Owls belong to the order Strigiformes and are a group of birds of prey that are found on every continent except Antarctica. There are around 250 species of owls that inhabit a wide range of habitats from rainforests to deserts. Like all birds, owls can become infected with avian influenza (bird flu) viruses. However, different owl species seem to have varying levels of susceptibility to highly pathogenic strains of the virus. Understanding if owls are at risk of contracting and spreading bird flu is important for both owl conservation and managing disease outbreaks in poultry and wild birds.
What is Avian Influenza?
Avian influenza refers to infection with any of the influenza type A viruses that are adapted to birds. These viruses occur naturally in wild waterfowl and shorebirds, who serve as a natural reservoir for the viruses and transmit them to other avian and mammalian species. Avian influenza viruses are classified into two categories based on their ability to cause disease: low pathogenic avian influenza (LPAI) and highly pathogenic avian influenza (HPAI).
LPAI viruses generally cause mild to no symptoms in infected birds. However, some LPAI strains are capable of mutating into HPAI viruses. HPAI viruses are extremely infectious and fatal in domestic poultry, resulting in outbreaks that require extensive control measures. Two subtypes of influenza A viruses that have caused significant HPAI outbreaks globally are H5N1 and H7N9.
Can Owls Become Infected with Avian Influenza?
Yes, owls are susceptible to infection with both LPAI and HPAI viruses. However, severity of disease varies between owl species. Numerous experimental studies have evaluated disease progression in captive owls inoculated with high doses of HPAI viruses. While some owls develop severe disease and high mortality, other species show few clinical signs and are more resistant to infection.
Several factors likely influence an owl’s susceptibility to avian influenza viruses:
Genetic variation between owl species
Research indicates that genetic differences between owl species affects their relative susceptibility to HPAI infection and disease. For example, snowy owls (Bubo scandiacus) and Eurasian eagle owls (Bubo bubo) experience 100% mortality within days of H5N1 HPAI virus inoculation. In contrast, great horned owls (Bubo virginianus) and barred owls (Strix varia) may not develop clinical signs and have significantly higher survival rates.
Variation in virus receptors
Influenza viruses must bind to specific host receptors to gain entry into cells and cause infection. Avian influenza viruses preferentially bind sialic acid receptors with an alpha 2,3-linkage, while human viruses bind receptors with an alpha 2,6-linkage. Owls have both receptor types in their respiratory and intestinal tracts, which likely facilitates avian influenza infections. However, receptor distribution and density varies between owl species, which may mediate HPAI virus binding and replication.
Differences in immune response
The innate and adaptive immune responses elicited against avian influenza viruses differ between owl species. Snowy owls show muted cytokine signaling, ineffective viral clearance, and impaired lymphocyte activation when infected with H5N1 compared to great horned owls. Weaker immune responses in snowy owls may explain their greater HPAI susceptibility.
Ecology and migratory patterns
Owl species with migratory ranges overlapping areas with HPAI outbreaks in waterfowl are at higher risk of exposure. Snowy owls and Eurasian eagle owls have vast migratory ranges and may encounter infected waterfowl more often. In contrast, more sedentary owl species with smaller home ranges likely have lower interaction frequency with waterfowl reservoirs.
Have Owls Been Infected with HPAI Viruses in the Wild?
There are a limited number of reports of natural HPAI infections in wild owls. However, serologic surveys provide evidence of prior HPAI exposure in some populations:
H5N1
– Between 2005-2009, H5N1 HPAI antibodies were detected in 10% of wild barn owls (Tyto alba) sampled in Austria, suggesting natural exposure.
– A 2010 serosurvey in Germany found H5N1-reactive antibodies in 10% of long-eared owls (Asio otus) and 6% of barn owls.
– In North America, a study found 1.5% of great horned owls had prior H5N1 exposure based on serologic testing.
H7N9
– A 2017 Chinese study detected H7N9 antibodies in 5% of brown fish owls (Ketupa zeylonensis) near a major outbreak area.
– However, larger scale surveillance found no evidence of H7N9 infection in owls in North America during the 2013-14 epidemic.
The presence of HPAI antibodies indicates owl species can be infected and mount an immune response. However, there are still substantial knowledge gaps regarding infection prevalence, disease severity, and virus shedding dynamics of wild owls infected with HPAI viruses.
What is the Role of Owls in Avian Influenza Ecology?
The role owls play in the epidemiology of HPAI viruses is complex and requires more research:
Susceptibility vs Mortality
While owls can become infected, many owl species may have high tolerance for avian influenza viruses. Unless illness is severe, infected owls likely persist in nature and could continually encounter waterfowl reservoirs. This ongoing spillover may facilitate viral adaptation in owls.
Shedding & Transmission
The duration and level of viral shedding from infected owls is uncertain. Prolonged shedding of HPAI viruses would increase windows of transmission risk to other birds. However, one study found viral shedding from experimentally infected great horned owls was significantly lower than other birds.
Predatory Behavior
As birds of prey, owls may contract HPAI viruses more readily when preying on infected waterfowl. Owls that hunt at wetlands bring them near large waterfowl populations where HPAI viruses persist. Predatory and scavenging habits can increase their exposure risk.
Broad Geographical Movement
Some owl species migrate long distances and occupy very large home ranges. Broad movement patterns mean owls could disperse HPAI viruses to new regions if infected during migration. However, there is currently limited evidence that owls contribute substantially to long range spread of HPAI viruses.
Overall, despite evidence confirming owl susceptibility to HPAI viruses, there is no indication at this time they are a major natural reservoir or driver of avian influenza ecology. Nonetheless, understanding their potential role can help guide wildlife surveillance and prevent spread of HPAI to new regions.
Are There Risks of Owl-to-Human HPAI Virus Transmission?
The risk of HPAI viruses like H5N1 and H7N9 jumping from infected owls to humans is considered low:
Barrier to human infection
Avian influenza virus receptors are adapted to infect birds. While certain mutations can facilitate human infection, most HPAI viruses lack adaptations enabling human transmission. There are no reported cases of humans contracting HPAI viruses directly from owls.
Limited Contact
Interactions between owls and people are infrequent compared to contacts between humans and poultry. Opportunities for zoonotic transmission from owls are far more limited than in domestic bird settings where HPAI outbreaks occur.
Appropriate Precautions
For wildlife professionals handling sick or dead owls, personal protective equipment can mitigate any infection risks. Proper precautions, including masks and gloves, can prevent any potential human exposures or transmission.
While the probability is extremely low, there are still unanswered questions about HPAI virus dynamics in different owl populations. Targeted surveillance and research can provide more insight on whether rare zoonotic transmission events are possible. But in most settings, owls do not pose a substantial public health risk for avian influenza.
Conclusion
In summary, experimental research clearly demonstrates that owls are susceptible to avian influenza virus infection, including highly pathogenic strains like H5N1 and H7N9. However, there is variation in disease severity and mortality rates among different owl species. In the wild, evidence confirms some owl populations have been naturally exposed to or infected with HPAI viruses through contact with waterfowl reservoirs. While they can become infected, owls are unlikely to be major drivers of avian influenza spread or significant sources of public health risk. There are still important knowledge gaps regarding HPAI infection prevalence, viral shedding dynamics, and pathogenetic mechanisms that require further scientific study in owls. Focused surveillance and research on avian influenza viruses in wild owl populations can help clarify their epidemiological role and guide conservation efforts. But current evidence suggests most owl species are spillover or dead-end hosts for HPAI viruses, rather than primary reservoirs sustaining transmission. Careful monitoring and more data are needed to fully understand if and how different owls may contribute to the ecology of avian influenza in wild bird populations.