Pigeons are common urban birds that can be found in most cities around the world. They live in close contact with humans and share many environments with us. Recently, there have been outbreaks of bird flu, also known as avian influenza, in domestic poultry and wild birds in some parts of the world. This has raised concerns about whether pigeons are at risk of contracting bird flu and potentially spreading it to humans. In this article, we will look at what bird flu is, whether it can spread to pigeons, and the implications if pigeons were to become infected.
What is bird flu?
Bird flu refers to influenza viruses that primarily infect birds. There are many different strains of avian influenza virus, most of which do not cause significant disease in birds. However, some strains such as H5N1 and H7N9 can be highly pathogenic, meaning they lead to severe illness and death in domestic poultry.
Influenza viruses have two key proteins on their surface – hemagglutinin (H) and neuraminidase (N). The H and N proteins allow the virus to enter and exit host cells. There are 18 known types of hemagglutinin and 11 types of neuraminidase, which can combine to form different subtypes of influenza A viruses. The most common strains currently circulating in wild birds and poultry are H5 and H7 subtypes.
Avian influenza viruses are found naturally in waterbirds such as ducks, geese and swans. These birds can carry the viruses in their intestines and respiratory tract and shed them in their saliva, nasal secretions and feces. Domestic poultry like chickens and turkeys become infected through direct contact with infected waterfowl or contamination from their feathers or feces.
Symptoms in birds
In birds, symptoms of bird flu can vary greatly depending on the strain of virus:
– Listlessness and lack of energy
– Decreased food and water intake
– Breathing difficulties
– Swelling around the eyes
– Coughing, sneezing and nasal discharge
– Greenish diarrhea
– Lack of coordination
– Drop in egg production
– Sudden death
In highly pathogenic strains like H5N1, the virus can spread systemically in poultry flocks and cause up to 100% mortality within just a few days. However, wild waterfowl often do not show signs of illness despite carrying avian influenza viruses.
Can pigeons get bird flu?
Pigeons are considered particularly resistant to avian influenza compared to other bird species. There are a few key reasons why pigeons seem less susceptible:
Receptors
In order for influenza viruses to infect a host, they must bind to specific receptors called sialic acid receptors on the surface of cells. Avian influenza viruses prefer binding to α2,3-linked sialic acid receptors, whereas human influenza viruses bind to α2,6-linked receptors.
Pigeons appear to have fewer α2,3 receptors in their respiratory and gastrointestinal tracts compared to chickens and ducks. This may make it more difficult for avian influenza viruses to gain entry into pigeon cells.
Body temperature
Pigeons have a higher normal body temperature compared to chickens – around 41-44°C versus 37-41°C. The warmer body temperature of pigeons may be less optimal for replication of avian influenza viruses, which prefer cooler environments.
Immune response
Studies have shown pigeons are able to mount an effective immune response against avian influenza viruses. Their immune system may be primed to recognize avian influenza viruses and quickly limit viral replication if exposed.
This innate resistance means pigeons are less likely to show symptoms of bird flu, even if they do become infected. However, it’s important to note they are not completely immune – there have been occasional cases of pigeons contracting highly pathogenic H5N1.
Evidence of bird flu in pigeons
Despite their relative resistance, there is some evidence that pigeons can become infected with bird flu under the right circumstances:
Experimental infections
Intentionally infecting pigeons with high doses of virus has induced bird flu infection in laboratory studies. When researchers experimentally infected pigeons with H5N1, some birds died while others showed minimal symptoms like weight loss and lethargy. These lethal experimental infections demonstrate pigeons are still susceptible to extremely high viral loads.
Natural outbreaks
There are a few reported cases of pigeons naturally contracting H5N1 during large bird flu outbreaks affecting poultry:
- In 2008, H5N1 was detected in a small number of pigeons at live bird markets in Hong Kong that were also experiencing widespread H5N1 outbreaks in chickens.
- In Iraq in 2006, scientists isolated H5N1 from a pigeon during a large poultry epidemic. However mass deaths of pigeons were not reported.
- In India in 2020, around 800 pigeons died suddenly in a few days across different cities in several states. Some of the dead pigeons tested positive for H5N1.
These situations indicate that while pigeons are generally resistant, they may occasionally contract bird flu during severe epidemics that provide a very high dose of virus. But evidence of symptomatic infection remains rare.
Asymptomatic carriage
There is more evidence that pigeons may carry avian influenza asymptomatically:
- Healthy, wild pigeons have tested positive for low levels of H5N1 in China and H7N9 in Taiwan.
- One study in Vietnam found over 20% of pigeons at live bird markets carried H5N1 asymptomatically.
- In Bangladesh, antibodies against H5N1 and H9N2 have been detected in pigeons, indicating previous exposure.
This suggests while the viruses may not actively replicate enough to cause disease, they can still persist in some pigeons for a period of time and be shed in their droppings. The duration and extent of asymptomatic shedding remains uncertain.
Could pigeons spread bird flu to chickens or humans?
The potential for pigeons to spread avian influenza is not completely clear. On one hand, it seems their resistance to illness and lower receptor binding reduces contagiousness to other birds and humans. But their frequent close contact with poultry and humans may still facilitate some transmission:
To chickens
– There are few studies demonstrating pigeons have directly transmitted H5N1 to chickens. But some experts consider it plausible if the chickens are unvaccinated.
– Pigeons shed very low levels of virus if infected, likely reducing transmissibility.
– More research is needed on how readily pigeons can pick up and transmit bird flu viruses to domestic poultry.
To humans
– There are no reported cases of humans catching H5N1 or other avian influenza strains directly from pigeons.
– The H5N1 virus likely needs to mutate substantially to become easily transmissible between pigeons, let alone have the capacity to jump to humans.
– Pigeons predominantly have avian-type receptors in their respiratory tracts, making human-type transmission less likely.
– However, people should still take precautions and avoid touching sick or dead pigeons barehanded. Proper cooking of pigeon meat and eggs also minimizes any risk.
So while pigeons may pose a theoretical risk, they have not been a significant source of bird flu spread to date based on the available evidence. But more research is still warranted to better understand their potential role.
Control measures if pigeons were infected
If a highly pathogenic strain of bird flu was identified in local pigeons, authorities would likely take measures to contain the outbreak and protect domestic poultry. Some possible control actions include:
- Culling infected flocks and safely disposing of carcasses to prevent further spread.
- Quarantining and testing pigeon flocks from the affected area.
- Temporarily closing live bird markets and pigeon racing or sporting events.
- Disinfecting any contaminated surfaces in coops, lofts, or bird handling areas.
- Restricting transport of all domestic and wild birds in and out of the control zone.
- Tracing contacts between infected pigeons and poultry flocks or markets.
For backyard pigeon owners, isolating and monitoring their birds for any signs of sickness would be advised. They may also be instructed to keep pigeons inside and avoid contact with other birds until the outbreak passes.
Proper biosecurity, poultry vaccination and surveillance would all be stepped up regionally to prevent further spread of the virus.
Conclusions
Pigeons appear to have an inherent resistance to avian influenza viruses, likely related to their receptor distribution, body temperature and immune response. This makes them unlikely to become sick or die from bird flu under normal circumstances. However, experimental studies show they can still harbour H5N1 asymptomatically and infect poultry under ideal conditions.
While pigeons may pose some theoretical risk for veterinary and public health, they have not been a major factor in influenza outbreaks so far. Nonetheless, precautions should be taken around any sick or dying birds to avoid potential exposure to avian influenza viruses. More research into the transmission dynamics between pigeons, poultry and humans would further clarify their role in the spread of bird flu.
