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Ebola Risk For U.S. Hog Herd 'Extremely Low' -- Vet

Just hearing or seeing the word "Ebola" these days can send shockwaves of fear through the population, regardless of how rational or irrational that fear may be. The disease has dominated the evening news and has many wondering, "What if it starts spreading here?"

These concerns have another facet altogether for those in the agriculture industry, specifically those with livestock. What if the hog herd gets infected? “Swine flu” and PEDv affected markets in the US… why wouldn't Ebola do the same?

While there's a remote chance that Ebola could work its way into the U.S. hog herd, the likelihood is "extremely low," one senior veterinary clinician says. Because there are different Ebola virus strains, generalizations are often inaccurate. The strain of Ebola virus hitting West African nations today has not been reported to naturally infect domestic swine.

Both humans and hogs are susceptible to Ebola virus infections, but disease expression is quite variable, depending on the strain of the virus that is currently classified in the family "Filioviridae." Six years ago, a strain of Ebola virus (Ebola-Reston) was found in hogs suffering from highly pathogenic PRRSV and PCV2 in the Philippines. This is an entirely different strain from the Ebola-Zaire strain, or Ebola hemorrhagic fever, hitting human hosts in Africa right now. The major difference is that the Ebola-Reston strain infects but does not cause severe disease in people or pigs. The Ebola-Zaire strain, on the other hand, has a mortality rate of up to 90% in humans, according to the World Health Organization (WHO).

The distinction between the two diseases is important to our understanding of the likelihood that an Ebola virus could enter the hog production system, and what impact it may have in the U.S. Ebola-Zaire -- causing severe disease in West Africa that could approach 1.4 million people by year’s end, according to the U.S. Centers for Disease Control (CDC) reports -- has not been found in pigs by natural infection. However, experimental infection of pigs with Ebola-Zaire has confirmed that pigs can become infected, sicken, and transmit the virus to other pigs or primate monkeys (macaques) in the same room. In contrast, Ebola-Reston can infect pigs and humans, is zoonotic, but does not cause symptoms of disease in either species. Indeed, Ebola-Reston does circulate in pigs in some Asian countries with no apparent disease in pig or human populations.

Currently, the likelihood that pigs in the U.S. will become infected with Ebola-Zaire is "extremely low," says Kent Schwartz, DVM, senior clinician of Iowa State University College of Veterinary Medicine Veterinary Diagnostic and Production Animal Medicine. The risk is low because of the low prevalence of infection in humans in the U.S. (less than 5 cases in over 300 million people), the low probability that a sick infected human shedding the virus would have access to a swine population, along with a host of other factors that would have to occur.  However, this reinforces the concepts we learned with “swine flu," that there is wisdom of in restricting the access of sick humans to swine populations.

"In general, various exotic or transboundary viruses reported or emerging in different animal populations around the world are on the radar of diagnostic laboratories, government agencies, animal industry groups, and swine veterinarians. Do we have a test in place today to detect each one of these agents distributed in all laboratories throughout the U.S.? That answer is no. But with today's communication capabilities and diagnostic technologies, such as PCR, sequencing, and the ability to do comparisons to known virus sequences, it wouldn't take long to confirm the presence of a new virus causing disease," Schwartz says. "The ability to diagnose a new introduction of a known pathogen like Ebola virus is pretty quick with the communication that exists within the swine industry and the resources within full-service veterinary diagnostic laboratories.”

The distinction between Asian Ebola-Reston strains and African Ebolavirus-Zaire strain is important to painting and accurate picture of the potential for widespread infection in the U.S. In the 2008 outbreak of Ebola-Reston in the Philippines, Schwartz says other factors were present that facilitated the spread of infection. "The Philippine virus, Ebola-Reston, was discovered almost by accident in pigs suffering from highly pathogenic PRRS and PCV2. When that Ebola-Reston virus was inoculated in pigs by itself, no disease symptoms occurred. During that period in the Philippines, it was also found that humans taking care of those pigs had antibodies for Ebola-Reston but they had remained free of symptoms of an Ebola infection, Schwartz says, adding, "With Ebola-Reston, disease symptoms are not (or rarely) expressed in either pigs or humans in Asia – that is, Ebola-Reston infections are asymptomatic in pigs and people. Transmission between pigs and humans can and does occur, however.”

In contrast, pigs have not been found naturally infected with Ebola-Zaire, but there is still cause for concern with this virus. Experimental inoculation of pigs with Ebola-Zaire in a very secure laboratory setting did result in infection and disease in inoculated pigs. In addition, other pigs in contact with the inoculated pigs became infected and shed virus; however those pigs in contact did not develop disease symptoms. In a subsequent similar study, nonhuman primates (macaques) in the same airspace with inoculated pigs became infected, so transmission from pigs to primates has been documented experimentally with Ebola-Zaire.

With the recent porcine epidemic diarrhea virus (PEDv) outbreak in the U.S., it may be natural to draw comparisons with Ebola. But, the differences between the two are immense, Schwartz says.

"With PED, diarrhea is readily apparent within 24-48 hours, which is a very short incubation period. The epidemic occurred in the U.S. because pigs were naïve to PED, the virus was very hardy and was shed in high numbers and therefore transmitted easily. For Ebola to enter a pig population, it probably would require a person with symptoms of Ebola to be in direct contact with pigs. It would be the people handling the pigs that would tip us off," he says. "There would first need to be transmission from people to pigs, which means the number of human cases of Ebola (the human disease prevalence) would have to increase dramatically to achieve a broad distribution in our agricultural community, to transmit to pigs, and create the potential for pigs to act as a source of infection. The chances of that happening are pretty remote.”

Even if Ebola-Reston or a similar non-pathogenic Filovirus was to infect the U.S. hog herd, recent history and the latest clinical data show the risk to humans to be low. Compared to PEDv, the epidemiology of Ebola is such that it will be difficult for Ebola to transmit through pig populations on a large scale.

"Is Ebola-Zaire a risk to the swine industry? The risk in my mind is very low. More concerning is people spreading it to people, not people spreading it to pigs or pigs to people. The locations of infected humans, the pathophysiology of Ebola infections in pigs, and overall epidemiology considerations of these Ebola viruses varies tremendously," Schwartz says. "In Africa, Ebola virus apparently jumps from wild animals to humans by direct contact of butchering and eating infected animals -- so if you're going to be butchering that animal and it still happens to be carrying virus asymptomatically -- there will be a risk. But, the likelihood is extremely remote that it will ever get that far in the U.S."

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