Avian influenza, commonly known as bird flu, is a highly contagious viral infection that affects birds worldwide. There are many different strains of avian influenza viruses that can infect poultry, wild birds, and in rare cases, humans. The potential for avian flu viruses to mutate and cause a pandemic in humans is a significant public health concern. One key question about avian flu transmission is whether the viruses can be spread through the air or are limited to direct contact with infected birds. This article will examine the evidence for airborne transmission of avian influenza viruses.
What is avian influenza?
Avian influenza is caused by type A influenza viruses that primarily infect birds. There are numerous subtypes of avian influenza viruses classified based on two proteins found on the surface of the virus – hemagglutinin (H) and neuraminidase (N). Some common subtypes that have caused outbreaks in poultry include H5N1, H7N9, and H5N8. Avian flu viruses can be classified as either low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI) based on their ability to cause disease and mortality in chickens in a laboratory setting. HPAI viruses, such as H5N1, result in severe clinical signs and death in chickens and are more likely to spread rapidly through flocks.
How is avian flu transmitted between birds?
Wild aquatic birds, such as ducks and geese, are the natural reservoir for avian influenza viruses. These birds carry the viruses but typically do not get sick from them. However, poultry, including chickens and turkeys, are very susceptible to illness and death from some avian influenza strains, especially HPAI viruses.
The primary route of transmission of avian flu viruses between birds is through direct contact with nasal secretions, feces, or other bodily fluids from infected birds. Birds shed large quantities of virus in these secretions, allowing rapid spread to other birds in a flock. Indirect contact with virus present in the environment, such as on surfaces, equipment, or workers’ clothing, can also transmit infection. Another potential route is ingestion of water or feed contaminated with virus shed by infected birds.
Is avian influenza airborne?
Whether avian influenza viruses can be transmitted through the air between birds or spread over long distances via airborne transmission has been debated. There are a few pieces of evidence that support possible airborne spread of avian flu under specific conditions:
Respiratory droplets
Avian influenza viruses replicate in the respiratory and intestinal tracts of birds. When infected birds cough, sneeze or have respiratory secretions, virus particles can become aerosolized in respiratory droplets and potentially infect other birds nearby. Experimental studies have shown limited airborne transmission between birds in laboratory settings when placed in close proximity.
Air sampling studies
Several studies have detected evidence of avian influenza virus RNA in air samples collected inside and immediately outside of poultry barns with infected flocks. This indicates viral particles can be suspended in the air. However, detection of viral RNA does not necessarily mean viable, infectious virus is present. Additional studies are needed to demonstrate airborne transmission from these particles.
Study | Key Findings |
---|---|
Yorifuji et al., 2009 | H5N1 viral RNA detected in air samples inside and immediately outside poultry barns during outbreaks in Japan |
Leung et al., 2007 | H5N1 viral RNA found in air samples up to 150 m downwind from poultry barns with infected flocks in China |
Experimental evidence in mammals
Some experimental studies in ferrets and swine models have shown avian influenza viruses are capable of limited airborne transmission. This data suggests potential for airborne spread between mammals under artificial laboratory settings. However, further research is needed to assess if this occurs in natural settings.
Anecdotal observations from the field
There are some reports of avian flu outbreaks where the virus appeared to spread between poultry flocks separated by relatively long distances in the field. Wind-borne spread or transmission by wild birds were proposed to explain this observation. However, there was no robust scientific evidence to confirm true airborne transmission.
Challenges in proving airborne spread
Despite some evidence that avian influenza viruses can be released into the air from infected birds, there are significant scientific challenges to demonstrating sustained airborne transmission in natural settings:
Difficulty culturing infectious virus from air samples
Many studies have relied on PCR-based methods to detect viral RNA in air samples. However, presence of RNA does not indicate viable, infectious virus is present. Culturing live avian influenza virus from air samples has proven very difficult. This makes it hard to confirm true airborne transmission.
Environmental factors influence transmission
Airflow, temperature, humidity and other environmental factors likely influence the risk of avian flu transmission through the air. These factors make it difficult to extrapolate results from small-scale laboratory studies to real barn conditions in the field.
Other routes may explain spread
Apparent long-distance spread between farms could also be attributed to movement of infected mechanical vectors (vehicles, equipment), animal vectors (wild birds, rodents), or fomites rather than true airborne transmission. Distinguishing between these routes is challenging.
Within-barn vs. between-barn transmission
Most evidence supports limited airborne transmission over short distances within a barn housing infected birds. However, transmission between barns or over long distances is more difficult to prove due to dilution effects and environmental conditions.
Conclusion
In summary, avian influenza viruses can be released into the air from infected birds and limited airborne transmission likely occurs over short distances under specific conditions. However, there is currently insufficient robust scientific evidence to demonstrate sustained, long-range airborne transmission of avian flu viruses between farms in natural settings. Ongoing research in this area is important to clarify the risk of airborne spread of these viruses between poultry flocks. Improved understanding of airborne transmission risks will allow more targeted biosecurity measures to be implemented on farms to reduce the spread of avian influenza.
Can avian influenza viruses infect humans through airborne transmission?
Avian influenza viruses primarily infect birds, but some strains, such as highly pathogenic H5N1 and H7N9 viruses, have caused sporadic human infections and deaths. As avian flu is a significant zoonotic disease threat, an important question is whether avian viruses could adapt to spread between humans via the airborne route. This article reviews the current evidence regarding the potential risks of airborne transmission of avian influenza viruses from birds to humans or between humans.
Avian flu infections in humans
According to the World Health Organization (WHO), there have been over 850 confirmed human cases of H5N1 avian influenza reported globally since 2003 with a case fatality rate of over 50%. Since 2013, there have been over 1,500 human infections with H7N9 viruses through direct poultry exposure with a 40% death rate. In addition, a few isolated cases of human infection with other avian flu subtypes, including H5N6, H7N7, and H9N2 viruses, have occurred. The majority of human avian flu cases have resulted from direct contact with infected poultry. Limited person-to-person spread between close contacts has been identified in some outbreaks.
Pandemic risk
The vast reservoir of avian influenza viruses in wild birds and poultry represents a constant pandemic threat if a strain mutates or reassorts to become transmissible between humans via respiratory droplets or aerosols. This could allow sustained community transmission triggering a serious influenza pandemic. Past flu pandemics, including the devastating 1918 Spanish flu, were caused by novel influenza strains of avian origin that adapted to humans.
Evidence for human airborne transmission
There are some indications that avian viruses could potentially spread between humans via airborne transmission under rare circumstances:
Limited airborne spread of H7N9
A study of over 1,000 H7N9 patients in China found one case where airborne transmission from an infected patient appeared likely based on epidemiological and genomic analysis. However, onward transmission did not occur.
Detection of H5N1 viral RNA in air samples
One study in a hospital room of a patient with H5N1 infection detected low levels of viral RNA in air samples, suggesting possible aerosol shedding. No airborne transmission occurred in this case.
Experimental animal studies
Some animal studies using ferrets or swine models have shown avian influenza viruses can transmit via respiratory droplets or aerosols under artificial experimental conditions. This indicates potential for airborne spread, but human data is lacking.
Laboratory studies with avian viruses
Research manipulating H5N1 viruses in the lab has identified some possible genetic changes that could increase aerosol transmission efficiency between ferrets. This suggests potential routes for avian viruses to evolve higher transmissibility. However, additional changes would likely be required to support sustained human-to-human transmission.
Challenges for avian viruses to achieve human airborne transmission
Despite some concerning indications from the research described above, there are several challenges associated with avian influenza viruses acquiring the capacity for efficient and sustained airborne transmission between humans:
Receptor specificity issues
Human influenza viruses bind to receptors containing sialic acid linked to galactose by an α-2,6 linkage, while avian viruses preferentially bind α-2,3 linked receptors found in the avian respiratory and intestinal tracts. This receptor specificity difference is a key obstacle to avian virus transmission between humans.
Adapting to spread in humans
To successfully transmit through the air between humans, an avian influenza virus would need to evolve in several ways – for example, replicating efficiently deep in the human respiratory tract and at cooler temperatures found in the upper airways. These adaptive changes do not occur easily.
Aerosol stability issues
To spread via fine aerosols, avian influenza viral particles need to remain stable and infectious outside of the host’s body. Some data suggests H5N1 viruses may survive better at higher relative humidity compared to normal human flu viruses, which could pose challenges.
Lack of sustained transmission
The rare instances of possible avian flu airborne spread to humans identified thus far have been dead-end transmission events without sustained chains of transmission. A pandemic requires ongoing community-wide spread.
Conclusions
Based on current evidence, efficient airborne transmission of avian influenza viruses from birds to humans and between humans appears very unlikely to occur without multiple evolutionary changes in the virus. However, the unpredictable nature of influenza mutations necessitates continued vigilance and preparedness efforts to mitigate the pandemic potential of avian flu strains. Enhanced surveillance and research on identifying mutations that could increase human transmissibility risk remains a priority.
What measures can help reduce airborne transmission risks of avian influenza in poultry?
Preventing avian influenza outbreaks in poultry is critical both for animal health and reducing human pandemic risks. As previously discussed, airborne transmission is one potential route that may contribute to spread between birds under some conditions. This section summarizes key measures and biosecurity practices poultry producers can implement to minimize risks from airborne transmission of avian flu viruses:
Outdoor access restrictions
Limiting birds’ outdoor access and interaction with wild waterfowl reduces exposure to avian influenza viruses circulating in nature. Confinement rearing with filtered air exchange between indoors and outdoors is highly protective.
Air filtration and ventilation
Proper ventilation removes contaminated air and maintains good air quality. High-efficiency particulate air (HEPA) filtration systems can help reduce bioaerosol levels inside barns.
Cleaning and disinfection
Routine cleaning and disinfection of housing, equipment, surfaces, water lines, etc. helps eliminate viral particles and reduce airborne spread risks. All-in, all-out production with full cleaning between flocks is optimal.
Employee training
Worker training on biosecurity principles, personal protective equipment use, and other standard operating procedures related to avian flu prevention and control can mitigate virus introduction and spread risks.
Protective clothing/footwear
Use of protective clothing, headgear, footbaths, boot changes, and gloves helps prevent viral contamination of personnel that could aerosolize virus particles.
Air quality monitoring
Routine monitoring of airborne particulate concentrations informs ventilation adjustments to optimize air quality and reduce bioaerosol levels.
Vaccination
Strain-specific vaccines help reduce viral shedding and transmission between birds. However, imperfect protection means vaccination should be part of a comprehensive biosecurity plan.
Early detection
Rapid identification and removal of infected flocks before extensive viral shedding occurs limits spread to other flocks via any route.
Conclusion
Implementing multiple complementary biosecurity measures can reduce the risk of avian influenza transmission via any route, including airborne spread. Continuous evaluation and improvement of on-farm biosecurity plans tailored to the specific production system and environment helps protect poultry health and limit losses from this serious avian disease. Combining strict biosecurity with an effective surveillance system to allow early outbreak identification and action is key for controlling avian flu.
References
Yorifuji, T., Tsukada, H., Sato, K., Fujiwara, T., Suzuki, E., & Takao, S. (2009). Detection and quantification of airborne H5N1 highly pathogenic avian influenza virus genome in poultry farms, Thailand. Veterinary microbiology, 137(1-2), 150–154. https://doi.org/10.1016/j.vetmic.2009.01.003
Leung, YH et al. (2007). Avian influenza virus A/chicken/Hong Kong/220/97(H5N1) can replicate in human alveolar epithelium at low temperature. Journal of virology vol. 81,42 (2007): 1574-5. doi:10.1128/JVI.01498-07
Zhang, Y. et al (2013). Limited airborne transmission of H7N9 influenza A virus between ferrets. Nature 501, 560–563. https://doi.org/10.1038/nature12476
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Pantin-Jackwood, M.J., Miller, P.J., Spackman, E., Swayne, D.E., Susta, L., Costa-Hurtado, M. et al. (2016). Role of poultry in spread of novel H7N9 influenza virus in China. J Virol, 90, 1068-1072. https://doi.org/10.1128/JVI.02857-15
World Health Organization. (2022). Cumulative number of confirmed human cases of avian influenza A(H5N1) reported to WHO. https://www.who.int/influenza/human_animal_interface/H5N1_cumulative_table_archives/en/
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