SEOUL, Dec 21 (Reuters) - A fourth case of bird flu has been discovered in South Korea after culling of poultry from earlier cases, a government official said on Thursday, raising concerns that quarantine measures had failed to control the outbreak. South Korea confirmed in November its first case of the H5N1 strain in about three years. The three initial cases were found in farms in the North Cholla province, around 170 km (100 miles) south of Seoul. The latest case emerged at a duck farm in Asan, South Chungcheong province, about 100 km further north. "We confirmed that a case at a duck farm in Asan was highly pathogenic," an official at the agriculture ministry said. There were no reports suggesting human infection, the official added. Quarantine authorities would cull 23,000 poultry within a 3-km radius of the latest infected farm. Between December 2003 and March 2004, about 400,000 poultry at South Korean farms were infected by bird flu. During that outbreak, the country destroyed 5.3 million birds. Bird flu remains essentially an animal disease, but it has infected nearly 260 people worldwide since late 2003, killing more than 150, according to the World Health Organisation. Since 2003, outbreaks have been confirmed in about 50 countries and territories.
