Officials inspect poultry farms in neighborhood of bird flu outbreak in southern Japan

An outbreak of deadly bird flu at a farm in southern Japan prompted health authorities and veterinarians to collect blood samples from poultry in neighboring farms to see if the disease had spread, officials said Wednesday.

The inspection in Miyazaki prefecture (state) comes a day after officials identified the virus that killed 4,000 birds at the original farm as the virulent H5N1 strain of bird flu virus. The National Institute of Animal Health, which had identified the strain, is running further tests to obtain DNA and pathogenic details of the virus.

Officials on Wednesday visited 11 of the 16 farms raising chickens within a 10-kilometer (6.2-mile) radius of the origin of the outbreak, said Miyazaki poultry official Masao Tanaka. The remaining five farms are currently without poultry and not subject to inspection.

Officials have slaughtered and burned all 12,000 chickens at the farm where the outbreak occurred and banned shipment of eggs and chickens beyond the radius.

Meanwhile, scientists at the National Institute of Infectious Diseases published a study that a Tokyo resident infected with the bird flu or other types of deadly influenza could spread it to as many as 120,000 others in Tokyo and four surrounding prefectures (states) within 10 days.

The simulation, based on activity patterns of 880,000 Tokyoites and infection through droplet transmission within a 1-meter (3.3-foot) reach, focuses on virus transmission and excludes analysis on fatality.

A first person who caught the bird flu in a trip to Southeast Asia would start spreading it to some 30 people on crowded trains and at work after developing full-blown symptoms following a three-day incubation period, said Yasushi Okusa, a senior researcher at the institute. Some 715 people would be infected on a sixth day, to 8,500 on an eighth day and 30,000 people on a ninth day, he said.

"Our study shows that an outbreak could spread in the Tokyo area at a much faster pace than in the U.S. or Britain, mainly because of commuting on crowded public transportation system in Japan," Okusa said. "An outbreak could also spread to broader areas than we expected, which makes it harder to confine the problem."

Bird flu remains hard for humans to catch. But experts fear the virus could mutate into a form that passes easily among people, potentially sparking a pandemic. So far, almost all human deaths from bird flu have been a result of direct contact with infected birds.

H5N1 has killed at least 161 people worldwide since late 2003, according to the World Health Organization.

If the collected samples from Wednesday's inspection in Miyazaki are tested negative, officials plan to start considering scaling back the quarantine restrictions, Tanaka said.

Miyazaki, about 900 kilometers (560 miles) southwest of Tokyo, is Japan's main chicken-producing region.

Bird flu was found in Japan in 2004 for the first time in decades. There has been one confirmed human H5N1 infection in Japan, but no reported human deaths. Japan has also had several outbreaks of other bird flu viruses not harmful to humans since then, leading to the culling of millions of birds.