Tracking the Causes of Mortality in American Crows in Quebec (2022 to 2024)
The American crow is a species that plays an important role in several ecosystems in Quebec, including agricultural, forested, peri-urban, and urban environments. Since the beginning of 2022, a total of 312 crow mortality incidents have been investigated by the Quebec regional centre. The American crow is actually the species most frequently submitted to our laboratory as part of the ongoing surveillance program. Fatal infectious diseases have been identified in 62% of the incidents where the cause of death was determined. Infections caused by three viruses are by far the most common causes of mortality.
Firstly, infections with the West Nile virus (WNV) have been documented as the cause of death in 118 crows, including 55 cases in 2024. Crows remain highly susceptible to this virus, which was introduced to North America more than twenty years ago. While young birds are overrepresented in WNV cases, birds over two years old make up about a quarter of the cases. It is important to note that due to the primary mode of transmission of this virus (via biting insects), infection cases are mostly observed from late May to early October.
A total of 45 cases of infection with a highly pathogenic avian influenza virus H5N1 have been diagnosed since the arrival of this virus in Quebec in 2022. Crows likely become infected by consuming carcasses of ducks or geese that have died from this infection. Infections in adult crows (over two years old) have been more frequently observed than in younger birds, suggesting greater exposure of adult crows to this virus (for example, through more frequent feeding on carcasses). The gradual decline in the number of cases in crows (25 cases in 2022, 11 cases in 2023, and 9 cases in 2024) suggests that herd immunity is developing in wild bird populations. Notably, this temporal decrease is especially observed in adult crows, which supports this claim.
The third viral agent associated with crow mortalities is an orthoreovirus causing viral winter enteritis of crows. This condition, first described in the early 2000s, has been documented as a cause of death in 18 incidents in Quebec since 2022. As the name suggests, this condition is mostly observed during the winter season. The congregation of birds in roosting sites during the fall and winter likely favours the transmission of this disease from one crow to another.
In addition to these viral infections, our examinations have identified conditions rarely described in this species, such as yersiniosis (caused by an infection with the bacterium Yersinia pseudotuberculosis) and sarcocystosis (a systemic infection by a protozoan).
The American crow, an abundant species found in most ecosystems in Quebec, serves as an excellent sentinel species for monitoring certain infectious agents, such as WNV and avian influenza, which could potentially infect people or domestic livestock. Moreover, tracking the causes of mortality allows us to detect new emerging conditions that could affect wildlife health.
Stéphane Lair, CWHC-Quebec
