Bird flu, also known as avian influenza, is a virus that infects domestic poultry and wild birds. There are many different strains of bird flu virus, some of which can be transmitted from birds to humans. Understanding how bird flu is spread is important for controlling outbreaks and protecting public health.
What is bird flu?
Bird flu is caused by influenza Type A viruses that occur naturally in wild aquatic birds worldwide. Wild birds can carry bird flu viruses in their intestines and respiratory tract and usually do not get sick from them. However, some bird flu strains are highly pathogenic, meaning they can cause severe disease and death in domestic poultry. Others cause low pathogenicity avian influenza, a much milder illness.
The main strains of bird flu that have caused concern for human health in recent years include:
- H5N1 – first emerged in China in 1996 and has caused the majority of bird flu infections in humans
- H7N9 – first reported in China in 2013 and continues to circulate seasonally
- H5N6 – emerged in China in 2013 and has infected a small number of people
- H5N8 – detected in 2016 and has mostly affected European countries
All known bird flu virus subtypes are named according to two proteins on their surface – hemagglutinin (H) and neuraminidase (N). Frequent genetic changes in these viruses occur naturally as they replicate, which allows new virus variants to emerge over time.
How are birds infected with bird flu?
Wild aquatic birds are the natural reservoirs for bird flu viruses. They shed the viruses in their saliva, mucous and feces, which can then spread to other birds.
Domestic poultry flocks become infected through direct contact with wild birds, or through contact with contaminated materials like manure, feed, water, or cages that have virus particles on them. The virus spreads easily between birds kept in close quarters through respiratory and fecal-oral routes.
Bird flu outbreaks often start when migrating wild birds introduce low pathogenic viruses into domestic poultry flocks. The virus can then mutate into highly pathogenic strains as it circulates among densely populated poultry.
Which species are affected by bird flu?
All species of birds are thought to be susceptible to bird flu infection, however some key differences have been observed between wild birds and poultry:
- Wild birds – Waterfowl and shorebirds are natural reservoirs of avian influenza viruses. Many species can be infected without showing symptoms. Migratory patterns allow for global spread of bird flu.
- Poultry – Chickens and turkeys are especially vulnerable to highly pathogenic bird flu strains which can cause severe disease with a mortality rate up to 100% in flocks.
- Gamebirds – Upland gamebirds like pheasants and partridge are susceptible but generally show low mortality rates.
- Pet birds – Birds kept in captivity like parrots, finches and aviary birds are at risk of infection during outbreaks.
The differences in disease severity are due to factors like genetic resistance, variations in virus receptors, and immune response capabilities across bird species.
How is bird flu transmitted to humans?
Humans can be infected with bird flu viruses through close contact with infected birds or contaminated environments, but human-to-human transmission is extremely rare.
The main routes of bird-to-human transmission include:
- Direct contact – Handling, slaughtering, defeathering, or preparing infected poultry for consumption, especially without proper protective equipment.
- Aerosol exposure – Inhalation of aerosols or dust containing fresh secretion from infected birds. This could occur in confined areas like barns or during processing of poultry.
- Consumption – Eating undercooked or raw poultry products contaminated with the virus.
- Contact surfaces – Touching bird droppings or surfaces contaminated with secretions from infected birds and then touching eyes, nose or mouth.
Person-to-person spread of bird flu is very uncommon. There a few isolated cases of human-to-human transmission, usually involving very close and prolonged contact between an infected person and a family member providing care.
What are the symptoms of bird flu in humans?
Symptoms of bird flu in humans can vary from mild to severe depending on the virus strain. Usually they start within 2 to 5 days after exposure.
Common symptoms include:
- Fever and chills
- Cough
- Sore throat
- Muscle aches
- Headache
- Shortness of breath or difficulty breathing
- Eye infections (conjunctivitis)
Rarely, some strains like H5N1 and H7N9 can cause severe viral pneumonia, acute respiratory distress syndrome, multi-organ failure, and death. Symptoms of severe infection include:
- High fever (>100.4°F)
- Difficulty breathing
- Rapidly worsening respiratory symptoms
- Pneumonia or ARDS
- Altered mental status
- Abdominal pain, vomiting, diarrhea
People at higher risk of severe illness include older adults, children under 5, pregnant women, and those with chronic medical conditions like asthma, diabetes, heart disease and HIV/AIDS.
Diagnosis and treatment of human bird flu
Specific laboratory tests are required to confirm human infection with bird flu viruses:
- RT-PCR testing – Respiratory samples like a nose and throat swab are analyzed using reverse transcription polymerase chain reaction (RT-PCR) assay to detect viral genetic material.
- Serology (blood tests) – Blood samples are taken to look for antibodies produced in response to the virus.
- Viral culture – Trying to grow the virus from respiratory, blood or tissue samples in the lab.
Doctors may order imaging tests like a chest X-ray or CT scan of the chest to assess pneumonia or ARDS in severe cases.
There are no specific treatments approved for bird flu infections in humans. Supportive medical care focuses on:
- Treating symptoms like fever using acetaminophen
- Respiratory support with supplemental oxygen, ventilators
- Treating secondary infections with antibiotics
- Maintaining fluid and nutrition levels
- Antiviral medications like oseltamivir may reduce severity if started very early in illness
Prevention of human bird flu infections
Prevention strategies for human bird flu aim to reduce exposure to infected birds and contaminated environments.
Recommended precautions include:
- Avoid contact with poultry that appear ill or birds that have died from unknown causes.
- Avoid travel to affected areas with known bird flu activity.
- Avoid direct contact with surfaces contaminated with bird droppings or secretions.
- Wear personal protective equipment like gloves, eye protection, face masks, and gowns when working with birds.
- Wash hands thoroughly with soap and water after touching birds or contaminated surfaces.
- Cook poultry products thoroughly to an internal temperature of at least 165°F to kill viruses.
- Get the seasonal influenza vaccine to reduce the dual infection risk.
Public health surveillance, vaccination and culling of infected poultry flocks, and restrictions on poultry movement during outbreaks also help limit human exposures.
Global significance of bird flu
Bird flu is considered a concern for global public health for several reasons:
- Some strains like H5N1 and H7N9 can be severe when transmitted to humans. Around 60% of human H5N1 cases have been fatal.
- Pandemic potential exists if a virus mutates to allow easy human-to-human transmission while retaining lethality.
- The widespread nature of influenza viruses in wild migratory birds enables global spread.
- Poultry trade and livestock movement facilitate transmission across countries and continents.
- Vaccination of poultry is only partially protective against some strains and logistically challenging.
- Countries with high poultry densities in close proximity to humans are especially vulnerable.
Ongoing surveillance, transparency about outbreaks, research into new vaccines and medications, and compliance with international health regulations are all vital for controlling bird flu threats.
Key Facts About Bird Flu Transmission
- Caused by Type A influenza viruses that occur naturally in wild aquatic birds
- Can be highly pathogenic or low pathogenic in birds
- Spreads from wild birds to poultry through contact with saliva, mucous, feces
- All species of birds can be infected but may show different symptoms
- Humans can be infected through contact with sick birds or contaminated environments
- Person-to-person transmission is very rare
- Symptoms in humans range from mild to severe pneumonia and ARDS
- Prevention involves minimizing exposure to infected birds and contaminated environments
- Considered a pandemic threat if a highly lethal human transmissible strain emerges
Conclusion
Bird flu remains an important global health issue given its pandemic potential. Understanding how avian influenza viruses spread in birds, the transmission risks to humans, and preventive measures is critical for controlling outbreaks. Ongoing surveillance, research and international cooperation are needed to reduce the threat.