MANILA, Jan 23 (Reuters) - At least one person has tested positive for the Ebola-Reston virus in the Philippines, where the disease has broken out in pigs at two farms north of the capital, the government said on Friday.

Health Secretary Francisco Duque told a news conference that there was little immediate health risk but experts warned the virus' jump to humans was a concern.

"This presents a negligible risk to human health," Duque said.

Experts from the World Health Organisation, World Organisation for Animal Health and the Food and Agriculture Organisation, all U.N. agencies, ended a 10-day field test at the two farms over the weekend after Ebola-Reston was found there last year.

It is the first time the virus has been found outside monkeys and the first time it has been found in pigs. The virus had previously jumped from monkeys to humans but it is the first case of a jump from hogs.

Duque said at least 50 workers in the two farms were exposed to the virus, but only one person tested positive. This person had not shown any symptoms, he said.

Experts said the jump was a concern even if the Ebola-Reston strain of the virus is not as deadly as other strains of the disease, which can cause incurable haemmorrhagic fever and have a mortality rate of 25 to 90 percent.

"Viruses jumping across species is always worrying," said Lo Wing-lok, an infectious diseases expert in Hong Kong.

"If it continues to do so, the virus will adapt to the human body or may mutate to become more transmissable among humans."

Although human cases of Ebola Reston in the past have been mild, Lo warned against any complacency.

"We can't say for sure that it is not dangerous to man, we have to follow developments very closely," Lo said. "In the past, the infections happened to a very small number of people.

"But this virus may get magnified in swine and we could get a higher-density virus in the environment and more cases of human infection can occur," he said. (Reporting by Manolo Serapio Jr. and Manny Mogato) (Additional reporting by Tan Ee Lyn) (Editing by Sanjeev Miglani)

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