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Avian Botulism

Avian botulism is a neuromuscular illness caused by a toxin that is produced by the bacteria Clostridium botulinum. There are seven types of botulism toxins (A-G). Wild birds are affected by type C and type E. These bacteria typically live in lakes, ponds or wetland substrates and sporadically produce toxins when certain environmental conditions develop. Environmental conditions thought to contribute to toxin production include high water temperatures and low oxygen levels in the water. Two invasive species may also play a role in the transmission of the toxin in the Great Lakes. These are the zebra mussel and a small fish called the round goby.


Botulism outbreaks in wildlife occur when invertebrates or fish ingest the bacteria. If sub-optimal water conditions lead to the deaths of these invertebrates or fish, the bacteria multiply in the dead animals and produce the toxin. Waterfowl, such as mallards, and fish-eating birds, such as loons and gulls, are then affected by the toxin contained within the invertebrates or fish that they eat. When these birds die, maggots consuming the carcasses pick up the toxin and any other bird or mammal that scavenges the carcasses can also be affected by the toxin.


The toxin interferes with nerve transmission to the muscles. Signs observed in affected birds will vary depending on how many toxins were ingested. Birds will have progressive muscle weakness and are observed to have difficulty flying or standing. The paralysis can eventually reach the muscles needed for breathing. Waterfowl may become unable to hold their heads up and drown.


In Wisconsin, botulism type C and botulism type E have both been documented to cause significant mortality in water birds and fish. Botulism type C is usually associated with waterfowl die-offs on smaller lakes and wetlands, while botulism type E is known to cause die-offs among fish-eating birds, such as common loons and gulls, in the Great Lakes ecosystems.

Because the Clostridium bacteria are naturally found in the environment there are no easy control methods for preventing outbreaks of botulism in wildlife. Wildlife mortality commonly occurs in the fall when water temperatures are high and water levels are low. On some smaller lakes, water level controls may help reduce the effects of botulism. Control of the invasive species linked with botulism may also help to reduce the occurrence of mortality due to botulism on the Great Lakes. Removing and properly disposing of carcasses during a botulism event decreases the availability of toxins and risks to additional wildlife.


Botulism in people is usually due to type A or B toxins from consuming home-canned foods that were improperly preserved or from damaged store-bought canned foods. Type C botulism is not known to affect people. Humans can become sick from type E from consuming affected fish, but proper cooking will inactivate the toxin. Besides cooking fish or waterfowl to the recommended temperature, it is never a good idea to eat fish or birds that appear sick or are found dead. Precautions should always be taken when handling carcasses, including wearing gloves and washing hands. Additionally, pets should be kept away from carcasses.