Avian botulism is a paralytic disease caused by ingestion of a toxin produced by the bacterium, Clostridium botulinum. Bacterium live in the soil and require warm temperatures, a protein source and an anaerobic (no oxygen) environment in order to activate and produce toxin. There are several types of toxin produced by strains of this bacterium. Type C toxin is most often the cause of botulism in waterfowl and type E may occur in some species of waterfowl and is frequent in certain other aquatic birds.
Important environmental factors that contribute to initiation of avian botulism outbreaks in wetlands include water depth, water level fluctuations, and water quality; the presence of vertebrate and invertebrate carcasses; rotting vegetation; and high ambient temperatures. The primary months for type C avian botulism outbreaks are July through September. Localized outbreaks are not uncommon throughout Arizona, especially in our urban lakes and ponds.
Bacteria grow in dead vertebrates and invertebrates. Birds either ingest the toxin directly or may eat invertebrates (e.g. chironomids, fly larvae) containing the toxin. A cycle develops in a botulism outbreak when fly larvae feed on animal carcasses and ingest the toxin.
The botulinum toxin causes respiratory failure, though birds often drown before this occurs due to muscle paralysis. Signs of paralysis include the inability to walk or fly, “limberneck” due to paralysis of the neck muscles, and paralysis of the inner eyelid.
Type C botulism in humans has not been associated with the botulism found in waterfowl therefore basic sanitary procedures should be used when handling dead waterfowl. Type E on the other hand is highly toxic to humans so guidance should be sought in proper sanitary methods.