Avian influenza, known informally as avian flu, is a bird flu caused by the influenza A virus, which can infect people.
It is similar to other types of animal flu in that it is caused by a virus strain that has adapted to a specific host.
Though influenza A is adapted to birds, it can also stably adapt and sustain person-to-person transmission.
The genes of the Spanish flu virus shows it to have genes adapted from both human and avian strains.
Pigs can also be infected with human, avian, and swine influenza viruses, allowing for mixtures of genes to create a new virus, which can cause an antigenic shift to a new influenza A virus subtype which most people have little to no immune protection against.
Avian influenza strains are divided into two types based on their pathogenicity: high pathogenicity (HP) or low pathogenicity (LP).
The most well-known HPAI strain, is H5N1.
Companion birds in captivity are unlikely to contract the virus and there has been no report of a companion bird with avian influenza since 2003.
Pigeons can contract avian strains, but rarely become ill and are incapable of transmitting the virus efficiently to humans or other animals.
Avian or bird flu is caused by viruses adapted to birds.
Outbreaks have become more common due to the high density and frequent movement of flocks from intensive poultry production.
From 2014 through 2015, United States poultry and egg producers experienced the largest outbreak of H5N2 in recorded history with approximately 51 million birds depopulated to control the spread of the disease.
Since early 2022, more than 60 million birds in 47 states have died either directly from a bird flu virus infection or been culled as a result of possible exposure to infected birds.
The virus has been detected in ducks, geese, gulls,pelicans, swans, vultures, crows, owls, eagles, and many other species of wild birds.
The list of mammals with confirmed infections in Europe and the Americas includes: badger, Blackbeard, bobcat, coyote, dolphin, ferret, fishercat, fox, leopard, lynx, opposum, otter, pig, polecat, porpoise, raccoon, dog, and skunk.
Mammals probably become infected with H5N1 while eating sick or dead birds with high virus loads.
The type with the greatest risk is highly pathogenic avian influenza (HPAI).
Caused by H5N1 stain of influenza A which is endemic in birds in large parts of Asia.
In the last 20 years, fewer than 900 confirmed human cases of H5N1 have been reported.
Avian influenza, doesn’t easily spread between people and other mammals.
Human infections have generally been dead end infections.
Strikes primarily children and young adults and is associated with a high mortality rate.
Evidence for bird-to-human transmission, possibly environment-to-human and nonsustained human-to-human transmission have been seen.
Genetic factors distinguish between human flu viruses and avian flu viruses include:
PB2 (RNA polymerase)
HA (hemagglutinin)
Out of the three types of influenza viruses (A, B, and C), influenza A virus can cause zoonotic infections, with a natural reservoir almost entirely in birds.
There are many subtypes of avian influenza viruses, but only some strains of five subtypes have been known to infect humans: H5N1, H7N3, H7N7, H7N9, and H9N2.
Most human cases of the avian flu are a result of either handling dead infected birds or from contact with infected fluids.
It can also be spread through contaminated surfaces and droppings.
While most wild birds have only a mild form of the H5N1 strain, once domesticated birds such as chickens or turkeys are infected, H5N1 can potentially become much more deadly because the birds are often in close contact.
H5N1 is a large threat in Asia-low hygiene conditions and close quarters.
Although it is easy for humans to contract the infection from birds, human-to-human transmission is more difficult without prolonged contact.
Strains of avian flu may mutate to become easily transmissible between humans.
As of November 29, 2006 258 human cases have occurred with human transmission not going beyond one person.
Has resulted in the death or destruction of more than 140 million birds at a cost of more than 10 billion dollars.
Viruses have the potential to cause severe respiratory illness with 369 human cases and 234 deaths as of 2008.
Spreading of H5N1 from Asia to Europe is much more likely caused by poultry trades than dispersing through wild bird migrations, being that in recent studies, there were no secondary rises in infection in Asia when wild birds migrate south again from their breeding grounds.
Infection patterns followed transportation such as railroads, roads, and country borders, suggesting poultry trade as being much more likely.
Strains of avian flu that have existed in the United States, have been extinguished and have not been known to infect humans.
Birds that have been put down because of avian influenza.
The virus is spread by contact between healthy and unhealthy birds.
Avian influenza is most often spread by contact between infected and healthy birds, though can also be spread indirectly through contaminated equipment.
The virus is found in secretions from the nostrils, mouth, and eyes of infected birds as well as their droppings.
HPAI (high pathogenic avian) infection is spread to people often through direct contact with infected poultry, such as during slaughter or plucking.
Though the virus can spread through airborne secretions, the disease itself is not an airborne disease.
Highly pathogenic strains spread quickly among flocks and can destroy a flock within 28 hours; the less pathogenic strains may affect egg production but are much less deadly.
Humans can contract the avian influenza virus from birds, but human-to-human contact is much more difficult without prolonged contact.
However strains of avian flu may mutate to become easily transmissible between humans.
Some strains of avian influenza are present in the intestinal tract of large numbers of shore birds and water birds, but rarely cause human infection.
Man made ecosystems contribute to modern avian influenza virus ecology:
integrated indoor commercial poultry,
range-raised commercial poultry,
live poultry markets,
backyard and hobby flocks,
bird collection and trading systems including cockfighting.
Indoor commercial poultry has had the largest impact on the spread of HPAI.
The highly pathogenic influenza A virus subtype H5N1 is an emerging avian influenza virus that is causing global concern as a potential pandemic threat.
H5N1 has killed millions of poultry in a growing number of countries throughout Asia, Europe, and Africa.
The coexistence of human flu viruses and avian flu viruses, especially H5N1, may provide an opportunity for genetic material to be exchanged between species-specific viruses, possibly creating a new virulent influenza strain that is easily transmissible and lethal to humans.
The mortality rate for humans with H5N1 is 60%.
The first human H5N1 outbreak occurred in 1997, there has been an increasing number of HPAI H5N1 bird-to-human transmissions, leading to clinically severe and fatal human infections.
Because a significant species barrier exists between birds and humans, the virus does not easily spread to humans.
Although millions of birds have become infected with the virus since its discovery, very few people have died from H5N1.
Culling is used in order to decrease the threat of avian influenza transmission by killing potentially infected birds.
Culling results in severe economic impacts especially for small scale farmers, and culling may be an ineffective preventative measure.
In the short-term, mass culling achieves its goals of limiting the immediate spread of HPAI, but impedes the evolution of host resistance which is important for the long-term success of HPAI control.
H5 subtype viruses contain hemagglutinin for which humans have limited immunity.
Viruses have the potential to cause severe respiratory illness with 169 human cases and 91 (54%) deaths s of Feb 13, 2006.
Clinical manifestations in humans range from asymptomatic infection or a mild upper respiratory tract illness to severe pneumonia and multiorgan failure.
Many viruses isolated from humans have been genotypically resistant to adamantanes and oseltamivir.
Fever a common symptom.
Leukopenia, lymphopenia and thrombocytopenia are common.
H5N1 viruses are evolving rapidly but the direction of their evolution is not presently clear as to the increase in mammalian virulence.
Difficult to spread from person to person and requires a prolonged period of contact to do so.
To cause a pandemic the virus would have to adapt to human hemagglutin receptors and acquire human transmissibility.
Patients given early treatment with oseltamivir have the best chance of surviving the infection.
Oseltamivir routinely provided to persons traveling to regions where disease is endemic.
Corticosteroids not effective in the management of this disease and may add complications.