MANILA, Philippines -- (UPDATE 2) Four more persons in the Philippines have tested positive for a strain of the Ebola virus that is not deadly to humans, taking the number of human cases to five, the government said Friday.

The Philippine government and World Health Organization (WHO) Friday refused to rule out the possibility that the five men so far confirmed infected -- four workers from pig farms in Bulacan, Pangasinan and Valenzuela City, and a butcher from a Pangasinan slaughterhouse -- may have contracted a non-lethal strain of the Ebola virus from infected pigs.

One farm worker was last week confirmed to have been infected by the disease.

Health Secretary Francisco Duque said Friday the five human cases were four pig farm workers and a butcher who tested positive for Immunoglobulin G (IgG), an antibody against the Ebola-Reston virus.

The presence of ERV IgG antibodies indicates that a patient may have contracted the virus but is effectively protected against it.

A UN team lead by the WHO said all the men -- who are aged between 22 and 52 -- are well and no longer carry the virus after being able to expel it from their systems.

If a link is proved between the outbreak in the animals and the men, it would be the first time humans have contracted the disease from pigs, although there was no firm evidence of that so far, officials said.

Julie Hall, a member of the UN investigating team, told a news conference that it was a "low-risk situation and an important situation for animal and human health."

However, she said she agreed with an earlier statement by Duque that a swine-to-human transmission of the virus "cannot be dismissed."

"There are still a lot of unanswered questions about this particular virus," Hall said.

"These are humans who are young, fit and healthy," she said, adding there was no telling what the virus could have done "if other individuals who do not have robust immune systems were to be infected."

The government quarantined farms in the Philippine towns of Pandi and Talavera after the Ebola-Reston virus was discovered in pigs in July 2008.

The UN sent the team of medical specialists to the Philippines last month after four pigs tested positive for a strain of Ebola that was first found in monkeys exported by the Philippines to a US laboratory in Reston, Virginia.

Ebola-Reston was first detected in 1989 in laboratory monkeys sent from the Philippines to Reston, Virginia, in the United States. Unlike its African counterparts, it has not proved deadly.

The other known Ebola strains are deadly to humans.

Two farms in Bulacan and Pangasinan provinces were quarantined and the health department and the UN team gathered blood samples from 77 people who may have been exposed to the virus.

Duque said three of the five men were from the quarantined farms, but all the men are "healthy and have not been seriously ill in the previous 12 months."

None of the five had been using personal protective equipment, he said.

Of the 77 samples tested at the Research Institute for Tropical Medicine (RITM), 37 were sent to the US Center for Disease Control and Prevention (CDC) for further tests.

The workers are not the first human case of the Ebola-Reston virus.

Twenty five people who came into contact with the infected laboratory monkeys in 1989 tested positive for the virus. Only one showed signs of sickness, suffering from flu-like symptoms, but quickly recovered.