The cycle of heavy rains and high heat spurred conditions in City Park’s Duck Lake that have killed more than 50 birds in the past week, according to the Denver Parks and Recreation Department.
For Duck Lake, the summer season can mean avian botulism season. Outbreaks have happened there for decades but have never spread to other lakes, said Alan Polonsky, environmental analyst at the Denver Department of Environmental Health. The last Duck Lake outbreak was in 2007.
“You could probably find botulism spores in any other lake or wetland in Colorado,” he said. “But it’s just a matter of having the right environmental conditions to create toxins.”
The Colorado Division of Wildlife tested some of the dead birds and confirmed that it was avian botulism, said spokeswoman Jennifer Churchill.
The bacteria is always present in the lake’s bottom but environmental factors can cause it to flourish. A period of high temperatures broken up by heavy rain will cause the bacteria to grow, and the rain will stir it up.
In addition to heat and movement, the bacteria require an oxygen-depleted environment, which is created when a lot of organic material is decomposing on the lake floor.
Birds that dive to feed — ducks and cormorants — are most affected because they ingest some of the bacteria with their food.
Other types of birds can get sick from eating things contaminated by the bacteria. For example, birds can be sickened if they eat insects that had come into contact with dead and decomposing birds.
Humans cannot be infected with avian botulism, said senior veterinarian at the Denver Zoo, Dr. Felicia Knightly.
Denver parks have collected 12 dead birds so far, said spokeswoman Jill McGranahan. In one day, Denver Zoo staff collected 39 birds, Knightly said.
“In addition to the obvious reason of why it is important to remove dead birds and other carcasses (fish),” Polonsky said, “they must be removed to not further contribute to the botulism cycle.”
The zoo staff is checking the lake every two to three days for infected birds.
Sick birds have limp necks, struggle to breathe and aren’t able to blink. They can’t fly or walk.
The zoo has treated four female mallard ducks in its hospital.
The zoo is home to many bird exhibits that could be contaminated by sick birds.
Sick birds are first fed a mixture that includes activated charcoal to block absorption of the toxins that are released by the bacteria. Usually within a day, the bird passes the charcoal and then is tube-fed waterfowl food until it recovers.
“We feel like we are doing something valuable,” Knightly said. “Providing safety for our animals while doing something for native wildlife.”
Officials do not want the public to touch sick birds, but if one is found, to report it to Denver parks officials.
The park has put up fencing around Duck Lake to keep the public from the birds.
Sarah Horn: 303-954-1638 or shorn@denverpost.com
