Avian botulism has killed at least 136 Laysan ducks, found dead over the past two weeks in the Midway Atoll National Wildlife Refuge, according to the National Wildlife Health Center.

"It's not uncommon to have birds affected by avian botulism," said Barbara Maxfield, U.S. Fish and Wildlife Service spokeswoman. "It's the first time on Midway Atoll. Wildlife has been affected here in Hawai'i by the avian botulism, but not with the Laysan ducks."

The first dead duck was found Aug. 10, and the death toll of adult and adolescent birds is up to 136.

Several carcasses were shipped to the National Wildlife Health Center's Honolulu field station, and from there, samples were sent to the NWHC in Madison, Wis.

Avian botulism toxin was found in all eight samples from the refuge, which is in the Papahanaumokuakea Marine National Monument.

The highest concentration was from a catchment basin near where the Laysan ducks nest. The catchment has now been drained and freshwater has been introduced to the wetlands in other nesting areas to dilute the botulism, Maxfield said.

The Laysan duck is on the endangered species list and the entire world's population is fewer than 1,000, all in the Hawaiian Islands, mostly on Laysan Island, Maxfield said.

Avian botulism, which is found in soil throughout the world, does not affect humans, she said. The outbreak affected a new duck population started in 2004 on Midway Atoll as a way to expand the Laysan population.

The Midway population was about 400 birds, and Laysan Island, which is more than 100 nautical miles away, has about 600 ducks. There is no risk of Midway's bout of avian botulism affecting the Laysan population, experts say.

The Papahanaumokuakea Marine National Monument is administered jointly by the U.S. Department of Commerce, the U.S. Department of the Interior and the state of Hawai'i.