Avian botulism has killed up to 7,000 birds and continues to spread in wildlife refuges south of Klamath Falls.
Dry and hot conditions have contributed to the rapid spread of the disease, according to the Herald and News.
Refuge workers have been removing dead birds to limit the infections of live birds.
Ducks have been most affected because they’ve concentrated at Tule Lake in California while Lower Klamath Lake remains dry.
The waterfowl cannot fly for about a month while they’re molting. That leaves them no way to leave the diseased marsh.