Scientists Puzzle Over Ebola-Reston Virus in Pigs

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Science  23 Jan 2009:
Vol. 323, Issue 5913, pp. 451
DOI: 10.1126/science.323.5913.451a

An international team of human- and animal-health experts is in the Philippines this month, studying the first known outbreak of Ebola-Reston virus in pigs. The virus, which is related to the Ebola virus that causes the highly fatal Ebola hemorrhagic fever, had previously been found only in monkeys and a few humans who had been in contact with the sick animals. It has not caused any known incidents of serious illness or death in humans. But experts are concerned “because this is new, because it is unexpected, because the virus is slightly different [from previous isolates], and because it is in pigs,” which live in close proximity to humans, says Julie Hall, an infectious disease expert for the World Health Organization (WHO) and a member of the investigative team.

“The finding is cause for further study but not further alarm,” says Stuart Nichol, a virologist at the U.S. Centers for Disease Control and Prevention in Atlanta. He says ongoing investigations may lead to a better understanding of Ebola viruses.

Ebola viruses belong to the Filoviridae family and come in five strains: Zaïre, Sudan, Côte d'Ivoire, Bundibugyo, and Reston. The Zaïre, Sudan, and Bundibugyo strains have caused outbreaks of Ebola hemorrhagic fever among humans in Africa, killing up to 90% of those infected. Ebola- Reston was first isolated in 1989 from cynomolgus macaques imported from the Philippines for medical research in the United States. Unusual numbers of the monkeys started dying while in a quarantine facility in Reston, Virginia. About 1000 monkeys died or were euthanized. Subsequently, 21 animal handlers at the Philippine exporter and four employees of the quarantine facility were found to have antibodies to the virus, indicating that they had been infected, but just one reported flulike symptoms. Further outbreaks in monkeys in the Philippines were reported in 1992 and 1996.

An increase in pig mortality on several farms in central Luzon, the Philippines' largest island, in 2007 and 2008 prompted an investigation by Philippine agencies. Last October, international reference laboratories studying samples supplied by the Philippines confirmed that the pigs were infected with a highly virulent strain of porcine reproductive and respiratory syndrome as well as the Ebola-Reston virus. Which virus is responsible for the increased mortality is not yet clear.

The presence of Ebola-Reston virus on pig farms increases the odds of human exposure and infection. Previous human infections occurred in young men, who happened to be employees at both the Philippine exporter and the Reston lab animal supplier, Hall says. “We now have that virus in pigs that live in very close contact not just with fit, healthy, young men, but with pregnant women, children, and people with underlying medical conditions,” Hall says. Initial laboratory tests on animal handlers and slaughterhouse workers who might have been exposed were negative, the Philippine Department of Health has reported.


A Philippines outbreak shows that pigs may host Ebola viruses.


At the request of the Philippine government, WHO, the United Nations Food and Agriculture Organization, and the Paris-based World Organisation for Animal Health assembled an 18-member team that began its 10-day investigation on 6 January. So far, they have more questions than answers. It is not clear whether the virus alone causes clinical illness in pigs, how easily it spreads among the animals, or how it invaded the separate farms. The implications of the slight genetic differences in this strain are also not known.

Answers to some of these questions should trickle in over the next several weeks. The international agencies and their local counterparts are planning further studies to determine, among other issues, whether the virus is in wider circulation in pigs and what its natural habitat might be. Meanwhile, the government is being cautious, quarantining the affected farms, even though there are no longer signs of illness among their pigs, and suspending all pork exports.

Gary Kobinger, a virologist at Canada's National Microbiology Laboratory in Winnipeg, says there have long been rumors of unusual die-offs of pigs before Ebola outbreaks among humans in Africa. “The question is: Is it possible that pigs are hosts that amplify and transmit the virus to other animals and humans?” he asks.

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