Pigs can get Ebola, but do they spread it?
KAMPALA, Uganda; NAIROBI, Kenya––Domestic pigs may already be among the vectors transmitting deadly Ebola viruses to humans, though no one knows for sure yet, warns a five-member International Livestock Research Institute team in a soon-to-be-published edition of the peer-reviewed scientific journal Transboundary and Emerging Diseases.
The International Livestock Research Institute is a multinational nonprofit organization that promotes agribusiness in the developing world. Despite the potentially scary implications for the pig industry of a positive finding of Ebola among domestic pigs, the institute favors and encourages the growth of pig farming in Africa.
A pre-publication edition of the International Livestock Research Institute scientists’ paper, “Assessing the Potential Role of Pigs in the Epidemiology of Ebola Virus in Uganda,” has been accessible online since August 27, 2015.
Working from International Livestock Research Institute facilities in Kampala, Uganda, and Nairobi, Kenya, veterinarian Christine Atherstone and colleagues C. Smith, P. Ochungo, K. Roesel, and D. Grace reviewed the available data from four Ebola outbreaks occurring in Uganda since the Ebola virus family was identified in 1976.
“Pigs are hosts for ebolaviruses”
“Recent epidemiological work has shown pigs are hosts for ebolaviruses,” Atherstone et al explained. Coinciding with the emergence of Ebola diseases, the team observed, “pig production in Uganda has undergone massive expansion. The combination of pork sector growth supported by development programs and Ebola virus risk prompted a foresight exercise,” which “warrants further research into potential zoonotic transmission in Uganda from pigs.”
To ANIMALS 24-7, Atherstone emphasized “the need to determine the actual risk of the African species of Ebola virus in pigs, particularly in areas where Ebola outbreaks have happened and where ecological niche modeling have identified as suitable for Ebola emergence.”
But are pigs actually infected?
“At present, we have no evidence that pigs are infected with Ebola in Uganda,” Atherstone continued: the confluence of circumstances suggesting that Ebola might occur there among pigs is an evidentiary step short of an actual positive finding of the virus or antibodies to it.
“Field work has begun to determine if pigs are infected and if there is evidence of transmission from pigs,” Atherstone said. “The goal of this, and other zoonotic disease work in pigs the International Livestock Research Institute is conducting in Uganda, is to help determine the country’s disease risk and the best measures for protecting Uganda’s public health and pig industry.
“Given the recent good news of an Ebola vaccine that is 100% effective, new tools to prevent and control Ebola are showing promise for the current outbreak in West Africa and future outbreaks,” Atherstone projected.
Optimism about new vaccine may be premature
But “the recent good news of an Ebola vaccine that is 100% effective” that Atherstone mentioned may not be quite what it seems, Wired magazine science writer Katie M. Palmer cautioned on August 4, 2015.
Further, the preliminary findings about the rVSV-ZEBOV vaccine published in the British medical journal The Lancet pertain to the development of a post-exposure vaccine for emergency administration to humans at potential great risk. These findings may not translate easily into the mass production of a prophylactic vaccine for pigs.
The number 100% “is why Doctors Without Borders is recommending distribution of the vaccine begin as soon as possible in the west African countries where Ebola is still killing people,” Palmer wrote. But, Palmer added, it is “based on incomplete data, so it doesn’t have the statistical clout it should.”
Reviewing the scientific, regulatory, and humanitarian considerations involved, Palmer concluded that while a vaccine reliable enough help slow the spread of the already waning 2014-2015 Ebola outbreak may have been developed, it is far from ready for deployment to populations who are not already in apparent imminent danger from exposures that have already occurred.
Accordingly, an actual finding of Ebola in pigs, if there is one, may not come at a point in the transmission and spread of the disease that will permit effective control before it spreads out of West and Central Africa.
“In case––following further required studies––pigs become implicated in the transmission of an African species of Ebola virus to humans, development of veterinary vaccines might need to be considered,” offered Program for Monitoring Emerging Diseases [ProMED-mail] animal disease and zoonoses moderator Arnon Shimshony.
Obvious though that may seem, relatively little has been done to develop veterinary vaccines against Ebola viruses amid the rush to try to prevent human deaths. What has been done has been done mainly in non-human primates, with the goal of protecting humans.
University of Cambridge researcher Peter Walsh has conducted vaccine trials in macaques and chimpanzees, hoping to find a way to protect wild chimps and western lowland gorillas.
But, lamented the September 2, 2015 edition of New Scientist, “An animal-welfare victory in the U.S. may prove to be a conservation catastrophe in Africa. Tests of a promising oral Ebola vaccine that could protect wild apes may be abandoned this month [September 2015] when a ban on the use of chimpanzees in biomedical research comes into force…For ethical reasons, chimps cannot be exposed to the virus to test vaccines,” New Scientist continued.
“However, human trials of Ebola vaccines being carried out in West Africa mean researchers can now compare antibody levels in vaccinated chimps with those known to protect humans,” New Scientist finished, meaning that experiments on humans may in this instance benefit non-humans.
Pigs as virus mixers
Meanwhile, though caution is always in order in identifying zoonotic disease vectors, the investigations by Atherstone et al address the risk that pigs could have a role in the spread of Ebola viruses comparable to their role in transmitting influenzas from wild waterfowl to humans.
Most notoriously, influenzas spreading from birds to pigs and then mutating to attack humans in 1918, 1957, and 1968 killed tens of millions of people, chiefly in Asia.
Usually, for thousands of years, avian influenzas shed among pigs did not mutate in a manner severely harmless to humans, or other mammals, including pigs themselves. But when the conditions were conducive to the spread of deadly mutations, disaster followed.
Symptoms in pigs often not obvious
As in the examples of swine flu outbreaks spreading to humans, diseases spreading from pigs to people often do not produce immediately obvious symptoms or excessive mortality among the porcine victims.
Viruses may incubate, mutate, and be shed and spread for months within pig barns, where the background mortality rate is often high for a variety of reasons, or among wild pig populations, where mortality is usually unobserved, before humans become aware that a disease outbreak is underway.
By the time medical investigators backtrack a human disease outbreak to pigs, both humans and pigs may be infected worldwide, and the infections may have passed back and forth between wild and domestic pigs several times.
Porcine epidemic diarrhea
The two most recent examples of previously little known pig diseases becoming pandemic in multi-national and intercontinental contexts involved non-zoonotic infections, meaning that they do not attack humans.
Porcine epidemic diarrhea, a coronavirus transmitted mainly through contact with feces, has raged across the U.S., Canada, and parts of Asia since mid-2013, killing more than seven million piglets in the U.S. alone.
Cases have also occurred in at least twelve European nations, including Ukraine, Germany, Spain, Italy, France, the Netherlands, Belgium, Portugal, Romania, Latvia, Lithuania, and Estonia.
African swine fever
African swine fever is a tick-borne virus which in the wild afflicts warthogs and bushpigs without causing visible symptoms. In domestic pigs African swine fever emerges as a hemorrhagic fever producing symptoms, ending in death, which might be confused with the symptoms of Ebola viruses.
“African swine fever spread from Africa to Portugal in 1957 as a result of waste from airline flights being fed to pigs near Lisbon,” recalled ProMed-mail moderator Arnon Shimshony in 2014. “Although this incursion of the disease was eradicated, a further outbreak occurred in 1960 in Lisbon, and African swine fever remained endemic on the Iberian peninsula until the mid-1990s.”
Further outbreaks hit France, Italy, Malta, Belgium, and the Netherlands. “The disease was eradicated from each of these countries,” Shimshony said, “but in Sardinia it has remained endemic since in 1982.”
African swine fever jumped to Brazil in 1978, but was eradicated by culling after 224 reported outbreaks.
Endemic in eastern Europe
That was all relatively close to the beginning of international jet air travel, high-volume intensive confinement pig farming, long-haul truck transport of pigs to slaughter, and the recovery of European wild boars from the habitat destruction of two world wars in the preceding human generation.
The next time African swine fever reached Europe, it appears to have become endemic before anyone knew it was there, both among wild boars and among large domestic pig herds raised indoors with minimal human contact in the former Soviet Union. As many as half a million domestic pigs have been culled since 2007 in response to more than 600 outbreaks of African swine fever in Russia, Ukraine, Belarus, Lithuania, and Poland, with little hint so far of containing it.
Time & place
In Uganda, Atherstone and team found reason for concern about pigs as a possible Ebola vector in “the lack of serological evidence for specific reservoir species, the number of human index cases unable to account for their source of infection, domestic pig habitat overlap with potential Ebola virus zoonotic host environments, reported interactions at the human-pig-wildlife interface that could support transmission, fever in pigs as a commonly reported problem by pig farmers, and temporal correlation of outbreaks with peak pork consumption periods.”
In other words, molecular evidence has not yet definitively established how Ebola viruses reach humans, but coincidences of place and time of infection hint that pigs may be involved.
Poverty, hunger, livestock
“Over the past three decades, the reported pig population in Uganda has increased 1500%, from 0.19 to 3.2 million in Uganda,” Atherstone et al wrote. “In 2011, Uganda had the highest per capita consumption of pork in East Africa at three-quarters of a kilogram per person per year. More than 1.1 million poor households in Uganda own pigs, mostly managed by women and children in backyard activities. Indeed, 80% of pig production in Uganda is carried out by smallholder crop-livestock farmers. There is a strong association between poverty, hunger, livestock keeping and zoonoses.”
Of the five currently identified Ebolavirus variants––Zaire, Sudan, Tai forest, Bundibugyo, and Reston––Atherstone et al observed that “Reston is considered nonpathogenic to humans but has been found to infect pigs (in the Philippines),” while “Most of the outbreaks so far, including the current major one, have been caused by Zaire.”
Ebola deaths up to 15,000
The current Ebola outbreak has killed more than 15,000 people in six African nations since March 2014, according to updated World Health Organization data released on September 2, 2015.
“While fruit bats are considered a natural reservoir [for Ebola viruses], the involvement of other species in the transmission cycle is unclear, especially for domesticated animals,” Shimshony wrote, introducing the findings by Atherstone et al to the ProMED audience of more than 60,000 public health and zoonotic disease professionals.
“Dogs and pigs are so far the only domestic animals identified as species that can be infected. While infections in dogs appear to be asymptomatic,” Shimshony said, “pigs experimentally infected with Ebola viruses can develop clinical disease, depending on the virus species and possibly the age of the infected animals.
“In experimental settings,” Shimshony continued, “pigs can transmit Zaire Ebola virus to naive pigs [i.e. pigs without previous exposure] and macaques.”
Atherstone et al recommended research into what role pigs might play in Ebola virus transmission; pig population dynamics as Ebola virus hosts; the risk factors for pig farming in proximity to wildlife known to carry Ebola viruses [bats and nonhuman primates]; the potential impact on pig production, human health, livelihoods and food security; disease course and outcome of the different Ebola virus infections in pigs; and how to share information about any risk that Ebola virus infections associated with pig production may present, “in ways that minimize adverse impacts on pig value chains, poverty, and livelihood.”
The U.S. Department of Agriculture currently does not allow any African nation to export any meat products to the U.S., either raw or processed. While this appears to have helped to keep African swine fever out of the U.S. (and Canada), this and other wildlife diseases, perhaps including Ebola viruses, could reach the U.S. through the traffic in bootlegged bushmeat.
“For years now,” wrote Gerard Flynn in the August 21, 2014 edition of Newsweek, “U.S. health officials and legislators have been expressing concern over the steady flow of bushmeat illegally imported into the country. Internal documents show that from 2009 to 2013, the U.S. Customs & Border Protection agency confiscated over 69,000 different bushmeat items, with seizures ranging from dried bat to smoked monkey. And that’s likely a mere sliver compared with what actually gets into the U.S. At least one estimate puts the number at 15,000 pounds every month.”
Pig disease control
African swine fever is known to currently occur, or have recently occurred, in Benin, Cameroon, Central African Republic, Chad, Kenya, Malawi, South Africa, Tanzania, and Zambia. The involved nations partially overlap the list of nations known to have had recent human Ebola virus cases.
Recommended ProMED-mail animal health liaison moderator Cristina Ramirez David on July 29, 2015, “In endemic areas of Africa, the most important factors to help control the spread of African swine fever is to control the natural tick vectors and wild pig reservoirs and limit their contact with domestic pigs.
“Rapid & efficient slaughtering”
“During disease outbreaks, the rapid and efficient slaughtering of all pigs and the proper disposal of carcasses and all waste material are critical,” Ramirez David said.
“In addition, cleaning and disinfecting affected farms, designating infected areas, conducting surveillance, and increasing the control of animal movements will help control spread. In the presence of any suspicious hemorrhagic pig disease, a differential laboratory diagnosis should be undertaken, as low virulent African swine fever strains do not produce significant lesions. Rapid communication of an outbreak is key to helping prevent the spread of the disease.”
The same or similar measures might help to prevent or limit the spread of Ebola viruses among pigs, and from pigs to humans.
Approaches have failed
Yet comparable approaches have failed thus far to prevent the spread of African swine fever in eastern Europe, failed to stop the spread of porcine epidemic diarrhea, and have not managed to eradicate foot-and-mouth disease from South Korea, located on a peninsula with relatively little possibility of the disease re-infecting the local pig population from abroad, if it ever was extirpated in the first place.
Only intensive vaccination has had significant success in quelling the spread of highly infectious pig diseases, but as yet there is no vaccination to stop either Ebola viruses or African swine fever. The current global population of about 980 million pigs on farms at any given time, along with the several million wild boar in Europe and feral pigs now ubiquitous in much of the U.S., are protected from these and many other highly contagious diseases chiefly by isolation and distance.
Pig diseases have jumped oceans
More than half of the pigs in the world are in Asia, chiefly China. About half the remainder are in the U.S. and Canada, far from the present sources of Ebola and African swine fever.
But the spread of the Pan-Asian foot-and-mouth disease strain from India east to Taiwan in 1999 and west to the Middle East and Europe by 2001 demonstrated how readily and seemingly unstoppably even a well-known pig disease can jump from continent to continent.
Once a highly contagious pig disease is no longer distant from intensive confinement farms holding thousands or even tens of thousands of pigs, the only protection the pigs have from diseases for which there are no vaccines is the “bio-security envelope” of their own barns. Wild and feral pig populations not only may have no protection at all, but may become local reservoirs keeping the new pathogen in the vicinity of pig barns, awaiting any bio-security breach.
Wild & feral pig reservoirs
In parts of eastern Europe, for example, African swine fever appears to have become endemic among wild boar. Pig barns become infected through wild boar feces contaminating feedstuffs, being accidentally tracked in by workers, or through workers’ contact with boar while engaging in sport hunting.
Trying to exterminate problematic wild and feral pigs is often recommended, but has rarely if ever been successful.
Offered David Thomson, general manager of operations at the National Agricultural Quarantine Inspection Authority in Papua New Guinea, via ProMED-mail in 2014, “Reduction or removal of susceptible feral or wild animal populations for disease control purposes is never, ever, as simple as it appears on paper.”
Thomson warned of the risk that trying to cull wild boars might “foster spread of disease as animals respond to the pressures via movement.”
There is no documented instance of any agency anywhere ever succeeding in even temporarily eradicating 90% of any sort of wild or feral pig population from a mainland habitat. Many U.S. states have attempted feral pig eradication for decades, yet no state with feral pigs has achieved a net reduction.
None of this bodes well for containing either African swine fever or Ebola virus in pigs, if either disease ever jumps to the Americas.