Labs in Wisconsin and at Oregon State University say that testing performed on at least a dozen ducks show preliminary findings that avian botulism was the cause of death of more than 1,200 birds at a Portland-area wetland, according to a Metro government news release Sept. 20.
Metro and Oregon Department of Fish and Wildlife staff have removed more than 1,200 dead or dying waterfowl from the Smith and Bybee Wetlands Natural Area.
Smith and Bybee Wetlands is a 200-acre natural area that attracts a variety of migratory birds.
In addition to migrating waterfowl, several shorebirds and a pelican also died. The Portland Audubon Society has been receiving and rehabilitating two dozen live affected birds, which will be released when they have recovered and are able to continue their migration.
Officials report there is little public health risk to humans from avian botulism. However, wetland officials will keep the area closed until the outbreak is declared over.
According to the USGS National Wildlife Center, 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 bacterium; 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.
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.
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