Nine pygmy rabbits died at Northwest Trek near Eatonville during during the past month as an unexplained disease swept through the endangered population.
Trek officials said the deaths constituted about half of the stock, which was being bred and reared in captivity for eventual release into the wild.
The cause of the rabbits' deaths is unknown, but "the situation is being handled as a highly contagious, acutely lethal disease," according to a news release.
They were of normal weight and eating well before their deaths, the release said.
There "were no visible signs of distress or abnormal behavior within hours" of their deaths.
The remaining rabbits were moved indoors and isolated in a clinic, where they're getting daily medical treatment.
The sudden deaths canceled Trek's plans to release some of the animals into Washington's Columbia Basin later this month.
Trek is one of several partners in the Pygmy Rabbit Recovery Program led by the State Department of Fish and Wildlife, the news release said. The Oregon Zoo and Washington State University also participate. Trek was scheduled to release at least five rabbits.
Animals from Oregon and WSU will be set free in Douglas County as planned Tuesday, the news release added.
Wildlife officials announced this week they planned to release 23 of the captively bred animals. They are to be placed in artificial burrows until they can dig their own.
The Columbia Basin Pygmy Rabbit is listed on the state and federal endangered species lists. To ensure the survival of some pygmy rabbits, animals from the Columbia Basin stock were crossbred with Idaho pygmy rabbits, according to a news release from the wildlife service.
The Trek rabbits, part of the breeding and rearing program, were not on public display.