National Wildlife Health Center

...advancing wildlife and ecosystem health

Avian Botulism

Avian botulism is a paralytic disease caused by ingestion of a toxin produced by the bacteria, Clostridium botulinum. This bacteria is widespread in soil and requires warm temperatures, a protein source and an anaerobic (no oxygen) environment in order to become active and produce toxin. Decomposing vegetation and invertebrates combined with warm temperatures can provide ideal conditions for the botulism bacteria to activate and produce toxin. There are several types of toxin produced by strains of this bacteria; birds are most commonly affected by type C and to a lesser extent type E.

Birds either ingest the toxin directly or may eat invertebrates (e.g. chironomids, fly larvae) containing the toxin. Invertebrates are not affected by the toxin and store it in their body. A cycle develops in a botulism outbreak when fly larvae (maggots), feed on animal carcasses and ingest toxin. Ducks that consume toxin-laden maggots can develop botulism after eating as few as 3 or 4 maggots.

  • Type C toxin: waterfowl, shorebirds, colonial waterbirds, and others
  • Type E toxin: gulls, loons, and others

Clinical Signs/Field Signs
Healthy birds, affected birds, and dead birds in various stages of decay are commonly found in the same area. The toxin affects the nervous system by preventing impulse transmission to muscles. Birds are unable to use their wings and legs normally or control the third eyelid, neck muscles, and other muscles. Birds with paralyzed neck muscles cannot hold their heads up and often drown. Death can also result from water deprivation, electrolyte imbalance, respiratory failure, or predation.

There are no specific lesions associated with this disease. Diagnosis of botulism is based on demonstration of the toxin in serum from sick birds, or tissue samples from dead birds such as clotted heart blood, stomach contents, or liver.

Wildlife Management Significance
Outbreaks occur from coast to coast in the United States and Canada, generally from July through September. Thousands of birds may die during a single outbreak.

Prompt removal and proper disposal of carcasses by burial or burning (in accordance with applicable ordinances) is highly effective in removing toxin and maggot sources from the environment. If possible avoid altering water depth by flooding or drawing down water levels during hot weather. This may increase invertebrate and fish die-offs, a protein source for the bacteria.

Providing mildly affected birds with fresh water, shade and protection from predators may help them recover from the intoxication. Botulism antitoxin is available but requires special handling and must be given early in the intoxication. Birds that survive a botulism outbreak are NOT immune to botulism toxin.

For more information please contact: The USGS National Wildlife Health Center, at 608-270-2400.

Duck with Botulism C

Public Health Significance

Botulism in humans is usually the result of eating improperly home-canned foods, which contain types A or B toxin. Type E toxin has been associated with improperly smoked fish. People, dogs, and cats are generally thought to be resistant to type C toxin, but a few cases have been reported in people and dogs. Thorough cooking destroys botulism toxin in food.

For a map of botulism outbreaks in wild birds through 2004:
  1. Go to the National Atlas
  2. Click on the yellow 'Map Maker' button in the menu
  3. Select: 'Biology' > 'Wildlife Mortality' > 'Avian Botulism'
  4. Click on the blinking 'Redraw map' button above the side menu

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Page Last Modified: Jun 20, 2018