Water temperatures in the 90s and runoff from recent squalls might have combined to create a petri-dish mix of toxins that killed 60 of the 200 mallard ducks that live along Havre de Grace's waterfront promenade.
State wildlife officials say the ducks and a Canada goose were found dead Monday and yesterday, and they suspect that naturally occurring botulism is to blame.
"It may not be that. You're never sure. But these incidents happen every year around the state, mostly in the Western Shore counties that border the [Chesapeake] Bay. I've seen them many times before," said Larry Hindman, waterfowl project manager for the Department of Natural Resources.
Two dying ducks were taken yesterday afternoon to the Maryland Department of Agriculture's Centreville Animal Health Diagnostic Lab for testing. Dr. William P. Higgins, director, said he would send blood serum samples to an out-of-state lab as part of his investigation.
"You have to detect the toxin in the serum. You don't see anything. There are no lesions or other markings," Higgins said.
Hindman said reports of botulism typically involve flocks in areas frequented by people. With its lighthouse, decoy museum and skipjack Martha Lewis, the promenade is a tourist attraction and gateway to the Susquehanna River.
The biologist suspects that of the six types of botulism, Type C is responsible for the deaths. The risk to humans is negligible.
Botulism can remain dormant in the soil for years. Under the right weather conditions, such as when water rises and quickly recedes, the spores germinate. Waterfowl nibbling on insects, mollusks and small shellfish containing the toxin often become ill within 48 hours.
"We call it 'limberneck disease' because the toxin affects their legs, wings and neck. They can't walk or fly and their wings droop and eventually they can't hold their heads up," Hindman said. "If you put them in the shade and give them water, sometimes they can fight through it."
Type C was first reported in California and Utah in 1910, when millions of birds died. The most recent major outbreak, in 1997, killed 1.5 million birds in Utah and Canada, according to the U.S. Geological Survey.
The Chesapeake Bay wetlands and tributaries have had about 17 outbreaks of avian botulism in the past 30 years, according to the agency.
Hindman said Havre de Grace officials have removed all the carcasses to prevent the spread of the bacteria and will continue to inspect the shoreline for sick birds.