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.

Wuhan seafood market closed after the new coronavirus was detected there for the first time in 2020. SISTEMA 12/Wikimedia Commons/CC BY-SA 4.0

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.

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 2019nCoV appears to be a recombinant virus between a bat coronavirus and an originunknown 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.

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.

Scientists speculate that the virus was then either transmitted directly to humans from bats, or passed through an intermediate host species, with SARS-like viruses isolated from Himalayan palm civets found in a live-animal market in Guangdong. The virus infection was also detected in other animals (including a raccoon dog, Nyctereutes procyonoides) and in humans working at the market.

The MERS coronavirus is a betacoronavirus that was first reported in Saudi Arabia in 2012. It turned out to be far more deadly than either SARS or the Wuhan virus (at least as far as current estimates of the new coronaviruss behavior). The MERS genotype was found to be closely related to MERS-like viruses in bats in Saudi Arabia, Africa, Europe, and Asia. Studies done on the cell receptor for MERS showed an apparently conserved viral receptor in both bats and humans. And an identical strain of MERS was found in bats in a nearby cave and near the workplace of the first known human patient.

However, in many of the other locations of the outbreak in the Middle East, there appeared to be limited contact between bats and humans, so scientists looked for another vector species, perhaps one that was acting as an intermediate. A high seroprevalence of MERS-CoV or a closely related virus was found in camels across the Arabian Peninsula and parts of eastern and northern Africa, while tests for MERS antibodies were negative in the most-likely other species of livestock or pet animals, including chickens, cows, goats, horses, and sheep.

In addition, the MERS-related CoV carried by camels was genetically highly similar to that detected in humans, as demonstrated in one particular outbreak on a farm in Qatar where the genetic sequences of MERS-CoV in the nasal swabs from 3 of 14 seropositive camels were similar to those of 2 human cases on the same farm. Similar genomic results were found in MERS-CoV from nasal swabs from camels in Saudi Arabia.

HIV, the viral cause of AIDS, provides an almost-textbook origin story of the rise of a zoonotic supervillain. The virus was genetically traced to have a chimpanzee-to-human origin, but it was found to be more complicated than that. The virus first emerged in the 1920s in Africa in what is now the Democratic Republic of the Congo, well before its rise to a global pandemic in the 1980s.

Researchers believe the chimpanzee virus is a hybrid of the simian immunodeficiency viruses (SIVs) naturally infecting two different monkey species: the red-capped mangabey (Cercocebus torquatus) and the greater spot-nosed monkey (Cercopithecus nictitans). Chimpanzees kill and eat monkeys, which is likely how they acquired the monkey viruses. The viruses hybridized in a chimpanzee; the hybrid virus then spread through the chimpanzee population and was later transmitted to humans who captured and slaughtered chimps for meat (becoming exposed to their blood). This was the most likely origin of HIV-1.

HIV-1 also shows one of the major risks of zoonotic infections. They can continue to mutate in its human host, increasing the risk of greater virulence, but also interfering with the production of a universally effective vaccine. Since its transmission to humans, for example, many subtypes of the HIV-1 strain have developed, with genetic differences even in the same subtypes found to be up to 20%.

Ebolavirus, first detected in 1976, is another case of bats being the potential culprit. Genetic analysis has shown that African fruit bats are likely involved in the spread of the virus and may be its reservoir host. Further evidence of this was found in the most recent human-infecting Bombali variant of the virus, which was identified in samples from bats collected from Sierra Leone.

It was also found that pigs can also become infected with Zaire ebolavirus, leading to the fear that pigs could serve as a mixing vessel for it and other filoviruses. Pigs have their own forms of Ebola-like disease viruses, which are not currently transmissible to humans, but could provide a potential mixing-vessel reservoir.

The Western world has been most affected by these highly mutable, multispecies zoonotic viruses. The 1957 and 1968 flu pandemics contained a mixture of gene segments from human and avian influenza viruses. "What is clear from genetic analysis of the viruses that caused these past pandemics is that reassortment (gene swapping) occurred to produce novel influenza viruses that caused the pandemics. In both of these cases, the new viruses that emerged showed major differences from the parent viruses," according to the Centers for Disease Control and Prevention.

Influenza is, however, a good example that all zoonoses are not the result of a mixing-vessel phenomenon, with evidence showing that the origin of the catastrophic 1918 virus pandemic likely resulted from a bird influenza virus directly infecting humans and pigs at about the same time without reassortment, according to the CDC.

The first 2 decades of the 21st century saw a huge increase in efforts to develop an infrastructure to monitor and potentially prevent the spread of new zoonoses. As part of a global effort led by the United Nations, the U.S. Agency for International AID developed the PREDICT program in 2009 "to strengthen global capacity for detection and discovery of zoonotic viruses with pandemic potential. Those include coronaviruses, the family to which SARS and MERS belong; paramyxoviruses, like Nipah virus; influenza viruses; and filoviruses, like the ebolavirus."

PREDICT funding to the EcoHealth Alliance led to discovery of the likely bat origins of the Zaire ebolavirus during the 2013-2016 outbreak. And throughout the existence of PREDICT, more than 145,000 animals and people were surveyed in areas of likely zoonotic outbreaks, leading to the detection of more than "1,100 unique viruses, including zoonotic diseases of public health concern such as Bombali ebolavirus, Zaire ebolavirus, Marburg virus, and MERS- and SARS-like coronaviruses," according to PREDICT partner, the University of California, Davis.

PREDICT-2 was launched in 2014 with the continuing goals of "identifying and better characterizing pathogens of known epidemic and unknown pandemic potential; recognizing animal reservoirs and amplification hosts of human-infectious viruses; and efficiently targeting intervention action at human behaviors which amplify disease transmission at critical animal-animal and animal-human interfaces in hotspots of viral evolution, spillover, amplification, and spread."

However, in October 2019, the Trump administration cut all funding to the PREDICT program, leading to its shutdown. In a New York Times interview, Peter Daszak, president of the EcoHealth Alliance, stated: "PREDICT was an approach to heading off pandemics, instead of sitting there waiting for them to emerge and then mobilizing."

Ultimately, in addition to its human cost, the current Wuhan coronavirus outbreak can be looked at an object lesson a test of the pandemic surveillance and control systems currently in place, and a practice run for the next and potentially more-deadly zoonotic outbreaks to come. Perhaps it is also a reminder that cutting resources to detect zoonoses at their source in their animal hosts before they enter the human chain is perhaps not the most prudent of ideas.

Mark Lesney is the managing editor of MDedge.com/IDPractioner. He has a PhD in plant virology and a PhD in the history of science, with a focus on the history of biotechnology and medicine. He has served as an adjunct assistant professor of the department of biochemistry and molecular & celluar biology at Georgetown University, Washington.

This article originally appeared on MDedge.com.

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2019-nCoV: Just a Stop on the Zoonotic Highway - Medscape