May 13, 2011 (CIDRAP News) – Canadian researchers have shown that an Ebola virus species that can kill humans can also infect pigs and spread among them, raising the specter of Ebola virus as a potential foodborne pathogen.
The researchers infected two groups of pigs with a high dose of the Zaire Ebola virus ("ZEBOV"), a strain that is up to 90% fatal in humans, according to a report published in the Journal of Infectious Diseases. The pigs became ill to varying degrees, with severe signs in some cases, and they transmitted the virus to other pigs that were previously unexposed.
A US expert who wrote a commentary on the findings said the probability of Ebola virus becoming a foodborne pathogen is small but can't be dismissed for now.
Pigs and Ebola have been linked once before, notes Daniel G. Bausch of the Tulane School of Public Health and Tropical Medicine in the commentary. In the Philippines in 2008, pigs on two farms were found to be infected with Reston Ebola, or "REBOV," a strain that does not sicken humans. That strain was originally identified in monkeys at a breeding facility in the Philippines. The Philippines pigs had a respiratory illness, but it was not clear if the disease was due to Ebola or to another virus the pigs were infected with, Bausch explains.
Severely sick pigs
In the new study, researchers from the Public Health Agency of Canada and the Canadian Food Inspection Agency used two groups of young pigs confined in high-containment cubicles. In the first experiment, six pigs were inoculated with ZEBOV in their nostrils, eyes, and mouths. Two other pigs, housed separately, were inoculated with saline solution as a control.
In a second experiment, three pigs were inoculated with the virus in the same way as the first group. A day later, four other pigs were put in the same cubicle with them to see if the virus would spread.
All the inoculated pigs became ill with a fever within 4 days, had labored breathing, and stopped eating, among other signs, according to the report. The clinical course of disease was less severe in the transmission experiment. In general, however, the virus reproduced abundantly, mainly in the pigs' respiratory tract, causing severe lung disease as shown on necropsy.
The pigs shed the virus from their oral and nasal mucosa for up to 14 days after infection, and in the transmission experiment, all the pigs placed with the inoculated pigs became infected.
The study "demonstrates that domesticated pigs are susceptible to ZEBOV infection following mucosal challenge and develop a respiratory disease with a clinical manifestation that can be severe," the report says. "This observation raises the possibility that pigs are capable of shedding relatively high viral loads into the environment."
The researchers comment that the disease in the pigs was primarily respiratory and therefore could be mistaken for other porcine respiratory diseases. In contrast, Ebola fever in primates typically affects multiple organs, they said.
From pigs to humans?
In the commentary, Bausch observes that in the case of the Philippines pigs infected with REBOV, six farm workers had serologic evidence of past infection with that virus. That finding suggests that pigs could serve as "accidental hosts" and transmit REBOV or ZEBOV to humans, he says.
But he adds that among more than 800 people exposed to the Reston strain, no one has been known to get sick, and there has been no sign of pigs playing a role in any Ebola outbreak so far.
As for the possibility of risk from Ebola-tainted meat, Bausch notes that ZEBOV transmission in Africa has been linked to the hunting and consumption of "bushmeat" (nonhuman primates as food). But infection in people who had contact only with meat procured by others has not been reported, and there is no risk from eating cooked food.
These facts "should assuage any sense of panic regarding EBOV [Ebola virus] as a potential foodborne pathogen, but the possibility cannot yet be dismissed," Bausch writes. One concern is that REBOV, though apparently harmless for healthy adult males, might be pathogenic for more vulnerable groups such as the elderly or immunocompromised people.
Bausch also comments that the susceptibility of pigs to Ebola also could be seen as an opportunity for bioterrorists wanting to disrupt the food supply or infect humans.
He concludes that it would be premature to restrict the movement of pigs as an Ebola precaution, but "targeting exporters in endemic and high-risk areas could be warranted if continued surveillance indicates a risk."
Kobinger GP, Leung A, Neufeld J, et al. Replication, pathogenicity, shedding, and transmission of Zaire ebolavirus in pigs. J Infect Dis 2011; early online publication May 12 [Abstract]
Bausch DG. Ebola virus has a foodborne pathogen? Cause for consideration, not panic. J Infect Dis 2011; early online publication May 12 [Full text]
May 13 Infectious Diseases Society of America news release