Avian influenza, commonly known as bird flu, is a highly contagious viral disease affecting birds worldwide. Avian influenza viruses are classified into two categories based on their ability to cause disease in poultry – low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI). LPAI strains cause mild symptoms in birds while HPAI strains can cause severe disease with high mortality rates in domestic poultry. Some strains of avian influenza viruses such as H5N1 and H7N9 can also infect humans.
When was avian influenza first discovered?
The first known outbreak of avian influenza occurred in Italy in 1878 when a disease called “fowl plague” swept across the region, killing large numbers of chickens, ducks and turkeys. The infectious agent was found to be a filterable virus in 1955. In 1981, the first influenza A virus was isolated from birds during an outbreak in Scotland. This virus was classified as an H5N3 subtype. The highly pathogenic H5N1 strain was first detected in geese in China in 1996. This was followed by outbreaks of HPAI H5N1 in poultry and humans in Hong Kong in 1997.
What are the major outbreaks of avian influenza in history?
Some of the major outbreaks of avian influenza viruses in history include:
- 1983-85 – H5N2 outbreak in USA resulting in destruction of over 17 million birds
- 1994 – H7N3 outbreak in Australia
- 1997 – H5N1 outbreak in Hong Kong poultry and humans
- 2003-04 – H7N7 outbreak in Netherlands resulting in one death
- 2004 – H5N1 outbreak in Asia spreading to over 60 countries in following years
- 2013-14 – H7N9 outbreak in China causing over 300 human infections
- 2014-15 – H5N2 outbreak in USA leading to culling of 50 million birds
- 2016-17 – H5N8 outbreak in Europe and Asia
The H5N1 outbreak in Hong Kong in 1997 was the first instance of human infections and deaths due to avian influenza. Since 2003, the H5N1 virus has devastated poultry flocks across Asia, Europe and Africa. The World Health Organization has reported over 850 confirmed human cases of H5N1 with a mortality rate exceeding 50%.
1997 H5N1 Hong Kong outbreak
In May 1997, a three-year-old boy in Hong Kong was hospitalized with respiratory illness and died. Later that month, two more cases of respiratory disease in children were reported. In all three cases, influenza A H5N1 virus was detected. These were the first documented human infections caused by H5N1. At the same time, outbreaks of highly pathogenic H5N1 were occurring at live poultry markets in Hong Kong. To control the outbreak, authorities ordered the slaughter of all poultry in Hong Kong, resulting in the culling of around 1.5 million birds.
H5N1 outbreak in Asia (2003-2011)
From 2003 to mid-2011, there were 566 confirmed human cases of H5N1 worldwide resulting in 331 deaths. The majority of these cases occurred in Asian countries including Indonesia, Vietnam, China, Cambodia, Thailand and Azerbaijan. This epidemic of H5N1 became the largest and deadliest outbreak of avian influenza in humans.
H7N9 outbreak in China (2013-2017)
In February 2013, a novel avian influenza A H7N9 virus emerged in China, causing severe respiratory illness in humans. As of September 2017, the WHO reported 1564 laboratory-confirmed cases of human infection with H7N9 virus, including at least 612 deaths. Almost all cases were exposed to live poultry or contaminated environments.
How do avian influenza viruses spread between birds?
Wild aquatic birds are the natural reservoir for avian influenza viruses. These viruses spread among birds through direct contact or through contact with contaminated surfaces. Migratory birds can carry avian influenza viruses to long distances, spreading them between countries and continents.
Domestic poultry like chickens and turkeys are especially vulnerable to avian influenza outbreaks. Once a virus enters a poultry farm, rapid bird-to-bird transmission can occur through:
- Respiratory secretions
- Saliva
- Nasal discharge
- Feces
Fomites including feed, water, equipment and cages contaminated with virus can also facilitate spread between birds. Strict biosecurity measures are required to prevent introduction and control spread of avian influenza in poultry farms.
How do avian influenza viruses infect humans?
Humans can get infected with avian influenza viruses through close contact with infected birds or contaminated environments. Transmission does not occur easily and sustained human-to-human transmission is very uncommon. When avian viruses gain the ability for human-to-human transmission, it can cause deadly pandemics.
The main routes of transmission of avian influenza from birds to humans are:
- Direct contact – Touching sick or dead infected birds
- Indirect contact – Touching contaminated surfaces or objects
- Inhalation – Breathing in aerosols or droplets with virus (e.g. in wet markets)
- Ingestion – Consuming raw or undercooked poultry products
People working closely with poultry, such as poultry farm workers and those involved in culling or processing infected birds are at highest risk of contracting avian influenza.
What are the symptoms of avian influenza in humans?
The symptoms of avian flu in humans range from mild flu-like illness to severe pneumonia and even death depending on the strain. Common symptoms include:
- Fever and chills
- Cough
- Sore throat
- Muscle and joint pain
- Severe respiratory illness (e.g. difficulty breathing, pneumonia)
- Eye infections (conjunctivitis)
- Nausea, vomiting, diarrhea
Some patients rapidly deteriorate and develop complications like respiratory failure, septic shock and multiple organ failure. Those infected with H5N1 or H7N9 viruses are at highest risk for severe disease and fatal outcomes.
How is avian influenza diagnosed?
Diagnostic tests are required to confirm avian influenza virus infection in humans. These include:
- Viral culture – Isolating the virus from respiratory samples like nasal and throat swabs
- RT-PCR – Detecting viral RNA in respiratory samples through reverse transcription polymerase chain reaction
- Serology – Measuring avian influenza specific antibodies in blood
Laboratory criteria for confirmation are either a positive viral culture or PCR for avian influenza virus.
How is avian influenza treated?
There are no specific drugs approved for treating avian influenza virus infections. However, in some severe H5N1 cases, antiviral medications like oseltamivir (Tamiflu) and zanamivir (Relenza) have been used. These help by reducing the duration and severity of symptoms.
Treatment is mainly supportive care to relieve symptoms and prevent complications:
- Oxygen therapy for respiratory distress
- Mechanical ventilation for respiratory failure
- Fluid management and maintaining electrolyte balance
- Medications to reduce fever (acetaminophen)
- Broad spectrum antibiotics for secondary bacterial infections
- Intensive organ support in ICU for multi-organ failure
What attempts have been made to control avian influenza?
Global efforts have been made to control avian influenza outbreaks in poultry and prevent human infections:
- Culling – Mass slaughtering of infected domestic and wild bird populations
- Quarantines – Restricting movement in and out of infected premises
- Surveillance – Monitoring wild and domestic birds for disease occurrence
- Worker safety – Use of protective clothing by personnel involved in culling and outbreak response
- Hygiene – Regular disinfection and cleanliness in farms, markets and slaughterhouses
- Vaccination – Immunizing poultry flocks and high-risk personnel against H5N1
- Public awareness – Educating people about risks, especially those working with poultry
These measures aim to prevent spread within and between poultry flocks and reduce human exposure to infected birds. However, avian influenza remains an ongoing public health threat due to virus persistence in wild birds and live poultry markets.
What is the global impact of avian influenza?
Avian influenza outbreaks have had devastating effects globally:
- Economic losses exceeding $50 billion due to trade bans, culling and decreased productivity
- More than 250 million birds culled to control outbreaks since 2003
- Over 850 confirmed human cases and 450 deaths due to H5N1
- Public health resources strained by continued monitoring and mitigation efforts
- Persistent threat of virus mutation resulting in deadly human pandemic
The widespread geographical distribution and endemic nature of some avian influenza viruses in bird populations makes eradication unlikely. Preventing avian-to-human transmission and preparedness for a potential influenza pandemic will continue to be global health priorities.
What is the historical significance of avian influenza?
Some key historical points about avian influenza include:
- Recognized as a disease entity affecting poultry since the late 19th century
- Caused the devastating panzootic of “fowl plague” through the 1880s and 1890s
- Emerged as a public health concern with H5N1 human infections in 1997
- H5N1 outbreak in Hong Kong in 1997 was the first evidence of direct avian-to-human transmission
- Has resulted in major losses for the global poultry industry
- Remains a potential pandemic threat due to circulation in wild birds and live markets
The 18th century Italian physician Giovanni Lancisi made the first connection between disease in birds and humans during an outbreak of human influenza in Rome that coincided with fowl plague in the same region. However, the influenza virus was yet to be discovered then.
Understanding the molecular virology, transmission patterns, and pathogenesis of avian influenza over the past century has shaped preparations for the next pandemic. The threat of an avian flu pandemic continues to be a major public health concern based on lessons learned from history.
Conclusion
In summary, avian influenza has a nearly 150-year history since its first recognition as a disease of birds in the late 1800s. Some of the major milestones and outbreaks include: the identification of influenza A virus in 1955; the emergence of the highly pathogenic H5N1 virus in the 1990s; human outbreaks of H5N1 and H7N9 in Hong Kong and China; and the global spread of H5N1 across three continents in the 2000s. Ongoing circulation of avian influenza viruses in wild waterfowl and efforts made towards pandemic preparedness highlight that this disease remains an important veterinary and public health problem worldwide.