Avian influenza, commonly known as bird flu, is a contagious viral infection that can infect not only birds but also some mammals including humans. Avian influenza viruses are divided into two groups based on their ability to cause disease: low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI). While LPAI causes mild symptoms in birds, HPAI results in severe illness and death in poultry. Both LPAI and HPAI viruses can at times transmit from birds to humans, but HPAI viruses are of greater concern for human health.
Understanding how avian influenza viruses can jump between avian and human hosts is key to mitigating future pandemics. Here are six major reasons that enable avian influenza transmission to humans:
- Direct Contact with Infected Birds
The most straightforward way humans can contract avian flu is through direct contact with infected birds. Exposure typically occurs at poultry farms, live bird markets, or backyard coops when proper biosecurity and hygiene practices are not followed. During culling operations, for example, the virus can be contracted through contact with bodily fluids of sick birds.
Touching surfaces contaminated by droppings or secretions of infected birds can also lead to transmission. Consuming undercooked poultry products like eggs and meat from infected birds is another route of infection. While sick birds shed the virus in saliva, mucus and feces, HPAI viruses can survive for long periods in the environment. Proper hand washing, protective clothing and cleaning and disinfection are crucial to prevent human infection through direct avian contact.
- Live Bird Markets and Intermediary Hosts
Live bird markets play a major role in avian influenza transmission to humans as they provide optimal conditions for the virus to amplify and transmit between different species. Large numbers of birds and other animals like pigs are kept in cramped cages at these traditional wet markets. Sick birds continuously shed virus into the environment, infecting other birds and facilitating transmission to humans.
Intermediary hosts like pigs and wild aquatic birds at live markets also enable ‘mixing vessels’ where avian viruses can reassimilate, mutate and sometimes become capable of infecting humans more easily. Closure and improved biosecurity at live poultry markets are critical interventions to reduce zoonotic transmission. During outbreaks, restricting human contact with possible intermediary hosts is also important.
- Mutations and Reassortment Events
Influenza viruses are prone to high mutation rates and can adapt rapidly. As they replicate, random errors introduce genetic changes in the viral genome. Most mutations are detrimental, but some can enable the virus to evade host immune responses and become more virulent.
Genetic shift or reassortment is another process where viruses swap gene segments and emerge as novel strains. Both mechanisms allow avian influenza viruses to acquire attributes that facilitate human infection. The ability to bind human-type receptors, adapted molecular features to replicate efficiently in mammalian cells, and resistance to human immunity are some concerning gains.
Continuous circulation in bird and swine populations allows more opportunities for such high-risk mutations to occur. Routine surveillance of avian and swine influenza viruses is therefore vital to detect emergent strains with pandemic potential as early as possible.
- Exposure at Bird Habitats
Wild aquatic birds like gulls, shorebirds and waterfowl are natural reservoirs of avian influenza viruses. These birds can carry influenza A viruses asymptomatically and transmit them to other birds they interact with at lakes, ponds and coastal areas. People who work with wild birds – like wildlife professionals, ornithologists and bird banders – are at higher risk as their activities involve handling sick or dead birds.
During migration seasons, wild waterfowl also spread viruses over long distances as they congregate near wetlands and bodies of water. Hunters coming in contact with infected migratory birds have been infected in past outbreaks. Limiting contact with wild birds and using proper protective equipment reduces risk for humans working or interacting closely with potential avian reservoirs.
- Aerosol and Airborne Transmission
Although influenza viruses mainly transmit through droplets and contact, aerosol transmission is more feasible compared to other respiratory viruses. Some HPAI strains like H5N1 and H7N9 demonstrate efficient aerosol transmission between ferrets in lab studies.
Airborne spread of avian viruses to humans is still rare, likely due to adaptation limits. But virus-laden particles from coughing birds or disturbed excrement can aerosolize and travel distances. This is concerning in densely stocked indoor poultry operations where workers inhale the same air for prolonged periods. Using adequate ventilation and protective N95 respirators helps lower risk. Monitoring workers for respiratory symptoms is also important to identify any aerosol-driven outbreaks early.
- Lack of Existing Immunity in Humans
A key factor that enabled past pandemics like the 1918 Spanish flu was a antigenically novel virus that humans had no prior immune memory against. Avian influenza viruses are in a similar position to cause outbreaks as most strains currently have limited ability to infect humans.
The H5N1 strain, for example, has infected less than a thousand people over 2 decades. So a virus like H5N1 emerging with mutations that adapt it for efficient human transmission could lead to an outbreak as severe as the Spanish flu due to lack of population immunity. Stockpiling H5/H7 vaccines and building vaccine platforms to rapidly produce new immunizations are ways to improve pandemic preparedness.
In summary, avian influenza transmission to humans is enabled by direct contact with infected birds, interactions at live bird markets, viral mutations that adapt strains for human infection, exposure to reservoir migratory birds, limited airborne spread, and lack of existing immune protection. A One Health approach that monitors avian and swine influenza globally, improves farm biosecurity, regulates live markets, builds stockpiles of vaccines and antivirals, and educates public health officials on pandemic risks is our best defense against a deadly avian flu epidemic in humans. With proactive efforts on these fronts, the world can be far better prepared to curb avian influenza transmission before it escalates into another public health crisis.