Outbreaks of sudden death in wild birds tough to solve

It is a sight no one wishes to see. This winter, more than 50 dead ducks were discovered in one of Calgary’s parks.

The people who found them were understandably upset. They were also concerned about the cause. Could it be poison? Or an outbreak of some type of infectious disease? Did a deadly bird flu reach Calgary? Could another wild animal have killed off all the birds? Were people and their pets at risk of the same illness?

These were all legitimate questions.

Outbreaks of sudden death in wild birds can occur for a number of reasons. In the case of poisoning, usually there are more than one animal species affected. For instance, wildlife can die if they scavenge an animal that was euthanized with drugs by a veterinarian.

Carcasses laced with strychnine can lead to dead coyotes, magpies, crows, eagles and other birds clustered around the carcass.

Infectious diseases are another major cause in instances where a large number of deaths among wild birds are reported. Some bacteria cause blood poisoning and are transmitted between birds when they are clustered at crowded ponds and lakes.

There are also types of botulism that affect birds. These bacteria produce a botulism toxin that paralyzes birds and when they die, the carcasses grow more bacteria in a vicious feedback loop. This is usually a disease of hot summer months.

Bird flu was a consideration in these ducks but much less likely. With overlap of the Asian and Pacific migration flyways, there is a chance that some of the deadly strains of avian influenza could find their way into North America. However, ducks tend to be carriers of this virus but don’t typically get sick. Nonetheless, it was important to check.

Finally, West Nile Virus can kill large numbers of birds, but it was the wrong time of year for the mosquito-transmitted virus to be the culprit in the Calgary case.

Working with Alberta Environment, I was able to obtain three duck carcasses for autopsy examination at the University of Calgary’s Faculty of Veterinary Medicine.

The first thing I noticed was the three ducks were remarkably thin with no visible fat stores and very little breast muscle left. None had eaten recently. And importantly for this investigation, there were no other visible signs of illness or disease.

They also had no evidence of trauma such as bite wounds, bruising or broken bones.

I collected and examined tissues from the ducks under the microscope as part of a through work-up. I was looking for the bird equivalent of pus in tissues, inflammation and bacteria, but none were present. This gave me further confidence that no diseases were present. There were also no changes suggestive of poisoning. The birds were also tested for avian influenza, which was negative.

Based on these results, it was my conclusion that the birds died of exposure and starvation. It had been an extremely cold February in Calgary and these birds were in the wrong place at the wrong time. Normally, they should have migrated south for the winter. But they found an area of open water and decided to overwinter in Calgary rather than make the journey to warmer climates. With little food, cold weather and crowding, these birds succumbed to their circumstances.

Human activities are partially to blame. By discharging warm water into ponds, lakes and rivers, we artificially create open water when there would naturally be none. Birds such as these ducks are drawn to the water when normally the frozen ice would trigger them to move on.

The value of examining sick and dead wild animals is that we gain a better understanding of the level of expected diseases and can find answers to mysteries such as this.

Dr. Jamie Rothenburger, DVM, MVetSc, PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine.
Twitter: @JRothenburger

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