MERS-CoV: A working theory for emergence from animals to man

January 12th, 2015

Camels may play an important role as an intermediate or “bridge” host for MERS-CoV, allowing the novel corona virus to move from a suspected bat reservoir to humans. As of Aug. 9, 2013, the virus had infected 94 people and killed 46 of them since beginning its sporadic emergence last year in the Middle East.

As described in a new Lancet study, researchers found antibodies for MERS CoV in camels in Oman, an Arab state on the southeast coast of the Arabian Peninsula. In a blood culture study, they found that 50 of 50 (100%) sera from Omani camels and 15 of 105 (14%) from Spanish camels had protein-specific antibodies against MERS-CoV. Though no MERS virus was detected, the presence of antibodies suggests the animals were able to mount an immune response and survive MERS infection. Camel meat serves as a source of protein and the Bedouins and other nomadic tribes have consumed camel milk for centuries. Moreover, camels are imported in the Middle East from Australia and Africa; with the southern regions of the latter continent being the home of a bat species (Neoromicia cf. zuluensis) that carries a virus that closely matches MERS, another group of researchers found.

Thus we now have a working theory for a classic progression of an emerging virus from an animal reservoir (African bat), to an intermediate host (exported African camel), and then directly into the human bloodstream. There have been a few thinly described encounters between camels in some of the human MERS cases, including one account of a man who treated a sick racing camel (which recovered) and another report of a man who “attended the slaughtering of a camel on October 24, 2012." Though the article gives no additional detail, camels are among the animals ritually sacrificed in the Islamic religious observation of Eid Al-Adha or the "Festival of Sacrifice."

It is certainly conceivable that killing and butchering camels could have allowed MERS to cross species lines, much like HIV moved from chimpanzees to humans during the slaughter and consumption of bush meat. However, camel exposures have not been described in the majority of MERS CoV cases, suggesting that other animal reservoirs might be contributing to the case count along with the limited episodes of clear human-to-human transmission. No MERS antibodies were found in European sheep, goats, and cattle tested in the aforementioned study.

In a recent interview with Nathan Wolfe, PhD, founder of the Global Viral Forecasting Initiative, we asked about the importance of detecting animal reservoirs of emerging viruses.

“It is very important because if we really understand the animal reservoirs and the nature of how these viruses are jumping into human populations, we are going to be much more likely to be able to catch future events, which is of course the Holy Grail for our field,” Wolfe noted.

That said, the author of The Viral Storm warned that viruses may continue to mutate and change even as they continue emerging from animals to man.

“You can have a situation where an agent crosses from an animal to human populations -- starts spreading within the human population -- and yet new strains, new genetic diversity might continue to come from animal populations to enrich the pool,” he said. “These viruses can effectively mate with each other and develop new variants, and that diversity ends up being quite critical.”

Camels have been suspected previously as an intermediate viral host, possibly allowing smallpox to move from rodents to humans.

“It’s interesting that we are really not 100% certain of the origins of smallpox, which is arguably the most important historical infection of human populations,” Wolfe said. “We simply don’t really know where it came from. The suspicion – because there is so much diversity in these viruses in rodents -- is that perhaps [smallpox] is a rodent virus. Once a virus has adapted to a mammalian system its probability of jumping into humans is greater. So domesticated animals can act as bridges and you can have pathogens that infect multiple animal hosts. These things really don’t respect species boundaries that well, or political boundaries for that matter.”