Although bacteria are often transmitted when insects feed on their wildlife hosts, transmission of the toxin that causes avian botulism can occur in a different manner. Avian botulism is the most signifi cant disease of migratory birds, especially waterfowl and shorebirds. Not surprisingly, it was formerly called ‘ duck sickness. ’ The bacterial complex known as Clostridium botulinum produces protein neurotoxins that cause food poisoning when animals (or humans) eat toxin - laden food. There are several different neurotoxins produced by Clostridium , with type C 1 being most common among birds, though loons and gulls are typically affected by type E toxin. Although over 250 species of birds have been found to experience botulism poisoning (scavengers such as vultures and crows seem to be resistant), it is filter feeding, dabbling, and fish - eating birds that are especially likely to ingest toxin. Apparently, the poison originates with invertebrates and plants living under anaerobic (depleted of oxygen) conditions in marshes and mud flats, because these bacteria only develop under anaerobic conditions. Decomposing vertebrate carcasses also support high levels of toxin production, producing a secondary form of poisoning called the carcass - maggot cycle of botulism . Decomposing animals are a very suitable substrate for growth of Clostridium , as decomposition also generates anaerobic conditions and the high temperatures that favor bacterial growth and toxin production. The dead animals are not attractive to waterfowl, but these birds will readily ingest any mature fly larvae (maggots) that disperse from the carcass, and most maggots do move a considerable distance as they leave the carcass and search for dry pupation sites.
Maggots developing in carcasses can have very high levels of toxin, and cause death to birds that feed upon them. Thus, all that is needed to initiate a prolonged cycle of botulism poisoning is initial death of some animals following a storm, collision with power transmission lines, algal poisoning, or feeding on invertebrates or decaying plant material rich in Clostridium. The bacteria normally found within the bodies of animals in wet habitats are then free to multiply in their carcasses, which are then assimilated into blowflies, which are fed upon by other birds, which then die and become available to more blowflies. Thus, outbreaks of botulism can originate in several ways, and persist for long periods. It is important to note that the animals are not killed because they became infected with the Clostridium botulinum bacterium, they die after ingesting their chemical metabolites, which happen to be poisonous neurotoxins. Thus, Avian Botulism is not really an infectious disease, but a biotoxin, and not quite comparable to most bacterial diseases.
