Bird flu, also known as avian influenza, refers to infection with influenza A viruses that occur naturally in wild birds. There are many different subtypes of bird flu viruses. Most subtypes do not infect humans, but some have been known to cause illness in people. The risk of bird flu spreading to mammals and humans depends on the specific strain of the virus.
What is bird flu?
Bird flu is caused by infection with influenza type A viruses that infect birds. Wild aquatic birds are the natural reservoir for these viruses. Occasionally, bird flu viruses can spread from birds to domestic poultry, such as chickens and turkeys. When this happens, it can cause significant illness and death in the infected flocks.
There are many different subtypes of bird flu viruses. They are identified based on two proteins found on the surface of the virus – hemagglutinin (H) and neuraminidase (N). Some examples of bird flu subtypes include H5N1, H7N9, and H9N2.
Most bird flu viruses do not infect humans or cause illness. However, some strains like H5N1 and H7N9 have been known to occasionally spread from poultry to people resulting in serious and sometimes fatal infections.
Can mammals get infected with bird flu?
Yes, some strains of bird flu are capable of infecting mammals under the right circumstances. Birds are the natural hosts for these viruses, but they have been documented to spread to and infect other animals in certain cases.
The main mammals that have contracted bird flu in past outbreaks include:
– Cats – Domestic cats and large cats like tigers and leopards are susceptible to some bird flu strains. Cats were infected with H5N1 during outbreaks in Asia and Europe.
– Dogs – Dogs do not seem to be as readily infected, but there have been rare cases of dogs becoming ill after exposure to H5N1 and H3N2 bird flu viruses.
– Pigs – Pigs are susceptible to both bird flu and human flu viruses. They may play a role in mixing genes of human and bird flu to create new strains. H5N1 and H3N2 viruses have infected pigs before.
– Humans – Certain bird flu strains like H5N1 and H7N9 are capable of infecting humans and causing severe disease. Over 850 human cases of H5N1 have occurred since 2003.
– Rodents – Small numbers of wild rodents have tested positive for H5N1 during large bird flu outbreaks in Asia. Guinea pigs can also be infected in laboratory conditions.
– Ferrets – Ferrets are often used in influenza research because they can be infected with both human and avian influenza strains. The viruses can replicate and transmit between ferrets.
– Mink – Farmed mink have proven susceptible to H5N1 and it has caused outbreaks on mink farms during bird flu epidemics. The virus causes severe respiratory disease in mink.
What mammals are at highest risk?
Some types of mammals appear to be more prone to contracting bird flu during outbreaks in birds:
– Cats – Domestic and large cats are very susceptible to H5N1 bird flu. The virus has been documented to transmit from birds to cats, as well as between cats on rare occasions.
– Pigs – Pigs can readily become infected with both bird and human influenza viruses, making them an important “mixing vessel.” This can lead to genetic re-assortment between strains.
– Mustelids – Animals in the mustelid family seem prone to severe disease from some bird flu viruses. This includes mink, ferrets, weasels, and badgers.
– Primates – Research monkeys have been found susceptible to experimental infection with H5N1. Some primates like macaques may develop quite severe disease.
– Humans – Humans remain at risk for infection from some strains of bird flu that are capable of overcoming the species barrier. Those in close contact with infected birds are most at risk.
Mammals that have less exposure to wild waterfowl or poultry seem at lower risk of contracting bird flu during outbreaks. However, any mammal may be susceptible under the right conditions on a case-by-case basis.
Which strains of bird flu can infect mammals?
The ability of different bird flu strains to infect mammals can vary:
| Bird flu strain | Known to infect mammals? |
|---|---|
| H5N1 | Yes – has infected cats, tigers, dogs, humans, and other mammals |
| H7N9 | Yes – has infected humans, ferrets, and pigs in lab experiments |
| H9N2 | Sporadic infections documented in pigs and humans |
| H7N7 | Has infected humans, pigs, cats, and other mammals in past outbreaks |
| H3N8 | Has adapted to infect dogs, horses, seals, and humans long-term |
As the table shows, some bird flu viruses like H5N1 and H7N9 are clearly capable of infecting multiple mammalian species, while others have only caused rare cases of mammal infection. The adaptability of each strain is different.
How do bird flu viruses spread to mammals?
Bird flu viruses can sometimes jump from birds to infect mammalian hosts through direct or indirect contact:
– Direct contact – Examples include predators eating sick birds, scavenging dead carcasses, or having close contact with infected poultry. Viruses transmit through saliva, mucus, feces or blood.
– Indirect contact – Mammals may pick up virus from contaminated environments, objects or aerosolized respiratory droplets. Things like shared water bowls, cages, or enclosures can facilitate transmission.
– Adaptation – Sustained mammal-to-mammal transmission requires the virus to mutate and gain the ability to bind receptors in the new host. This may take time.
– Reassortment – When mammals like pigs are infected with human and avian viruses simultaneously, the strains can mix and produce a novel pandemic virus.
For established mammal infection, the virus must overcome the species barrier by adapting to successfully gain entry, replicate, and transmit between the new hosts. Some flu viruses like H5N1 are better at breaching this divide than others.
Have entire mammal populations been infected?
There are a few examples of bird flu viruses infecting a significant portion of a mammal population during past outbreaks:
– Mink farm outbreaks – H5N1 caused large outbreaks in mink farms in the Netherlands in 2020-2021, infecting tens of thousands of mink. The virus transmitted between the semi-aquatic mink.
– Tiger cases – In Thailand 2004, 147 captive tigers at a zoo were infected and died from H5N1 after being fed raw chicken carcasses. The virus efficiently spread tiger-to-tiger.
– Human clusters – Though not entire populations, some clustered cases of H5N1 and H7N9 transmission between immediate human family members have occurred.
– Horse flu – H3N8 adapted from birds to stably infect dog and horse populations in the 2000s. It routinely circulates in these hosts as “equine influenza.”
– Bat colonies – Some evidence that bat colonies may harbor H9N2 long term, with occasional spillover infections back into poultry.
In these cases, the viruses were able to exploit dense populations and close quarters to transmit efficiently between hosts of the same mammal species. Sustained transmission remains rare.
What allows bird flu to infect new mammals?
There are a few key factors that facilitate bird flu viruses infecting novel mammalian hosts:
– Receptor compatibility – Hemagglutinin proteins on the virus surface must be able to bind cell receptors in the new host. Some strains already have this ability.
– Exposure opportunity – The mammal must have contact with a sufficient dose of virus shed from infected birds or other hosts.
– Vulnerable hosts – Animals that are immunosuppressed, stressed or have comorbidities may be more prone to infection.
– Viral mutations – Adaptation through random viral mutations that confer the ability to replicate in the cells of the new host.
– Reassortment potential – Some mammals like swine can harbor both avian and mammalian viruses, allowing gene segments to mix.
– Species density – Close quarters and crowded populations support efficient transmission once a virus spills over.
– Suboptimal biosecurity – Lapses in biosecurity, personal protective equipment, or containment protocols increases spillover risk.
Both host and viral factors determine if an avian influenza virus can successfully jump the species barrier and establish itself in a new mammalian population.
What is the zoonotic risk from bird flu?
Zoonotic risk refers to the potential for a disease to transmit from animals to humans. Some strains of bird flu do pose a zoonotic threat:
– H5N1 – Highly pathogenic H5N1 poses the greatest zoonotic risk from birds. Over 850 human cases have occurred since 2003 with a >50% fatality rate.
– H7N9 – Has caused over 1,500 human infections with a ~40% death rate. Less deadly than H5N1 but still dangerous.
– H9N2 – Considered low pathogenic but can infect humans sporadically, particularly children. Causes mild flu-like illness.
– Other strains – H7N7, H10N7 and additional subtypes have occasionally infected humans but do not transmit easily.
– Pigs as mixing vessels – Pigs co-infected with human and bird flu can theoretically recombine viruses and amplify pandemic risk.
Factors that increase zoonotic risk include close human proximity to waterfowl reservoirs or infected poultry, exposure to diseased animal products, insufficient biosecurity and personal protection for those working with birds. Individuals exposed to sick poultry are at heightened risk of infection.
Have bird flu pandemics started from mammals?
Major flu pandemics are primarily thought to trace back to avian influenza strains that at some point crossed over and adapted to humans. Mammals likely served as interim hosts and “mixing vessels” along the way:
– 1918 Spanish flu – Suspected to have originated as an avian virus that crossed to humans via an unknown mammalian intermediary host. Potentially a pig or horse.
– 2009 H1N1 swine flu – Emerged from reassortment of avian, human and swine influenza strains co-circulating in North American pig herds.
– H2N2/H3N2 pandemic strains – Emerged after avian viruses mixed with seasonal human flu strains in pigs and reassorted genes.
– H5N1/H7N9 risk – Pig co-infections remain a concern for potential creation of pandemic viruses by reassortment with human strains.
No major flu pandemics are known to have started exclusively in a mammalian population without links back to an original avian progenitor virus. However, mammals clearly play a key role as mixing vessels for recombination events along the way.
Do outbreaks in mammals precede human pandemics?
Sometimes significant outbreaks in mammals have served as warning signs foreshadowing a potential human pandemic:
– Swine outbreaks often precede human flu pandemics as key mixing vessels and intermediary mammalian hosts along the path from birds to humans.
– Mink outbreaks with H5N1 and H7N9 may signal risky strains with zoonotic potential that can spread in mammals. Both viruses have infected humans.
– H3N8 bird flu established circulation in horses and dogs prior to infecting humans long-term as “canine influenza.”
– Animal handlers infected – Early spillover events to farm workers or zookeepers can provide advance notice of potential mammalian transmission.
– Laboratory animal studies – Ferrets, guinea pigs and non-human primates can indicate which viruses may pose the greatest threat for enhanced mammalian transmission.
However, most mammal outbreaks of bird flu viruses have eventually hit a dead end without sustaining extensive transmission. Close surveillance is still warranted to identify strains of greatest concern for potential human pandemics.
How are mammal infections studied in the lab?
Scientists use lab animal models to study how avian influenza viruses adapt and transmit in mammals:
– Ferrets – Used most commonly to mimic human flu infection due to similar lung physiology and receptor distribution. Allows virus transmission studies.
– Guinea pigs – Support viral replication studies in mammalian lung tissue. Used to test vaccine efficacy.
– Mice – Useful for screening large numbers of avian virus strains and access to genetic knockout models. Does not model human transmission well.
– Pigs – Valuable for studying natural reassortment of avian, swine and human viruses and assessing vaccine approaches.
– Non-human primates – Macaques develop very human-like disease with H5N1 and are used to test therapies and pathogenesis. Requires special facilities.
– Dogs and cats – Employed to evaluate risks to pets and develop veterinary vaccines for H3N8, H5N1 and other zoonotic strains.
Mammal studies are restricted to biosafety level 3 or 4 containment labs due to the risks involved. Data generated helps assess pandemic potential and develop interventions.
Conclusion
In summary, occasional spillover of certain avian influenza strains from birds to mammals does occur in nature. Some mammals like swine allow reassortment with human viruses, raising concerns about pandemic generation. Others such as domestic cats are at high risk of infection during bird flu outbreaks. However, the vast majority of strains do not transmit easily between mammals or pose serious zoonotic threats. Active surveillance remains key to staying ahead of strains that demonstrate enhanced ability to jump between mammalian hosts. Understanding transmission barriers and pathways from birds through intermediate mammalian hosts continues to be a priority for pandemic preparedness.
