The worst outbreak of avian botulism in 40 years at the Klamath Basin National Wildlife Refuge has killed more than 40,000 waterfowl and shorebirds, biologists say. The outbreak is a product of sustained hot weather, warm water, receding wetlands from lack of freshwater and crowding. “This has turned out to be a really bad year for botulism,” said John Vradenburg, a supervisory biologist for the refuge. “Every wetland in the basin is dealing with some level of botulism outbreak.” Amid the epidemic, with three airboats, crews have rescued more than 2,000 birds for rehabilitation and release. Vradenburg said it will take consecutive freezing nights to quell the bacteria, and hopes cold weather arrives soon, as it often does here in early September. Avian botulism does not affect humans, but it can paralyze waterfowl such as ducks and geese and shorebirds such as stilts and grebes. It is a bacteria that lives in the soil, where sustained warm weather, receding water, and decaying vegetation can cause a flare-up. Birds can be infected through their food, and it can be spread by birds that have died from the disease and have become infested with maggots, which other birds might eat. The outbreak ignited in late July, just as many waterfowl started molting — shedding and then replacing their feathers — and for a month, lost the ability to fly to clean water. Additionally, some 50,000 ducklings hatched during summer and are unable to fly for about two months. “If the ducks can’t fly, they can be stuck in a contaminated area,” Vradenburg said. The Klamath Basin features a complex of national wildlife refuges that straddle the California-Oregon border. Although only 13,000 acres of permanent wetland habitat remains, it is considered the “heartbeat of the Pacific Flyway” by the California Waterfowl Association, where millions of migratory birds stop to rest, feed, and water, and resident ducks will nest. It gets limited water from the Klamath River, In the past two weeks, waterfowl and shorebirds also started arriving in large numbers from points north on the Pacific Flyway, which stretches from Alaska to the southern tip of South America. In one area alone, about 2 square miles on Tule Lake, Vradenburg estimated there were 75,000 waterfowl and shorebirds. Yet miles of designated wetlands of the refuge are dry due to drought and limited water deliveries. An aerial survey shows a network of canals, some with stagnant water and others with cracked earth, that border historic wetlands resembling desert stubble. This forces birds to crowd in relatively tiny spaces where they can find water. To provide some relief last month, the Tule Lake Irrigation District worked with the Department of the Interior and several conservation groups, most notably the California Waterfowl Association, to allow a small amount of water to flow into the refuge along the Stateline Highway that runs parallel to the California-Oregon border.