Emerging viruses that spread to humans from an animal host are commonplace and represent some of the deadliest diseases known. Given the details of the Wuhan coronavirus (2019-nCoV) outbreak, including the genetic profile of the disease agent, the hypothesis of a snake origin was the first raised in the peer-reviewed literature.
SISTEMA 12/Wikimedia Commons/CC BY-SA 4.0
Wuhan seafood market closed after the new coronavirus was detected there for the first time in 2020.
It is a highly controversial origin story, however, given that mammals have been the sources of all other such zoonotic coronaviruses, as well as a host of other zoonotic diseases.
An animal source for emerging infections such as the 2019-nCoV is the default hypothesis, because “around 60% of all infectious diseases in humans are zoonotic, as are 75% of all emerging infectious diseases,” according to a United Nations report. The report goes on to say that, “on average, one new infectious disease emerges in humans every 4 months.”
To appreciate the emergence and nature of 2019-nCoV, it is important to examine the history of zoonotic outbreaks of other such diseases, especially with regard to the “mixing-vessel” phenomenon, which has been noted in closely related coronaviruses, including SARS and MERS, as well as the widely disparate HIV, Ebola, and influenza viruses.
Mutants in the mixing vessel
The mixing-vessel phenomenon is conceptually easy but molecularly complex. A single animal is coinfected with two related viruses; the virus genomes recombine together (virus “sex”) in that animal to form a new variant of virus. Such new mutant viruses can be more or less infective, more or less deadly, and more or less able to jump the species or even genus barrier. An emerging viral zoonosis can occur when a human being is exposed to one of these new viruses (either from the origin species or another species intermediate) that is capable of also infecting a human cell. Such exposure can occur from close proximity to animal waste or body fluids, as in the farm environment, or from wildlife pets or the capturing and slaughtering of wildlife for food, as is proposed in the case of the Wuhan seafood market scenario. In fact, the scientists who postulated a snake intermediary as the potential mixing vessel also stated that 2019‐nCoV appears to be a recombinant virus between a bat coronavirus and an origin‐unknown coronavirus.
Coronaviruses in particular have a history of moving from animal to human hosts (and even back again), and their detailed genetic pattern and taxonomy can reveal the animal origin of these diseases.
Going batty
Bats, in particular, have been shown to be a reservoir species for both alphacoronaviruses and betacoronaviruses. Given their ecology and behavior, they have been found to play a key role in transmitting coronaviruses between species. A highly pertinent example of this is the SARS coronavirus, which was shown to have likely originated in Chinese horseshoe bats. The SARS virus, which is genetically closely related to the new Wuhan coronavirus, first infected humans in the Guangdong province of southern China in 2002.