Credit Calistemon
#18,426
In 2006, when H5N1 was beginning to make inroads into Europe, there was understandably a good deal of concern over the ability of pigeons - which are ubiquitous city-dwellers - to carry and/or transmit avian influenza.
In the April 2006 edition of the CDC's Journal of Emerging Infectious Diseases, in an article entitled Avian Influenza H5N1 in Naturally Infected Domestic Cats, they wrote of a domestic cat that died after eating an infected pigeon. Other pigeons had reportedly died in the area.It wasn't an idle concern, since earlier that year we'd seen reports that a 14 year old pigeon handler in Iraq had died of the H5N1 virus, and in West Jakarta, a 39 year old man died after reportedly cleaning pigeon feces from blocked roof gutters at his home.
We report H5N1 virus infection in a domestic cat infected by eating a pigeon carcass. The virus isolated from the pigeon and the cat showed the same cluster as the viruses obtained during the outbreak in Thailand. Since cats are common house pets, concern regarding disease transmission to humans exists.
At the same time, we were seeing assurances that pigeons were not susceptible to HPAI H5 infection, including an article on pigeons and the bird flu virus, published in (now defunct SEED magazine) on May 22, 2006, entitled The Invincible, Flu Immune Pigeon and an Expert Q&A by the BBC on Bird Flu which included this exchange:
While the evidence suggests pigeons have played only a small role in H5N1's spread, in the middle of the last decade they were linked to avian H7N9 (see Eurosurveillance: Genetic Tuning Of Avian H7N9 During Interspecies Transmission & human cases here, here & here).
In 2014, the journal Avian Pathology published Natural infection with highly pathogenic avian influenza virus H5N1 in domestic pigeons (Columba livia) in Egypt by Shimaa M. G. Mansour, Reham M. ElBakrey, Haytham Ali, David E. B. Knudsen & Amal A. M. Eid., which details a 2011 H5N1 outbreak (with 50% mortality) among free range pigeons in Egypt.
The authors called this `. . .the first description and characterization of HPAIV in naturally infected pigeons in Egypt. Our findings reveal that pigeons can indeed be susceptible to H5N1 HPAIVs and could be a source of infection to other birds and humans.'
In conclusion, RP25 is a novel reassortant avian influenza virus, and poses a potential threat to human. These results highlight the importance of persistent surveillance for H5 subtype AIV in birds.In 2017's Vet. Micro.: Experimental Infection Of Mandarin Ducks & Pigeons With HPAI H5N8, researchers successfully inoculated Mandarin ducks and domestic pigeons with the HPAI H5N8 virus - and while they shed the virus - they displayed no clinical signs of infection.
It should be noted, that many of the laboratory experiments exonerating pigeons were conducted nearly 20 years ago, with older clades of the H5N1 virus.
While our current concern is HPAI H5 clade 2.3.4.4x, the authors demonstrate that pigeons can carry multiple LPAI and HPAI viruses, making them potentially potent `mixing vessels' for influenza.
by Ning Cui 1,2,Peipei Wang 3,Qinghua Huang 1,2,Zihao Yuan 1,2,4,Shuai Su 4,*,Chuantian Xu 1,2,* andLihong Qi 1,2,*Viruses 2025, 17(4), 585; https://doi.org/10.3390/v17040585 (registering DOI)Published: 18 April 2025AbstractPigeons (Columba livia) are usually kept as free-ranging or racing birds, and they have direct contact with livestock, poultry, and humans. Therefore, they may have an important role in the ecology of influenza virus among various species. In the present study, we bring together all available sequence data of pigeon avian influenza virus (AIV) from public databases to address the current understanding of the genomic characteristics and emergence of each subtype of AIV in pigeons.
Collectively, we identified 658 pigeon AIV strains in 21 countries across the world, which were mainly distributed in Europe, Asia, and North America. H1 (2), H2 (1), H3 (8), H5 (71), H6 (16), H7 (16), H9 (543), and H11 (1) AIV subtypes have been identified in pigeons.
In addition, we interrogate features of the H5, H6, H7, and H9 subtypes of pigeon AIV, which are relatively common in pigeons. It is particularly noteworthy that the H5 AIV strains identified in pigeons are all classified as HPAIV. For the first time, this study presents a complete overview of the multiple AIV subtypes that have been circulating in pigeons, providing information on their distribution and genomic characteristics. This study will help to understand the molecular evolution of AIV in pigeons.(SNIP)
Figure 1. Global prevalence of pigeon AIV. (A) Subtypes and temporal distribution of pigeon AIV. The temporal distribution of the isolates of each subtype is indicated with different colors of tips. (B) Global geographical distribution of different subtypes of pigeon AIV strains. The proportion of isolates for each subtype category is visualized in a pie chart. The size of the pie chart is proportional to the number of isolates in each continent. (C) Summary of different subtypes of pigeon AIV strains. The y axis denotes the number of strains. All of the public data used in this study were up to date as of 30 December 2024.
(SNIP)
3.5. H5 Virus
(excerpt)
The results suggested that Clade 2.3.4.4b included the dominant H5 HPAIV strains were prevalent in pigeons. Indeed, H5 Clade 2.3.4.4.b led to a massive number of outbreaks worldwide in wild and domestic birds, and it has evolved, adapted, and spread to species other than birds, with potential mammal to mammal transmission [23,24]. For example, genomic analysis revealed 99.7% nucleotide identity between H5N1 viruses circulating in pigeon flocks and those infecting dairy cattle during the 2023 US outbreak [25]. Considering the high risk of morbidity and widespread transmission of the H5 subtype of AIV, especially the H5 Clade 2.3.4.4.b, epidemiological investigation and immunization implementation for the virus in pigeons should also be emphasized.
3.10. Mixed Viruses
The mixed infection of AIV provides the necessary prerequisite and opportunity for viral gene recombination to generate novel genotype viruses. Mixed infection of different subtypes of AIV strains has also been identified in pigeons (Table S1). A/pigeon/Fujian/1.17_FZHX0111-C/2017 contains mixed viruses of the H6N6 and H7N8 subtypes. A/pigeon/Jiangxi/10.19_NCDZT62A4-OC/2018 contains mixed viruses of H6N2 and others. A/pigeon/Jiangxi/JXA130010/2013 is a mixed virus of the H7, H9, N2, and N9 subtypes.
Combined with the aforementioned statistical data on pigeon AIV, pigeons can serve as “virus blenders” capable of simultaneous infection with multiple AIV subtypes. Such mixed infections significantly promote genetic recombination, leading to the emergence of novel strains and increasing pandemic risks.
4. Conclusions
Overall, we demonstrate that the H1, H2, H3, H5, H6, H7, H9, and H11 subtypes of AIV strains have been identified in pigeons. H9 and H5 AIV are the most abundant subtypes reported in pigeons. It is particularly noteworthy that the H5 AIV strains identified in pigeons are all classified as HPAIV. Therefore, it is imperative to pay close attention to epidemiological investigation and immunization implementation for pigeons. For the first time, this study presents a complete overview of the multiple AIV subtypes that have been circulating in pigeons, providing information on their distribution and genomic evolution.
As most AIV subtypes do not cause obvious symptoms in pigeons and only limited surveillance has been conducted, the prevalence of AIV, especially LPAIV in pigeons, is likely to be higher than we document here. From the viewpoint of the sequence analysis of pigeon AIV, free-ranging pigeons always play a negligible but non-zero role as an interspecies bridge in the ecology of influenza virus dynamics.
While the role of pigeons in the ecology and evolution of HPAI H5 isn't entirely clear, it is pretty obvious they aren't as `Invincible and Flu Immune' as once believed. Whether our understanding was incomplete 20 years ago - or avian viruses simply moved on - is hard to say.
But it does remind us that anything we say about H5N1 (or any other emerging threat) today, is always subject to change.
What may be considered a `low threat' today could suddenly loom large tomorrow. By the same token, we've seen threats (including H5N1) suddenly, and inexplicably, retreat.
The only thing we can truly count on is there will be more surprises ahead.
And we need to be ready to meet them.
