WEST FORK, Ark. (AP) — State officials say all the chickens exposed to a strain of the bird-flu virus at a Tyson Foods Inc. contract farm in northwest Arkansas have been killed and buried.

Jon Fitch, director of the state's Livestock and Poultry Commission, says workers finished the job just after midnight Wednesday. Fitch says Tyson has provided more than 1,000 blood samples from other birds for testing, but no other exposures have been found.

Springdale-based Tyson began killing 15,000 hens from a flock that tested positive for antibodies of H7N3, a less virulent strain of the virus. The H5N1 bird-flu virus has killed 240 people worldwide and scientists worry it could mutate into a form that spreads easily among people.