Indonesia is testing pigs in several regions for bird flu after discovering the virus in swine on densely populated Java island, an official said on Monday.

The finding last week heightened fears that the sometimes deadly virus could spread to humans in the world’s fourth-most populous country and Southeast Asia’s biggest economy.

Pigs can carry human flu viruses, which can combine with the avian viruses, swap genes and create virulent new strains, health experts say. They fear the virus could mutate into a form that can pass easily among humans and trigger a global pandemic that could kill millions.

Indonesia is the world’s most populous Muslim country and pig farming is not widespread. Islam regards pigs as unclean animals.

Tests have been conducted in several places, including the resort island of Bali, but so far only one area in West Java has shown positive tests for Avian Influenza in pigs. “It’s being carried out, but until today the results of the (other) sites show negative,” the director of animal health at the agriculture ministry, Tri Satya Putri Naipospos, said.

“The pigs are not showing signs of being unhealthy, and thus we can’t say they’re infected. So far, three (pigs in West Java) are positive carrying the strain,” she told Reuters.

She declined to give details of the location where the positive tests were made or elaborate on the kinds of measures the government was taking, saying she feared public panic.

But she said authorities were taking necessary biosecurity and surveillance measures to keep the situation under control.

“Let the government work on this first,” she said.

Naipospos said authorities had not decided whether to conduct a mass cull of pigs.

So far there have been no reports of people in Indonesia being infected by the bird flu virus. Since 2003, 36 Vietnamese, 12 Thais and four Cambodians have been killed by the H5N1 strain.