Wildlife biologists are pondering what to do after several major outbreaks of botulism felled millions of waterfowl this summer. The disease, caused by an aquatic microbe, brought the largest bird die-off in North America in recent memory.
Dead duck. Female pintail shows "limberneck" from nerve poison.
GARY WOBESER/CCWHC
Water birds pick up the disease, which peaks in summer months, by eating insect larvae and other invertebrates that carry the Clostridium botulinum neurotoxin. Paralyzed, they often suffocate or drown. Avian botulism has probably been around for at least a century in western regions, but now it's popping up "in almost every state," says Tonie Rocke of the National Wildlife Health Center in Madison, Wisconsin. Among the hardest hit are northern pintail ducks. Included in the summer toll--as revealed by tests on carcasses--are more than 5000 ducks and other birds at a refuge in Illinois; close to 100,000 birds at Great Salt Lake in Utah; and perhaps half a million birds at Old Wives Lake in Saskatchewan. This type of botulism is not known to harm people.
Some outbreaks of avian botulism are inevitable, because spores of C. botulinum are present in most wetlands. The microbe turns deadly when it has a good protein supply--decaying organic matter, dead bugs, or bird carcasses. Outbreaks spread when birds eat toxin-drenched maggots feeding on dead birds, explains wildlife pathologist Gary Wobeser of the University of Saskatchewan. Factors like temperature, pH, and salinity also matter. Wetter weather in the past few years as well as water that contains more pesticides (which create masses of dead invertebrates) and sewage may help explain the recent rise. But finding the source of a particular outbreak is "like trying to go back to a forest fire to find the spark," Wobeser says.
Wildlife managers are moving to minimize future botulism episodes. Rocke's group will soon publish a set of "predictive models" based on a study of U.S. wetlands that would point managers to susceptible wetlands and enable them to take steps, such as altering water flows, to avoid outbreaks. Wobeser hopes to convene a dozen U.S. and Canadian experts next month to discuss research needs and strategies such as altering water flows to change the pH or salinity.
