JAKARTA, Indonesia, June 15 -- A farm worker in eastern Indonesia has tested positive for bird flu, marking the country´s first human case of the virus that has already killed at least 54 people elsewhere in Southeast Asia, health officials in Indonesia said Wednesday.

The worker from southern Sulawesi island is healthy and currently shows no symptoms of the illness but two tests at a Hong Kong laboratory confirmed that he had been infected by avian influenza, health officials said.

The laboratory results make Indonesia the fourth country to register a human case of bird flu, which international health experts warn could easily undergo genetic change, sparking a global pandemic.

Since 2003, the highly lethal disease has struck chickens, quail and other birds in 18 Indonesian provinces on seven islands, prompting the government to order a massive campaign to vaccinate poultry against the virus.

Indonesian health experts, however, have sought to ease public anxiety about the outbreak over the last year by saying the local virus was slightly different than the strain in other Asian countries and had demonstrated no capability to infect people.

Muhammad Nadhirin, chief of epidemiological surveillance at the Indonesian health ministry, confirmed that the Sulawesi laborer had tested positive for bird flu, adding that officials were now hoping to conduct further tests on his sample.

The farm worker was initially tested in late March after the epidemic spread to Sulawesi, killing at least 25,000 chickens. That outbreak prompted officials to limit the transfer of poultry off the island and take blood samples from laborers, veterinarians and others exposed to sick chickens. In total, 81 people were tested and all but one of the samples came back negative, officials said.

Efforts to complete a second round of testing in Hong Kong were prolonged in part because the farm worker had left his job and health investigators had to track him back to his home village elsewhere on the island.

The second test, finally completed earlier this month, confirmed that the laborer had been infected by bird flu but the concentration of antibodies was relatively low, officials said. That finding meant the worker was no longer carrying the virus but it was impossible to determine how long ago he had caught it, according to Hariadi Wibisono, a director of disease eradication at the health ministry.

Since late 2003, more than 100 people have been infected by bird flu in Vietnam, Thailand and Cambodia. In Vietnam, where the outbreak is most serious, government health officials have previously reported at least five cases in which people had the disease but showed no symptoms.

Klaus Stohr, head of the World Health Organization´s influenza program, said last month it is not unprecedented for otherwise healthy poultry workers to test positive for bird flu. During a 1997 outbreak in Hong Kong, about 10 percent of workers in live poultry markets tested positive for the virus, health officials said.

Even if a person shows no symptoms, the infection still poses a serious health risk. Health experts worry that a person could catch both avian influenza and an ordinary flu bug at the same time, presenting a chance for the viruses to swap genetic material and produce an entirely new lethal strain that can easily be passed among people. If this happens, WHO officials warn that tens of millions of people could die worldwide.

Researchers have also determined that bird flu has infected Indonesian pigs, another ominous development because a similar genetic change can occur in swine, which like people, can catch both avian and ordinary flu viruses.

Chairul Anwar Nidom, an influenza expert at Airlangga University in Surabaya, said in a February interview that he had discovered bird flu in blood samples taken from pigs on the outskirts of Jakarta. These pigs were from farms near a poultry outbreak in West Java province.

But despite Nidom´s efforts to warn agriculture officials about his findings, the Indonesian government months took weeks to follow up before conducting its own tests. Indonesia notified the World Animal Health Organization late last month that the virus had spread to pigs.