Science Center Objects

Botulism is a natural toxin produced by a bacterium ( Clostridium botulinum ) commonly found in the soil. There are several types of botulism toxin some of which can affect humans who eat improperly canned foods. Birds get their own kind of botulism (Type C in Hawaii) that does not affect humans. Botulism type C is concentrated in aquatic invertebrates that filter feed sediments or water. When birds eat the invertebrates, they get a concentrated package of toxin. A bird-to-bird cycle can also exist where maggots feeding on dead birds can concentrate the toxin and can then be eaten by and poison other birds.

What does it do?  Botulism is one of the most potent toxins known to science (very little is needed to kill). Botulism binds to nerve endings and interferes with muscle movements. Typical clinical signs in birds with botulism include weakness, lethargy, inability to hold up the head or to fly. For waterfowl, this can be catastrophic because inability to hold up the head leads to drowning.

Laysan Duck

Laysan Duck. (Credit: James H. Breeden, USGS. Public domain.)

What does it affect?  Avian botulism most often affects waterfowl. In Hawaii, birds commonly affected include shovelers, Hawaiian coots, Hawaiian gallinules, Hawaiian stilts, black crowned night herons, and various migratory ducks. Shovelers are particularly sensitive indicators of botulism because they are such efficient filter feeders and thus are most likely to accumulate sufficient toxin to kill. Avian botulism does not affect humans.

What are the field signs?  Dead birds or birds manifesting clinical signs such as inability to fly, walk, or hold up their head. Typically, birds will die in good body condition although exceptions exist for birds that are poisoned over a longer period of time and become emaciated. Because botulism is a poison that kills fairly rapidly, most birds do not show any gross lesions at necropsy.

Where and when does it occur?  In Hawaii, there is no seasonal pattern to avian botulism (it appears to occur year-round). In some cases, botulism die-offs in waterfowl can occur just prior to fish kills in a wetland. On the main Hawaiian islands, botulism has been found on Kauai, Oahu, Molokai, Maui, and Hawaii. Recent outbreaks of botulism have occurred on Midway Atoll in the Northwestern Hawaiian Islands affecting critically endangered Laysan ducks. Any place where there is water and where waterfowl congregate is a potential area for botulism.

How do you detect it?  Although the presence of suggestive clinical signs and absence of gross lesions in dead birds would render a suspicion of botulism, confirming the presence of botulism type C as a cause of death in birds requires laboratory tests. The sample of choice is the heart (which contains a lot of blood where the toxin is found) and which must be submitted to the laboratory frozen or fresh.

How do you manage it?  Botulism is one of the few wildlife diseases we can actually manage effectively. Although we don't know all the environmental triggers that cause Clostridium botulinum to start producing toxin, we do know that if mortalities are detected early enough, certain management techniques, if implemented quickly, can rapidly stop and mitigate the magnitude of waterfowl mortality. These measures include: 

  1. Carcass pick up and removal: Clostridium botulinum needs protein to produce botulism toxin. Because animal carcasses are an excellent source of protein, removing them reduces the resources the bacterium needs to produce toxin and can help reduce or eliminate toxin production. Because many birds sick with botulism will hide in vegetation, it is critical that carcass retrieval be thorough. A single carcass left in the wetland can prolong an outbreak.
  2. Water management: Draining or flooding the wetland can change the environmental conditions sufficiently so as to stop the production of toxin by C. botulinum.
  3. Hazing birds away from the source: If the two management techniques above fail, hazing birds away from the wetland until toxin production stops is an option. Because hazing birds constitutes a “take” under the endangered species act, and because most wetlands in Hawaii have endangered waterfowl, this action should be taken in consultation with the US Fish & Wildlife Service.