Overview

Bird flu is a viral infection that affects various bird species including poultry birds like chicken. Outbreaks of bird flu can cause economic and environmental devastation as the poultry food supply becomes contaminated.¹ Furthermore, some strains of bird flu have the potential to not only infect birds but also transmit across species to humans in a process known as zoonotic transmission. Bird flu is an important disease that should not be overlooked, and understanding the root sources of bird flu and how to counter and respond to outbreaks can help decrease the risk of dangerous outbreaks.

Transmission of bird flu in poultry

Bird flu has many different modes of transmission between birds.² Firstly, it can be transmitted via physical contact, which makes cramped conditions such as poultry farms more susceptible to faster outbreaks of bird flu, resulting in the entire bird population in the farm becoming contaminated, leading to large economic losses. To reduce the risk of bird flu contamination between farms, farmers have to cull their chicken and poultry livestock when they become infected, which not only results in economic losses but can also affect local and global food prices.

Secondly, the viral infection can also be transmitted via contaminated surfaces such as cages, farm equipment, and clothing.² This means that bird flu can also spread between population centres via physical carriers such as people, vehicles and other birds and animals. Contaminated surfaces must therefore be sterilised before reusing after an outbreak.

Thirdly, bird flu can also be spread via body fluids such as saliva, droppings, and nasal secretions.² This type of transmission is important for understanding how wild birds can also act as vectors that transport the infection from one location to another. Infected flying and migrating birds can end up spreading bird flu via this mechanism.

Fourthly, the viral infection can also be transmitted through the air via airborne transmission.² Respiratory droplets and dust can act as carriers that spread the infection further, similar to how the COVID-19 virus spreads.

Bird flu is a highly infectious disease that can quickly transmit across a population and cause devastating environmental and economic losses. Understanding the methods of transmission that the virus uses to spread is important in understanding how to overcome and minimise risk from such infectious outbreaks.

Common sources of bird flu outbreaks

Wild migratory birds

Wild and migrating birds can be an important source of infection amongst livestock populations. Wild birds may come into contact with an infected population of poultry livestock, and become infected via air, physical contact, or body fluids.² Carrying the infection, migrating wild birds can travel kilometres away, contaminating surfaces, water sources, and food, and can come into contact with other birds including poultry. Some species of birds, such as waterfowl, can carry the virus without displaying any symptoms, making it harder to identify which wild birds are carrying the virus and which are healthy.

Contaminated water sources

Infected birds can contaminate water sources with the bird flu virus. Ponds, lakes, and reservoirs are often hotspots for wildlife activity, and infected birds can contaminate the local environment around water sources, spreading the disease to the other birds that use the pond for water.³

Wild birds can not only contaminate natural water sources but can also contaminate the water sources used by poultry. As birds migrate, they could share the water sources that are meant for poultry such as chickens, contaminating the water source in the process and coming into contact with many other birds. They can also contaminate water sources without drinking from them via their excrement.

Infected poultry movement

Large outbreaks of bird flu can be further exacerbated by the transport of infected birds between farms or markets which can spread the disease further. If infected birds are relocated to another farm, or are sent to a market, the bird flu outbreak can become more widespread and harder to control.

Some countries have live poultry markets where live birds are taken to the market to be sold. In these cages, the poultry birds are in close proximity to each other, leading to cross-infection. Furthermore, all cages, trucks, cars and other equipment used during the transport of infected poultry may become contaminated and infect other birds.

Human activity on farms

Farmers that work with poultry may unintentionally become carriers of bird flu, and infect and contaminate other birds. Although some strains of bird flu can transmit cross-species to humans, most strains of the virus cannot harm humans. Despite that, humans can still act as a carrier for the disease, and the virus can hitchhike on human skin and clothing and be transmitted to other birds.⁴

Hence, biosecurity practices are an important step to ensure that the risk of bird flu outbreaks is reduced. Proper hygiene and disinfection when handling poultry livestock can reduce the risk of bird flu outbreaks.

Improper disposal of infected birds

In the case of an outbreak, infected and dead birds need to be disposed of quickly and effectively to limit the spread of the disease. Birds infected with bird flu are culled (killed), and the bodies are then disposed of. The bodies must not be left exposed and must be treated as soon as possible. This is because the infected exposed corpse can attract wild animals and scavengers, which may end up spreading the disease. The infected corpses could also contaminate the soil and water if not disposed of.

There are many different methods of disposal of the dead, but still infected, bodies. One of the most effective ways is with high-temperature incineration, where the bodies are burnt. This is extremely effective as the high temperature destroys any pathogens such as the bird flu virus, and specialised incinerators should be used to ensure complete combustion and prevent airborne spread.⁵

Shared farm equipment

The practice of sharing farm equipment may help reduce the costs of operating a farm. However, in the event of an outbreak, it can spread easily between farms that are sharing equipment. Contaminated equipment that is being shared can quickly spread an outbreak to other farms, and so if equipment has to be shared, it is recommended to practice adequate hygiene and sterilise all equipment being used.

Risk factors that increase outbreaks

Bird flu has many different sources where an outbreak can occur, but some risk factors can elevate the risk of the severity of an outbreak. These risk factors often result in a much more rapid escalation in bird flu outbreaks if they occur, and hence are much more vulnerable to such outbreaks of bird flu. 

For example, high-density farming is often labelled as a potential risk factor for bird flu.⁶ In high-density farming, large numbers of poultry birds are in close quarters to increase the space efficiency of farms and increase output. However, the close proximity between the birds makes these farms more vulnerable to outbreaks, as it is easier for the virus to spread between birds in close proximity. High-density industrial farms are also often very high capacity, generating huge quantities of goods. As a result, a bird flu outbreak in a high-density farm can not only be difficult to control but can also lead to huge economic losses.

Even farms that do not engage in industrial high-density farming may still be at risk of bird flu outbreaks. Free-range systems in farming allow poultry birds more freedom and space to move around, and have access to open-air spaces. However, this still comes as a risk since although the birds are not as densely packed together, the open spaces that they enjoy increase the contact between the poultry birds and wild birds, potentially introducing flu strains that wild birds may be carrying.

Summary

In conclusion, bird flu is a potentially very dangerous disease that can cause widespread damage to agricultural industries and the environment. Bird flu is very infectious and has many sources that can cause an outbreak, and hence combating bird flu requires adapting and reducing many factors that increase the risk of bird flu. Bird flu in poultry is usually introduced from external sources such as wild animals, contaminated farm equipment and people, which suggests that there must be adequate quarantine and hygiene practices in place to reduce the spread and damage of bird flu outbreaks.