What is it?
Avian botulism is a disease that affects birds as a result of eating toxin produced by the bacterium Clostridium botulinum. The bacteria is found in the sediments of lakes throughout the world and can be present but inactive until environmental conditions become favorable for reproduction. Avian botulism is not caused by the ingestion of inactive spores as inactive spores can be found in the tissues of healthy lake inhabitants including bugs as well as healthy birds.
The bacteria becomes toxic when environmental conditions are favorable and it becomes infected by a particular virus. This infection triggers the production of toxin by the bacteria. While the specific environmental conditions that cause outbreaks are not exactly known, some of these conditions include warm temperatures, low dissolved oxygen, and high pH levels.
What species are susceptible?
Waterfowl and shorebirds are the bird species most often affected by avian botulism in the United States. However, almost all birds and some mammals are susceptible to the illness. Birds get sick when they forage in the water and eat the toxin itself or eat animals that have toxin in their systems. Dogs and cats are rarely affected by botulism, but as a precautionary measure should avoid any contact with affected birds.
Is there a human health risk?
Avian Botulism is caused by Type C or E toxin. Humans are susceptible to Type A or B, making any health risks to humans minimal.
Occurrence in Denver
Avian botulism outbreaks occur throughout the U.S. and typically occur in the Metro Denver area between late June and September, the hottest months of the year. This is likely due to the increased activity of bacteria and the virus that triggers toxin production with increasing water temperatures.
Anecdotally, Denver staff has noted bird deaths consistent with avian botulism symptoms for more than 13 years. Avian botulism has been documented as the cause of death for waterfowl at Denver’s City Park by the Colorado Division of Wildlife. Most known cases within the City and County of Denver have been associated with City Park, and likely originated from Duck Lake. The extremely heavy use of Duck Lake by roosting cormorants and feeding waterfowl likely contributes to this problem through loading of organic matter associated with their waste. Bird waste includes ammonia and other organic constituents that can reduce the amount of oxygen deeper in the water. Bird waste also provides material for bacteria to feed on.
New developments in 2010 included potential botulism deaths at Smith Lake in Washington Park, and at Lollipop Lake in Garland Park. . It is possible that these birds contracted the disease at City Park and died at these lakes. Efforts to collect all dead birds and rehabilitate inflicted ones will help minimize chances of long term problems at Smith and Lollipop Lakes.
Signs of Botulism
Birds suspected of having avian botulism are often reported as lacking the ability to use their muscles. Affected birds often have trouble holding their necks and heads up and may also appear to struggle to swim across the water. Unfortunately, death by drowning often is the end result. Birds suffering from later stages may also be found on the shoreline, lethargic and relatively immobile.
Prevention
Once botulism becomes established in a water body, it is very difficult to eliminate. Management actions that can help minimize likelihood of the disease include minimizing fluctuations in water levels during the hot summer months, and prompt removal of animal carcasses. This includes waterfowl, fish, and anything else that provides breeding habitat for flies and food for bacteria.
What is the City and County of Denver doing to prevent outbreaks?
Multiple agencies under the lead of the City’s Department of Parks and Recreation and the Denver Zoo have been working on a plan to ensure that park improvements include renovations that will improve lake conditions. The current Duck Lake renovation includes sediment removal, shoreline stabilization, and re-landscaping to discourage goose waste accumulation. These efforts should help lower temperatures, remove existing organic matter in the sediment, and decrease goose waste and slow the accumulation rate of organic matter into the future. This effort entailed several years of planning and developing the funds for implementation.
The Department of Parks and Recreation has also streamlined efforts to control algae and vegetation. This will help maintain better water quality conditions and diminish the environmental conditions that lead to Avian Botulism. Prompt removal of animal carcasses and minimizing dramatic changes in water depth during the summer months are on-going outbreak prevention practices.
Treatment
When found quickly enough, sick birds can sometimes be treated and survive botulism. Local bird rehabilitators (Wild B.I.R.D.) and the Denver Zoo have taken in and treated sick birds. In 2007, thirteen birds were treated and twelve successfully released.
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