Four more people in the Philippines have been discovered infected by the Ebola-Reston virus and the possibility of pig-to-human transmission cannot be dismissed, the government said yesterday.

It was not a major health risk, Health Secretary Francisco Duque told a news conference, adding that the government was however widening testing of people who might have been in contact with sick pigs at hog farms placed under quarantine since October 2008.

"The Ebola-Reston virus is both an animal and human health issue, but we still consider this as a low risk situation to human health," Duque said.

He said experts from the World Health Organisation (WHO), Food and Agriculture Organisation (FAO) and World Organisation for Animal Health (OIE) had been asked to determine the source and risk of the Ebola-Reston virus in pigs and impact on food security.

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.

Ebola-Reston virus was found in the Philippines as early as the late 1980s and 25 people were found infected after contact with sick monkeys. But only one developed flu-like symptoms and later recovered.

Last week, the government said at least 50 workers in the two farms were exposed to the virus but only one person tested positive. This person has not shown any symptoms and has remained healthy.

Mr Duque said four new cases had been found and these people possibly had direct contact with sick pigs. Three of them were farm workers while one was a butcher, he added.

"At this time, the possibility of pig-to-human transmission cannot be dismissed," Mr Duque said.

"Contact tracing of all five positive individuals is ongoing. It is a standard procedure to determine health risks to humans who are in close contract with positive individuals."

He said all the four men who were found positive remain healthy and have not been seriously ill in the past 12 months.

Davinio Catbagan, director of the bureau of animal industry, said tissue samples from 140 pigs from the two quarantined farms had been sent abroad for tests. Earlier tests on more than 1,000 blood samples from pigs in the two affected farms had yielded negative results.

"No current unusual pig illness has been detected in these two farms," Mr Catbagan said, adding only two pigs died when the Ebola-Reston virus was discovered in October of last year.

The country has more than 13 million heads of swine and the discovery of Ebola-Reston in two farms was isolated, the government said.

The government has called on the public to report unusual pig deaths or illness, cautioning consumers from buying cheap pork and to fully cook meat bought from reputable sources.

Mr Duque said they also asked commercial and backyard farms to practise safe farming and biosecurity measures to prevent and contain any future outbreaks.

Q&A

What is the Ebola Reston virus?

There are five distinct species of the Ebola virus: Zare, Sudan, Cte d'Ivoire, Bundibugyo and Reston. The Zare, Sudan and Bundibugyo species have been associated with large Ebola hemorrhagic fever (EHF) outbreaks in Africa with high mortality rates of between 25 per cent and 90 per cent while Cte d'Ivoire and Reston have not.

How is Ebola-Reston different from the Ebola virus that was responsible for many deaths in Africa?

The Reston species can infect humans but no serious illness or death in humans have been reported to date. During outbreaks of the Reston strain in monkeys in the 1990s, a small number of people (around 25) were found to have antibodies against Ebola Reston. This means they had been infected by the virus and their body had produced an immune response. However, only one person had mild, flu-like symptoms. This person fully recovered. The other people who tested positive for antibodies did not have any symptoms or illness.

What's different this time?

This is the first time that Ebola-Reston has been found in pigs and five people in contact with pigs have tested positive for Ebola Reston antibodies, which means that they were probably infected by pigs, although there is no proof of this. Pigs are worrisome because they are mixing vessels for many types of viruses and bacteria and if left uncontrolled, experts fear Ebola-Reston could mutate into a form that is more transmissible among people.

Who and what are at risk?

Unlike monkeys, pigs are farmed for food and far more people are exposed to them, which puts them at risk of getting infected if the epidemic in pigs is not under control.

What precautions can be taken?

Basic good hygiene practices and food handling measures. Ebola viruses are normally transmitted via contact with the blood or other bodily fluids of an infected animal or person. In all situations, even in the absence of identified risks, meat handling and preparation should be done in a clean environment (table top, utensils, knives) and meat handlers should follow good personal hygiene practices (e.g. clean hands, clean protective clothing). In general, hands should be regularly washed while handling raw meat.

Pork from healthy pigs is safe to eat as long as either the fresh meat is cooked properly (70˚C in all parts of the food, so that there is no pink meat and the juices run clear), or, in the case of uncooked processed pork, national safety standards have been met during production, processing and distribution. Meat from sick pigs or pigs found dead should not be eaten and should not enter the food chain or be given to other animals.