Winter Mortality of American Crows: Orthoreovirus in Ontario

In late November of 2022, CWHC began receiving calls from the Chatham-Kent region regarding an alarming number of dead crows, with no apparent cause of death. We received six crows from this area over the next few weeks for examination, but the reports continued well into the new year. These specimens came from the south-western region of Chatham along the north bank of the Thames River with reports specifically from the Village on the Thames, the walking trail near Enbridge, and apartment buildings on Grand Avenue West. In addition to this crow die-off, we also began receiving a handful of calls from the Woodstock area. They too were seeing similar patterns, with several intact birds found dead but in the north end of Woodstock, near Roth Park. With over 300 birds reported dead in Chatham, and over 30 in Woodstock, what could be causing these crows to die in such large numbers?

Nine crows from these two locations were submitted to CWHC ON/NU for examination. All of the crows were tested for avian influenza; eight were negative, and one tested positive for the highly pathogenic (H5N1) strain of avian influenza. All were examined, and the eight that tested negative for avian influenza were all found to have intestines lined by and filled with blood and necrotic material. Three of these crows also had an enlarged and friable spleen. Two of the crows were examined microscopically and both had evidence of necrosis scattered throughout the spleen, and abundant necrosis lining the mucosa and filling the lumen of the intestinal sections examined. Spleen and intestine from one crow were sent for virus isolation and an orthoreovirus was isolated from this tissue.

When most people think of diseases that affect crows, West Nile virus likely jumps to the front of their mind, but unfortunately for our crows, there’s another virus that hits them hard in the winter. Since 2001, there have been reports of large numbers of crows dying during the cold winter months. This coincides with the time of year when crows live in the highest density, which, in combination with  environmental survival of the virus during cold weather, provides an ideal situation for transmission of this orthoreovirus infection. Although there have been other reoviruses detected in other bird species (including a strain which causes arthritis in poultry), this specific reovirus only appears to negatively affect crows. There is currently no known danger for humans or other animals from this virus.

Unfortunately, this isn’t the only virus that has been affecting our crow populations recently. In addition to orthoreovirus and West Nile virus, the highly pathogenic strain (H5N1) of avian influenza, which has been circulating in wildlife since December 2021, has also been found to cause the death of crows across the province. Thankfully, there are millions of crows in Ontario, and we have not yet noticed any population level impacts associated with the presence of these three viruses – but it is something that we will have to continue to monitor.

Anyone encountering these sick or dead crows should take precautions and not handle them. Orthoreovirus does not appear to pose a danger to humans, but the H5N1 avian influenza virus, will occasionally infect people, so it is recommended that if someone encounters a dead crow, they contact us at the CWHC or their local municipality/humane society. Dead wildlife should only be handled by the public if they have been instructed to by one of these organizations and they understand the potential risks and are wearing the appropriate personal protective equipment to keep themselves safe.

Submitted by CWHC Ontario/Nunavut Regional Centre

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