Avian diseases are a significant topic in One Health. Salmonella, coccidia, avian influenza, and West Nile virus, to name a few, are well known for their potential to In this context, one group of pathogens might not be as “famous”, but may also be a threat to domestic and some wild birds – have you ever heard of orthoreoviruses?
Orthoreoviruses are well known in poultry production for being an environmentally resistant pathogen that can lead to a variety of clinical manifestations depending on the strain. The most well-known presentation in poultry is associated with joint disease, which impairs movement and can decrease both egg and meat . Vaccines can protect against classical strains of the virus, but cross-protection is limited and new vaccines need to be created for new strains that emerge. Hence, this disease is of special concern to poultry farmers, but not only domestic birds are prone to disease – some wild birds are also affected by certain orthoreoviruses, and corvids seem to be especially vulnerable.
We had at least four orthoreoviruses-associated cases in corvids (three crows and a magpie), diagnosed in 2021, 2022, 2023 and 2026. After their death, they were submitted for diagnostic assessment through our wildlife disease surveillance program. They underwent an autopsy for a macroscopic evaluation of internal organs and sampling of important tissues for histopathology, where microscopic lesions could be evaluated.
These were juvenile birds with fair to poor body condition; occasionally, lesions in the gastrointestinal tract (like reddened cecal tonsils with a congested serosal surface) were also visible macroscopically. Microscopically, lesions in the gastrointestinal tract were more remarkable, including (but not limited to) what our pathologists call a . Some also showed signs of having contracted secondary infections. Tissue samples were submitted for PCR testing and Orthoreovirus was detected in all four cases.
Interestingly, these mortalities occurred between late-August and late-September even though corvid Orthoreovirus is primarily known to cause large-scale winter mortalities in crows. Since the disease is transmitted via the fecal-oral route, it spreads easily when crows congregate at communal roosts in the winter. However, other findings indicate that there is no seasonality to the disease. We have not confirmed any large-scale winter die-offs in corvids caused by orthoreoviruses in Saskatchewan, though other regional centres of the CWHC have investigated such cases and identified Orthoreovirus as the cause.
Orthoreoviruses have been isolated from other wild bird species without significant disease presentation, so the apparent sensitivity of crows to one strain of this pathogen is interesting. Orthoreoviruses can change (or reassort) relatively quickly, which is why orthoreoviruses have caused disease in domestic birds that had previously been vaccinated against classical strains of the virus.
While there is no known risk of virus transmission to humans or other animals, their impact on poultry production indirectly affects humans by impacting food production. This is a prime example of a wildlife disease that is not well understood and caused by a pathogen that can have a significant impact on wildlife, has connections to animal production and, indirectly, to humans’ wellbeing – and this is One Health.
Written by Beatriz Garcia de Sousa
References
Forzán, M. J., Renshaw, R. W., Bunting, E. M., Buckles, E., Okoniewski, J., Hynes, K., Melendez, R., Ableman, A., Laverack, M., Fadden, M., Dastjerdi, A., Schuler, K., & Dubovi, E. J. (2019). A Novel Orthoreovirus Associated with Epizootic Necrotizing Enteritis and Splenic Necrosis in American Crows (Corvus Brachyrhynchos). The Journal of Wildlife Diseases, 55(4), 812–822. https://doi.org/10.7589/2019-01-015
Rafique, S., Rashid, F., Wei, Y., Zeng, T., Xie, L., & Xie, Z. (2024). Avian Orthoreoviruses: A Systematic Review of Their Distribution, Dissemination Patterns, and Genotypic Clustering. Viruses, 16(7), 1056. https://doi.org/10.3390/v16071056
Huhtamo E, Uzcátegui NY, Manni T, Munsterhjelm R, Brummer-Korvenkontio M, Vaheri A, et al. Novel Orthoreovirus from Diseased Crow, Finland. Emerg Infect Dis. 2007;13(12):1967-1969. https://doi.org/10.3201/eid1312.070394. Novel Orthoreovirus from Diseased Crow, Finland – Volume 13, Number 12—December 2007 – Emerging Infectious Diseases journal – CDC
Kalupahana, A. W. (2017). Characterization of orthoreoviruses isolated from American crow (Corvus brachyrhynchos) winter mortality events in eastern Canada. Charlottetown, PE: University of Prince Edward Island. ir-21146-PDF (1).pdf