Bird flu, also known as avian influenza, is a viral infection that can affect birds and some mammals. There are many different strains of bird flu virus, most of which do not affect humans. However, some strains, such as H5N1 and H7N9, have caused concern due to their ability to occasionally spread to people and cause severe illness. Bird flu is most commonly spread to humans through contact with infected poultry. Outbreaks in domestic poultry flocks can have devastating impacts on the poultry industry and the environment.
What is bird flu?
Bird flu refers to illness 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. Avian flu viruses do not normally infect humans. However, sporadic human infections with avian flu viruses have occurred.
Types of bird flu viruses
There are many different subtypes of avian influenza A viruses. The current circulating bird flu viruses include:
- H5N1 virus. First detected in humans in 1997, the H5N1 bird flu virus is deadly to domestic fowl and can be deadly to humans. Transmission to humans has occurred from direct contact with infected birds.
- H7N9 virus. First reported in China in 2013, the H7N9 virus has infected more than 1,600 people, with a death rate of about 40%. The virus transmits to humans through contact with infected birds.
- H5N6 virus. The H5N6 subtype has caused sporadic human infections in China. Most cases are the result of direct contact with infected poultry.
- H9N2 virus. H9N2 is moderately widespread and causes mild clinical signs in birds. H9N2 has infected a small number of humans.
In birds, bird flu viruses cause two main forms of disease — low pathogenic and highly pathogenic. Highly pathogenic bird flu causes severe disease and death in poultry species.
How bird flu spreads
Wild aquatic birds can carry bird flu viruses in their intestines and shed them in their feces. If the viruses infect domesticated birds like chickens and turkeys, the animals can die from the infection.
Bird flu spreads in several ways:
Through migrating birds
Wild birds migrate along established flyways and can carry bird flu viruses with them. These migrating birds can then pass the viruses to domestic birds. Many outbreaks affect poultry farms located near migratory flyways and bodies of water.
Between farms
The movement of infected poultry, poultry products, equipment, and people from farm to farm is another method of flu transmission. Viruses can also be tracked into poultry houses on shoes and clothing.
Through markets
Live bird markets and wet markets allow interaction between domestic poultry and wild birds creating an environment where viruses can rapidly spread between birds. This poses a major challenge to containing avian flu outbreaks.
Human transmission
Humans can also inadvertently carry bird flu viruses between locations on shoes, clothing, and equipment. In rare cases, bird flu viruses have developed the ability to spread from person-to-person, typically through close contact. However, this transmission has been limited, inefficient and unsustained.
How bird flu outbreaks affect poultry
Bird flu outbreaks can cripple poultry industries. When an outbreak hits a poultry operation, the virus quickly spreads between birds kept in close quarters. Some key impacts include:
High mortality rates
Certain bird flu viruses, like H5N1, can have mortality rates exceeding 90% in some domestic poultry flocks. Sudden widespread death of chickens and turkeys is often the first sign of an avian flu outbreak on farms.
Culling
Authorities may order entire flocks to be culled (killed) to contain avian flu outbreaks. More than 50 million birds were culled during the H5N1 outbreaks between 2003-2006. Culling efforts aim to stop disease spread but can be financially devastating to producers.
Trade restrictions
Countries will restrict poultry imports from countries experiencing bird flu outbreaks. These trade barriers can severely impact the export market.
Consumer confidence
Public perception may view poultry products as risky even if the chances of disease transmission to humans is extremely low. This avoidance of poultry meat and eggs can negatively impact producers.
| Bird flu outbreak | Area affected | Number of birds culled |
|---|---|---|
| H5N8 in South Korea | South Korea | 38 million |
| H5N1 in China | Mainland China | Over 35 million |
| H5N1 in Vietnam | Vietnam | 45 million |
Effects of bird flu on wild birds
Bird flu viruses primarily circulate in wild waterfowl and shorebirds without causing significant illness or death. However, some virus strains like H5N1 are deadly to certain wild bird species. Impacts may include:
Death of migratory birds
Since 2005, H5N1 has killed millions of wild birds during outbreaks across Asia, Europe and Africa. Waterfowl, raptors and scavenging species like crows appear most vulnerable. Significant mortality has occurred in endangered bird species.
Disruption of migratory patterns
Widespread bird flu mortality and altered flock behavior may disrupt migratory patterns and habitats. Changes in the distribution and timing of migrations can impact ecosystems.
Accelerated spread
Migrating birds likely played a key role in the rapid spread of H5N1 between Asia, Europe and Africa. Wild birds can readily transmit viruses between distant locations.
Re-assortment of viruses
Co-infection of the same host by different influenza viruses can result in gene segment mixing and new virus strains. Wild aquatic birds facilitate virus reassortment and may serve as reservoirs for novel viruses.
Loss of native species
Mortality in threatened and endangered avian species, like California condors, whooping cranes, Hawaiian crows and oriental storks, is a major concern during outbreaks. Even localized losses can devastate small populations.
How bird flu impacts humans
Bird flu viruses do not usually infect people. But sporadic human infections occur through direct poultry contact. Concerning strains like H5N1 and H7N9 have higher death rates when transmitted to humans. Impacts include:
Severe pneumonia
Symptoms are often flu-like – fever, cough, sore throat. Pneumonia and acute respiratory distress syndrome can develop in severe cases. Multi-organ failure is also possible.
Antiviral resistance
H5N1 and H7N9 display resistance to older antiviral drugs like adamantine derivatives. Newer neuraminidase inhibitors may improve clinical outcomes.
No vaccine exists
Several H5N1 vaccine candidates are in development. However, no licensed vaccines for H5N1 or H7N9 currently exist for human use. Preventive vaccines are available for poultry.
Public health response
Public health officials monitor outbreaks closely to identify any sustained human-to-human spread. Preparedness activities include stockpiling antivirals and prototyping diagnostic tests and vaccines.
| Bird flu strain | Human cases since 2003 | Deaths |
|---|---|---|
| H5N1 | 870 | 455 |
| H7N9 | 1,625 | 625 |
Environmental contamination
Bird flu viruses can persist in water and manure at low temperatures, especially in cold climates. Environmental contamination may play a role in outbreaks and virus transmission.
Water sources
Avian influenza viruses can remain infectious for long periods in water. Lakes, ponds and other water supplies can become contaminated with bird flu viruses shed by wild waterfowl. This viral persistence facilitates transmission.
Poultry litter
The H5N1 virus can survive for weeks in poultry manure and litter, even at moderately high temperatures. Reusing contaminated organic wastes may infect flocks. Proper composting can help inactivate viruses.
Live markets
Wet markets allow open slaughter of live poultry in crowded conditions with poor sanitation. This provides an ideal environment for avian flu transmission and viral contamination of surfaces.
Soil transmission
While less studied, some research detected viable H5N1 viruses in soil samples up to six days after experimental contamination. Soil persistence may depend on many factors like temperature, pH and organic content.
Prevention and control strategies
A multifaceted approach is required to prevent and control the spread of avian influenza viruses in poultry and wild birds.
Poultry biosecurity
Biosecurity measures help prevent contact between domestic flocks and wild birds. This includes housing birds indoors, restricting access to poultry houses, and decontaminating equipment.
Routine surveillance
Regular testing of commercial and backyard poultry flocks for avian influenza viruses allows for early detection and containment of outbreaks.
Restricting live markets
Limiting or closing live bird markets, which allow customer contact with live poultry, reduces disease transmission.
Wild bird monitoring
Sampling wild waterfowl allows tracking of influenza viruses circulating in waterfowl populations and identification of novel strains.
Worker training
Education on biosecurity and early outbreak reporting helps poultry workers prevent and quickly respond to signs of infection.
Culling infected flocks
Quickly and humanely killing infected domestic birds limits viral loads in the environment and prevents spread. Proper carcass disposal is essential.
Avian flu vaccines
Vaccines are available to help protect poultry flocks against common circulating H5 and H7 bird flu viruses. However, vaccines must match the outbreak strain.
Conclusion
In summary, avian influenza viruses can have devastating impacts on domestic poultry industries, wild bird populations, and trade. The environment plays an important role in outbreaks through persistence in water, poultry litter and soils. Controlling avian flu requires ongoing surveillance of poultry and wild birds paired with biosecurity measures and education campaigns. With continued vigilance, the threat of bird flu can be reduced to protect poultry, public health and the environment.
