A case of bird flu was reported in Pyeongtaek, Gyeonggi Province near Seoul on Tuesday. It comes two weeks after the first reported case on April 1 in Gimje, North Jeolla Province, from where the outbreak has since spread to South Jeolla Province. Two weeks after the first outbreak, a total of 20 cases were reported of the highly pathogenic H5N1 strain of virus that can be transmitted to humans.

According to the Ministry for Food, Agriculture, Forestry and Fisheries, test results of the National Veterinary Research and Quarantine Service showed that chickens at a poultry farm in Pyeongtaek, which reported their sudden death on Monday, tested positive for the H5 strain.

Quarantine authorities have already culled a total of 23,000 chickens from this farm and prevented movement of all chickens and ducks within a 10-km radius. If further tests confirm the chickens died from a highly pathogenic strain of H5 that is transmissible to humans, 315,000 chickens raised in the nine poultry farms within 3 km will also be culled as a preventative measure.

But quarantine authorities have failed to come up with a persuasive answer to the basic questions why bird flu broke out and how the virus spread. Nor have they explained why the epidemic comes in April, much later than the usual November-February period.

They have offered presumptions only, the first of which is that the outbreak was caused by droppings of migratory birds that have not yet returned to Russia and other areas, which they usually do in February, apparently due to global warming or the peculiar phenomenon of migratory birds turning sedentary.

Yoon Moo-boo, a professor emeritus of the Department of Biology at Kyunghee University, says the first is conceivable. "It's possible to hypothesize that winter arrives late in Russia, the home of winter migratory birds, as a result of global warming, and that therefore migratory birds arrive in Korea later, and leave later too."

Others speculate that the first outbreak of bird flu at the chicken farm in Gimje on April 1 may have something to do with Chinese, Mongolian and Vietnamese workers there. The three countries have all seen heavy earlier outbreaks. But this is improbable since bird flu has also broken out at other poultry farms without foreign workers.

Quarantine authorities presume that the first outbreak in Gimje did not spread to Jeongeup but the cases occurred separately. Kim Chang-seob, the ministry's chief veterinary officer, said, "Despite the geographical proximity of Gimje and Jeongeup, both in North Jeolla Province, it's hard to believe that the epidemic in one place spread to the other given that both occurred almost simultaneously."

However, they speculate it may then have been spread to other areas in both North and South Jeolla provinces by vehicles. After moving chickens and ducks was banned in Gimje on April 3, some chicken and duck distributors crossed the quarantine line and traveled to other areas.

In other words, there was a loophole in the measures. Quarantine authorities do not rule out that the flu epidemic will spread to South Chungcheong Province, since distributors' vehicles were confirmed to have gone to Nonsan and Cheonan in the province.

That still leaves the question why bird flu struck Pyeongtaek, an area more than 200 km from the Jeolla provinces where there has been no confirmed contact with vehicles from the other areas.