Bird flu, also known as avian influenza, is a respiratory disease caused by influenza A viruses that spread among poultry and wild birds. Occasionally these bird flu viruses can infect humans as well. There are several subtypes of bird flu viruses, with the H5N1 and H7N9 subtypes being of most concern for human infection.
When bird flu viruses infect humans, they can cause a range of respiratory symptoms. This is because like all influenza viruses, bird flu viruses predominantly infect and damage cells lining the respiratory tract. Understanding how bird flu affects the respiratory system can help guide treatment and prevention strategies against this potentially serious infectious disease.
How does bird flu spread to humans?
Bird flu viruses are carried by wild aquatic birds like ducks and geese. These wild birds don’t usually get sick from the viruses. However, bird flu can spread rapidly at poultry farms where large populations of birds are kept together in confined spaces. When infections spread at poultry farms, large numbers of chickens, turkeys, and other domesticated birds can become sick or die from the virus.
Humans can catch bird flu through close contact with infected birds or contaminated environments. Transmission to humans usually requires direct contact with sick or dead infected poultry. Contact with their mucous, blood, feathers or feces can transmit the virus. Less commonly, humans may catch bird flu through contact with contaminated surfaces.
Consumption of raw or undercooked poultry products can also transmit bird flu viruses. However, properly cooking poultry products to an internal temperature of 165°F kills the virus.
Very rarely, limited human-to-human transmission of bird flu has occurred. This has involved very close contact between family members or health workers caring for those sick with bird flu. Sustained transmission between humans has not occurred yet.
How do bird flu viruses infect respiratory cells?
Influenza viruses like bird flu have spherical or filament-shaped virions enveloped in a lipid membrane. Two key proteins help the virus penetrate and infect host cells: hemagglutinin (HA) and neuraminidase (NA).
Hemagglutinin enables the virus to attach to sialic acid receptors on host cell surfaces. Human-adapted influenza viruses bind α2-6 linked sialic acids which are abundant on epithelial cells lining the human respiratory tract. In contrast, avian-adapted influenza viruses preferentially bind α2-3 linked sialic acids found deeper in the lungs.
After attaching via hemagglutinin, the virus particle is taken up into the cell via endocytosis. Acidification inside the endosome triggers a conformational change in HA that fuses the viral and endosomal membranes.
This releases the viral genome segments into the host cell cytoplasm. The negative-sense viral RNAs are transported into the nucleus and transcribed into positive-sense mRNAs and replicated into new genomic viral RNAs.
Newly synthesized viral RNAs and proteins assemble at the host plasma membrane. Neuraminidase helps cleave sialic acids to release new virions that bud from the infected cell. The virus can then spread to infect other cells.
What respiratory cells does bird flu infect?
Avian influenza viruses can infect a variety of cell types lining the human respiratory tract. These include:
- Epithelial cells lining the nose, sinuses, throat, and larynx
- Epithelial cells lining the trachea, bronchi and bronchioles
- Alveolar epithelial cells lining small air sacs in the lungs
- Alveolar macrophages in the lungs
Of these cell types, alveolar epithelial cells deep in the lungs seem to be most susceptible to highly pathogenic avian influenza virus subtypes like H5N1. However, all influenza viruses can infect cells throughout the respiratory tract.
How does bird flu damage the respiratory system?
Bird flu can damage the respiratory system through both direct viral effects and host immune responses:
Direct viral effects
- Viral infection impairs and kills epithelial cells lining the trachea, bronchi and alveoli
- Loss of cilia on airway epithelial cells prevents clearing of mucus and debris
- Difficulty breathing due to fluid buildup and airway plugging with dead cells and fibrin
- Pneumonia as alveoli fill up with fluid and inflammatory cells
- Acute respiratory distress syndrome (ARDS) can develop in severe cases as fluid builds up and the lungs fail
Immune responses
- Infiltration of immune cells like neutrophils and macrophages causes inflammation and lung damage
- Inflammatory cytokines secreted to combat infection can worsen tissue damage
- Lung damage from frustrated phagocytic activity of neutrophils and macrophages
Overall, bird flu can reduce lung function by damaging airway epithelial cells, triggering buildup of fluid and cells in alveolar air spaces, and provoking injurious inflammation. This impairs oxygen exchange and can lead to severe pneumonia and ARDS.
What are the symptoms of bird flu in humans?
Bird flu causes symptoms similar to seasonal influenza but is often more severe. After an incubation period of 2 to 8 days, common symptoms include:
- Fever and chills
- Cough
- Sore throat
- Muscle aches
- Headache
- Shortness of breath
- Nausea, vomiting, diarrhea (more common in children)
In severe cases, pneumonia and ARDS can develop rapidly. Warning signs include breathing difficulty, bluish discoloration of the lips, respiratory failure requiring mechanical ventilation, and septic shock.
How is bird flu diagnosed?
Diagnosing bird flu requires laboratory testing, including:
- Polymerase chain reaction (PCR) tests on respiratory samples like nasopharyngeal swabs to detect viral RNA
- Viral culture to isolate influenza A virus from specimens
- Serology blood tests to detect antibodies against the virus
- Immunohistochemical staining to detect viral antigens in tissues
Diagnostic testing is important for confirming bird flu cases. It also allows subtyping of the virus to detect novel and potentially pandemic strains like H5N1.
What is the treatment for bird flu?
There are no specific treatments approved for bird flu infections. However, the following supportive therapies are used:
- Oxygen therapy and mechanical ventilation for respiratory support
- Fluids and medication to maintain blood pressure in septic shock
- Broad spectrum antibiotics for secondary bacterial infections
- ECMO (extracorporeal membrane oxygenation) in some severe ARDS cases
Experimental antiviral medications like oseltamivir and zanamivir may reduce severity if given early. Corticosteroids are controversial but may help control lung inflammation. More research is needed to find optimal treatments for bird flu infections.
How can bird flu be prevented?
Preventing bird flu involves controlling transmission at its source in poultry as well as protecting humans through good hygiene and vaccination:
Poultry control measures
- Culling infected flocks
- Restrictions on importing/transporting live poultry
- Testing and surveillance of poultry
- Biosecurity at farms to prevent spread
- Proper slaughtering and cooking to kill viruses
Human prevention methods
- Avoiding contact with sick or dead poultry
- Washing hands thoroughly with soap
- Properly preparing raw poultry products
- Annual flu vaccination to prime immune responses
- Antiviral medication for high-risk groups during outbreaks
Coordination between agriculture, wildlife, and public health agencies is key for control. Pandemic preparedness plans are essential in case a highly transmissible strain emerges. Ongoing research and development is needed for better antiviral drugs and bird flu vaccines to protect humans.
Conclusion
Bird flu or avian influenza remains an important public health threat, especially emerging strains like H5N1 and H7N9 that can be severe in humans. These viruses predominantly infect and damage epithelial cells lining the respiratory tract. This can lead to pneumonia, ARDS, and respiratory failure.
Control efforts focus on preventing spread in poultry populations and transmission to humans. Improved medical treatments and vaccines for high-risk groups are also vital. Continued surveillance and research on bird flu viruses is critical for getting ahead of the next potential pandemic strain.
