Avian botulism
Dead birds floating in the water at Willard Spur WMA

Avian botulism

Dead birds floating in the water at Willard Spur WMA

Avian botulism at Willard Spur WMA

Avian botulism is a paralytic, often fatal, disease of birds that results from the ingestion of a toxin produced by the bacterium Clostridium botulinum. Seven types of toxins have been identified, only three of which are known to cause avian botulism in North America. The toxin is produced after the germination of bacterial spores, which can persist in the environment for years during adverse conditions. A virus or phage, which is known to infect the bacterium, is the actual carrier for the gene that encodes for the toxin protein. Botulinum spores and the phages that carry the toxin gene are very prevalent in wetlands, and it is believed that optimal environmental conditions for spore germination, bacterial growth and replication, and a means for toxin transfer to birds, are the critical factors contributing to avian botulism outbreaks.

Waterfowl and shorebirds are the bird species most often affected by type C avian botulism in the United States, although almost all birds and some mammals are susceptible to botulism. Foraging behavior is probably the most significant host determinant for botulism. Humans are regarded as being fairly resistant to type C botulinum toxin. Thorough cooking destroys botulinum toxin in food.

Avian botulism in Utah

Avian botulism occurs almost yearly in Utah, and can result in extensive losses, typically between July and September. Major waterfowl botulism outbreaks in Utah have occurred at the Great Salt Lake and estimated loss was in the hundreds of thousands. In 1997, estimated loss from a botulism outbreak at the Great Salt Lake was 514,000 waterfowl.

Field signs

Lines of carcasses coinciding with receding water levels often indicate major botulism outbreaks. Botulism mainly occurs at the waters edge and few sick or dead birds are found very far from the shoreline. Healthy and sick birds of multiple species are commonly found mixed together during a botulism outbreak, along with carcasses in various stages of decay. Inability to sustain flight and paralysis of leg muscles, indicated by the bird propelling itself across the water with its wings, followed by paralysis of the inner eyelid and neck muscles, which results in inability for the bird to holds its head erect, are two of the most easily recognizable signs of avian botulism. Death by drowning often follows.

Control and management

Prevention of avian botulism outbreaks involves control efforts to minimize fluctuating water levels during hot summer months and prompt removal and disposal of animal carcasses.

Safety tips for hunters and others who encounter dead birds

Avian botulism outbreaks are not typically associated with human illness. However, while the risk of avian botulism transmitting to humans is low, it is still important to practice the following safety precautions if you encounter sick or dead birds:

  • Harvest waterfowl that are actively flying (typically an indication that they are healthy).
  • Avoid harvesting or consuming birds that appear sick or weak.
  • Cook meat thoroughly, as heat will denature the toxin.
  • Botulism can affect dogs if they consume the meat, so keep your dogs away from sick, dead or dying birds.

What to do if you encounter sick birds

  • Do not handle or touch the birds. Do not attempt to catch them and do not feed them — doing so can harm them further by providing them with the incorrect food.
  • Keep a safe distance away to avoid stressing them further.