TOKYO, Jan 18 (Reuters) - A biological test showed an outbreak of bird flu at a poultry farm in southwestern Japan last week was due to a highly virulent type within the H5N1 strain of the virus, a farm ministry official said on Thursday. Earlier tests had shown that the virus had the N1 component, making it distinct from the less lethal H5N2 strain. But a biological test under the guidelines of the World Organisation for Animal Health was required to determine how highly poisonous the virus was, as not all types of the H5N1 strain are equally virulent, the ministry official said. In the test, eight birds were injected on Tuesday with the virus taken from dead birds at the farm, and by Thursday all eight had died, confirming that it was highly virulent. Further genetic tests will allow experts to analyse the results, the ministry said in a later statement. There have been no reported cases of human infection or additional outbreaks in poultry in Japan. Almost 4,000 birds died from the disease at the affected farm, and authorities killed the farm's remaining 8,000 chickens on Sunday.
