Four more Filipinos have been shown to have contracted the Ebola Reston virus, a finding that increases the likelihood that pig-to-human transmission took place, health officials said Friday.

Philippine Health Secretary Francisco Duque III said there now are five known cases of possible pig-to-human transmission of the virus after a backyard hog raiser from a northern Manila suburb became the first confirmed case this month.

An initial report said that none of the five men wore protective clothing and all were exposed through direct contact with sick pigs, according to Duque.

The rare virus - first discovered among monkeys south of Manila in 1989 - has not been known to cause serious illness among humans. At least 25 people in the Philippines have been infected with the virus by monkeys, but only one victim exhibited mild flu-like symptoms.

Three of the Ebola virus' five subtypes are associated with deadly hemorrhagic fever in humans; two other subtypes, including the Reston, are not, according to the World Health Organization.

WHO's Dr. Julie Hall said the five cases in the Philippines "increases the likelihood" of pig-to-human transmission of the virus, but investigators were not yet certain. She said all five people are now virus-free.

"They are not infectious to others, they therefore do not need to be quarantined," she said.

While the virus appears to pose low risks to humans, Hall said the government must implement strict measures such as quick reporting of sick or dying pigs and prevention of the sale of illegally slaughtered meat to keep the virus from spreading among the swine population.

The Philippine government invited experts from WHO, the Food and Agriculture Organization and World Organization for Animal Health early this month to conduct a study on the health risks of the virus, first found in pigs in the Philippines in October.

The discovery not only marked the first time the virus has been found outside of monkeys, but also the first time it has been found in swine, a food-producing animal.

Health officials were trying to locate anyone who may have had contact with the five men.