliva, nasal secretions or droppings, as well as through predation/scavenging. There are several ways by which the disease is spread, including by contaminated poultry, their excretions and secretions and contaminated object.
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