The results of this study provide evidence that raptor species in the upper Midwest, United States have been infected with and survived infection with HP H5N1. The three most represented species that tested positive for antibodies to H5 and N1 included: bald eagles (65/93, 69.9%), red-tailed hawks (Buteo jamaicensis) (7/57, 12.3%), and great horned owls (Bubo virginianus) (5/34, 14.7%). These three species also had the highest prevalence of RT-PCR positive test results at the time of admission during active patient surveillance conducted at TRC in 2022, during the height of transmission of the virus9. Bald eagles had the highest H5 and N1 seropositivity compared to all other species tested. This could be due to a multitude of factors including the impact of natural history and diet on exposure risk, and differences in disease susceptibility between species9,13,21,22. Other species that tested seropositive to IAV subtypes H5 and N1 include rough-legged hawks (Buteo lagopus), red-tailed hawks, turkey vultures (Cathartes aura), and great horned owls, all of which have been reported to feed on carrion or hunt wild waterfowl which are likely routes of exposure22. Some of the species that tested seropositive in this study included very few representative individuals, which precludes the ability to draw any conclusions about the regional population. For example, seroprevalence in rough-legged hawks was 25%, but only four individuals were tested and only one bird tested positive for antibodies to H5 /N1.
The results of this study are particularly interesting when compared to a similar study conducted at TRC in 2012 that assessed the seroprevalence of IAV NP antibodies in six species of raptors16. This prior study used the same initial screening test and was sampling from the same wild raptor population that is included in this study. At the time of the 2012 study, the Gs/GD lineage of HP IAV (H5Nx) had not yet been detected in North America, as the first introduction occurred in November 20141. Bald eagles in this study were found to have the highest prevalence for IAV NP antibodies at 5.1% (22/406). A single seropositive individual was found in the following species: great horned owl and Cooper’s hawk (Accipiter cooperii). The results of the 2022–2023 serosurveillance presented here show a marked increase in IAV NP seroprevalence in bald eagles 69.9% (65/93) and in other raptor species, which correlates with the ongoing HP H5N1 outbreak at the time of sample collection. The landscape of IAV in wild birds has changed significantly in the past 10 years highlighting the impact, rapid spread, and changes in HP H5N1 risk to species with previously low rates of IAV exposure.
The species most frequently sampled at TRC in this study was the bald eagle. The vast majority of the bald eagles which had IAV NP antibodies were aged as greater than one year at the time of admission (66/67, 98.5%). Additionally, the single juvenile with IAV NP antibodies did not have antibodies specific to H5 or N1. The older age classes could potentially have had existing immunity prior to the start of the outbreak, therefore making it more likely that they would survive compared to naïve younger birds; however, based on the previous low prevalence of antibodies in this species, a significant impact between age classes would not be expected. Differences in IAV seroprevalence between age classes has been documented in other species23,24. The difference in seroprevalence between adults and younger birds may have implications in disease epidemiology over time. As HP H5N1 continues to circulate over multiple breeding seasons, it is possible that there will be different impacts on different age classes, potentially impacting population structure and recruitment13. Further research is needed to evaluate serology trends related to age and disease prevalence in the environment to learn more about the behavior of this disease in wild raptors.
One of the limitations of surveillance conducted at wildlife rehabilitation centers is the bias towards unhealthy or injured wildlife20. To help address this bias, samples were collected from apparently healthy migrating raptors at Hawk Ridge Bird Observatory (HRBO) to supplement the samples obtained from rehabilitation patients. Wildlife rehabilitation center populations are also biased towards animals living near humans, as that increases the probability of them being found and brought into care20. Despite these limitations, there are benefits to sampling from a wildlife rehabilitation center. Wildlife rehabilitation centers may admit animals from a wide geographic region, which may not be able to be mimicked with targeted trapping in a few locations. This advantage is highlighted well in this study, as samples were obtained from birds recovered across the entire state of Minnesota as well as surrounding states. In addition, wildlife rehabilitation centers can collect samples from species that are notoriously hard to capture, handle and sample. This is especially relevant when it comes to adult raptors which are naturally more dispersed on the landscape and do not typically congregate in large groups during migration. Additionally, viral genotypes of HP H5N1 detected in birds sampled at TRC during 2022 represented the same genotypes detected in wild birds sampled within the Mississippi flyway, confirming that during active disease outbreaks wildlife rehabilitation centers are viable resources to augment wider surveillance efforts9.
A limitation with extrapolation of these results to a larger population is the lack of HP H5N1 experimental data in raptor species. The details around infectious dose, severity of disease, and survival rate for HP H5N1 under controlled conditions are unknown in raptors as are the potential effects of existing immunity of these responses. While this paper provides evidence of raptors surviving infection with HP H5N1, it could not be determined when infections occurred if surviving birds developed clinical disease. It also is unknown if there will be a subsequent decrease in severity of disease in birds attributable to this acquired immunity or how long such potential protection will last.
The results of this study are consistent with raptors surviving infections with HP H5N1. The data also are consistent with a higher rate of HP H5N1 infection in bald eagles than can be estimated from virologic testing of dead or moribund birds. There is a clear need for continued surveillance for both active HP H5N1 infections and seroprevalence in raptor species and additional research in order to better understand the effects and the potential trajectory of this outbreak in wild raptor populations.