Crows in the Snow: A Wintery Tale of Reoviruses

“Winter is Coming, We Know What’s Coming With it.”

-Jon Snow (Game of Thrones)

On a sunny Saturday afternoon in early January, I dropped in to the Pittock Conservation Area, near Woodstock to look for dead crows.  Crows had been dying in the area and the conservation area beside a lake was a popular roosting area.  I didn’t have much luck, but did find one dead crow in a pine plantation.  Walking back up the road to my car with the crow in my hand, I met a man out for a walk.  Discovering that he lived nearby and came here often, I asked him if he had seen many dead crows.  “It’s the wrong time of year” was his reply.

If he was thinking about West Nile Virus, he was certainly correct.  However, there are other things that kill crows.  Since 2001, crows have been observed to be dying in substantial numbers from a different virus that attacks them during the winter period while they are congregated on their winter roosts.  This is the time of year when crows live in the highest density, circumstances that are ideal for the transmission of a contagious disease.  The virus in question has been identified as a Reovirus; other reoviruses have been found in corvids in Europe and others are the cause of arthritis in poultry.


Field Trips, Dead Crows, and Disease

We encountered the disease for the first time in 2004, also in the Pittock Conservation Area.  The Oxford County Public Health Unit submitted a number of birds to us and eventually we made a field trip to collect some fresh samples.  The birds develop a severe, transmural necrotizing enteritis and splenitis.  In some cases, the majority of the spleen has been destroyed.  As well as a number of dead birds, weak and sick birds were found in the woods, able to fly short distances only.

Cases have been reported over a broad geographic range in North America.  In the winter of 2008, there were reports of this disease from many parts of New York state, New Jersey and Ohio.  That same year, there were cases in Ontario, again in Woodstock and also in the Ottawa area. The disease has been seen sporadically here in Ontario since then until this winter, which has produced the highest number of cases since 2004.

Work on identifying the virus has been done by Anil Kalupahana, a Ph.D. student working with Dr. Fred Kibenge at the Atlantic Veterinary College in PEI.  Although the disease has been investigated here and in several laboratories in the USA, nothing has yet been published in the scientific literature.  A number of different reoviruses have been described in wild birds, affecting hooded crows, magpies, woodcock and eider ducks, so it will be interesting to see how this crow Reovirus is related to the others.

The crows are now dispersing to their breeding sites and the disease is likely over for this year.  It is important to remember, though, that although it may be the wrong time of year for one disease, it may be exactly the right time for another.

Article contributed by Dr. Doug Campbell, the staff wildlife pathologist for the CWHC’s Ontario/Nunavut Region.

Photo credits: Dr. Dale Jefferson, Data and Communications Officer CWHC National Office.

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