Avian Botulism: Overview

Radio-marked female Mallard; Photo: Trent Bollinger, Canadian Cooperative Wildlife Health Centre

History
Causes
Frequently Asked Questions

• Where does it occur?
• What are the signs?
• Is it always fatal?
• Are other species susceptible?
• What can be done

History

Avian botulism has had destructive effects on birds for a considerable time, and undoubtedly predates written records. During the early part of this century, a die-off of "millions" of waterfowl was encountered in the Great Salt Lake area of the United States.  Because early observations occurred on alkaline lakes in areas of western North America, the phenomenon was suspected of being a form of alkali poisoning and became known as Western Duck Sickness. It wasn't until a quarter of a century later that the cause of these die-offs was determined to be botulism poisoning.

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Causes

For avian botulism to occur, a virus must first infect type C Clostridium botulinum bacteria, causing the bacteria to produce toxin, which then must be ingested by a bird.

The botulism spore, or resting stage of the bacteria, is commonly found in wetland soils, and can survive for years, withstanding freezing and drying. Many birds inadvertently eat these spores while feeding and the spores live in their digestive systems with no effect on the birds' health. When a bird dies, its decaying carcass often offers three conditions that type C botulism bacteria need to grow and produce toxin: high temperatures, protein rich material, and an absence of air.

Botulism toxin is transferred to birds by maggots and possibly other invertebrates that feed on the decaying carcasses. The botulism toxin does not harm the invertebrates but it accumulates in their tissues to levels where one maggot can kill a duck. Large numbers of maggots on a bird carcass can be attractive to live birds that then become poisoned by ingesting toxic maggots. The cycle repeats itself, and each time the cycle often involves more and more birds.

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Where Does It Occur?

Outbreaks of avian botulism occur intermittently, generally on water bodies with little or no outflow. Botulism occurs most often in mid-to-late summer, on shallow lakes, during periods of hot dry weather. Incidents are frequently associated with a lack of oxygen in the water that often occurs after blue-green algae has bloomed.

Botulism outbreaks can be started by the presence of bird or other animal carcasses. Disease, starvation,severe weather, and collisions with power lines or other structures kill animals that then serve as ideal material for the development of botulism.

Most major avian botulism outbreaks occur in western North America, although they can appear in other areas throughout Canada, the United States and Mexico. In Canada, recent major botulism outbreaks occurred at Whitewater Lake, Manitoba; Old Wives Lake, Saskatchewan; and Pakowki Lake, Alberta. However, many lakes and marshes in these provinces have a history of recurrent problems.

Reports of avian botulism from other continents are becoming more common, with more than 15 countries in South America, Europe, South Africa and Australasia having reported botulism outbreaks.

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What are the Signs?

Avian botulism poisoning causes progressive paralysis. Affected birds gradually lose control over their muscles and initially they appear weak and limp. Symptoms include the inability to fly, followed by an inability to walk. At this latter stage, the bird often can propel itself by using its wings to "paddle" over land or water. Eventually the bird loses control over the muscles in its neck and it cannot hold its head upright. One of the common names for botulism intoxication is "limberneck." If this stage of the condition occurs when the bird is over water, it will drown. Ultimately, the bird dies from suffocation.

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Is It Always Fatal?

The effect on the bird depends directly on the quantity of toxin ingested. Under favorable environmental conditions - access to clean water, fair weather conditions and no predation - birds can recover from low doses of the toxin. Birds that do recover from avian botulism do not show any increased tolerance for the poison and are not resistant to subsequent exposures. There is some evidence that birds may be protected for short periods of time with a vaccine.

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Are Other Species Susceptible?

Type C botulism toxin is not the same as the botulism associated with human food poisoning. Humans, dogs and cats are not usually exposed to Type C. Thorough cooking will destroy any toxin present in food items. On a few occasions, cattle with access to wetlands where botulism is present have contracted botulism poisoning. These cases have occurred only where a massive die-off of waterfowl was ongoing. It is believed that the cows ingested toxic maggots floating on the water. In such unusual situations, it is recommended that cattle be provided with an alternative source of water, to ensure adequate quality of drinking water.

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What Can be Done?

Environment Canada, Alberta Sustainable Resource Development, Manitoba Conservation, Ducks Unlimited Canada, the University of Saskatchewan, and the Canadian Cooperative Wildlife Health Centre are working cooperatively on botulism research and management in Western Canada, with assistance from agencies such as the U.S. Fish and Wildlife Service, California Waterfowl Association, U.S. Geological Society and Delta Waterfowl Foundation.

Although botulism cannot be prevented, the overall objective of current botulism research is to minimize or reduce bird deaths. Research efforts are focusing on the impact of botulism on populations and ecological causes of outbreaks. Research to date has concluded that carcass cleanup operations are not effective in reducing duck mortality. The number of waterfowl killed by an outbreak is much greater than current capabilities to locate and collect the carcasses, especially on larger water bodies.

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This fact sheet was prepared by the Avian Botulism Task Force associated with the Prairie Habitat Joint Venture of the North American Waterfowl Management Plan.