Tuesday, August 17, 2021

Science Perspective: The Animal Origin of SARS-CoV-2

Credit FAO


We've a new perspective article, published today in the Journal Science, that looks at the emergence of the SARS-CoV-2 virus in China in late 2019, and speculates on the likely path the virus took to becoming a pandemic. 

This is obviously a contentious subject, with many convinced that COVID emerged from a Wuhan lab, while others are equally convinced it probably jumped on its own from a bat, either directly to a human or first to an intermediate animal host, and then to humans.

In 2002, we saw another SARS-CoV Betacoronavirus (subgenus Sarbecovirus) emerge in China, sparking a global epidemic (see SARS And Remembrance), which was fortunately contained. That virus - which appears to have originated in bats - was probably introduced into humans via `wet markets' and `wild flavor' cuisine.  

Practices that - while outlawed in many countries - continue today (see Despite Crackdown, `Wild Flavor’ Trade Continues In China  and Bushmeat,`Wild Flavor’ & EIDs).

Although surveillance and reporting on spillover events remain suboptimal, we see scattered reports of avian flu, human anthrax infection, strep suis (see Tiết Canh - An Incredibly Bat Idea, bubonic plague (see China: Inner Mongolia Reports Another Plague Case), Nipah, and Ebola linked to animal contact and/or consumption nearly every year around the globe.

Many of these zoonotic pathogens are carried by bats without ill effect, can be passed on to intermediate hosts, which then may pass the virus on to humans.  Once infected, humans can then spread the virus widely, and sometimes even to new animal hosts (see COVID Variant B.1.1.7 & Companion Animals).

Agree or disagree with their scenario, you'll want to follow the link to read the full article, as I've only posted some excerpts (Bolding mine).  When you return I'll have a brief postscript. 

Spyros Lytras1, Wei Xia2, Joseph Hughes1, Xiaowei Jiang3, David L. Robertson1


Trading of animals susceptible to bat coronaviruses is the likely cause of the COVID-19 pandemic


Overall, SARSr-CoV animal-to-human transmission associated with infected live animals is the most likely cause of the COVID-19 pandemic. However, the massive scale of cold-chain supply, particularly following disruption to the meat industry in China caused by ASFV-associated culling, suggests that frozen susceptible-animal carcasses, either for human or animal consumption, should not be discounted as playing a role in the emergence of SARS-CoV-2. This will especially be the case if the progenitor population of SARS-CoV-2 is found further away from Wuhan, because live-animal trafficking is much more likely to involve more proximal locations to the city, e.g., the prefectures of Hubei province. Serology, sampling and interviewing of the individuals (e.g., trappers, traders, and farmers) connected to the sources of wildlife sold in the Wuhan markets in October and November 2019 would be a sensible next step in future investigations.

Once in the human population, SARS-CoV-2 has spread surprisingly rapidly for a new human pathogen. Contrary to classical expectations for a host species jump, SARS-CoV-2 is highly capable of human transmission, including frequent asymptomatic transmission and amplification through superspreader events. This initial ”success,” at least prior to the emergence of variants of concern, is unlikely to be due to early adaptation to humans but rather can be attributed to the relatively generalist nature of SARS-CoV-2 (14), evidenced by frequent transmission to mammals: minks, cats, and others.

Worryingly, recent experimental evidence has found that the pangolin-derived sarbecoviruses (presumably acquired from exposure to horseshoe bats or other infected animals after illegal trafficking into China) can also infect human cells and have spike proteins that are even better at facilitating entry into human cells than that of SARS-CoV-2 (15). Collectively this points to a further risk of spillover that extends to the more divergent members of the lineage that SARS-CoV-2 emerged from and implies frequent spillovers from bats to other susceptible wildlife.

Humans are now the dominant SARS-CoV-2 host species. The danger is that SARS-CoV-2 could spread from humans to other animal species, termed reverse zoonosis, as is suspected for white-tailed deer in the United States. The promiscuous infection of various host species by the sarbecoviruses means that future spillovers of SARSr-CoVs from wildlife are very likely, and current vaccines may not be protective against novel variants. The sampling intensity of sarbecoviruses needs to be urgently increased to gain a better understanding of this spillover risk.

The recent finding of sarbecoviruses, not dissimilar to SARS-CoV-2, dispersed in Southeast Asia emphasizes the urgency of monitoring coronavirus diversity. Humanity must work together beyond country borders to amplify surveillance for coronaviruses at the human–animal interface to minimize the threat of both established and evolving variants evading vaccines and to stop future spillover events.

While I don't completely discount the possibility of a lab-leak origin for SARS-CoV-2, we've been expecting another `SARS-like' spillover from bats - likely via an intermediary host - since the mid-2000s.  

It was the scenario behind 2019's The JHCHS #Event201 (Fictional) CAPS Table Top Exercise, and throughout the last decade we looked at numerous `bat-borne' zoonotic threats, including:
Emerg. Microbes & Infect.: Novel Coronaviruses In Least Horseshoe Bats In Southwestern China 
PNAS: SARS-like WIV1-CoV Poised For Human Emergence

Sci Rpts: Avian & Human Influenza Compatible Receptor Cells In Little Brown Bats

Study: Hotspots For Bat To Human Disease Transmission

Six weeks before the Wuhan outbreak, in African Swine Fever's (ASF) Other Impacts; Pharmaceuticals, Bushmeat, and Food Insecurity, I even speculated that China's ASF outbreak could  lead to increased `bushmeat' consumption, which in turn might spark another SARS-like outbreak (one of the possible scenarios discussed in today's article).  

Admittedly, more of a lucky guess than prescience, since we've been expecting a SARS redux - or the emergence of Virus X - for years.   

Regardless of how SARS-CoV-2 emerged, today's perspective is a reminder that nature's laboratory is open 24/7 - and future spillovers are all but guaranteed.  We either prepare as if it will happen, or we will be caught flat footed and unprepared again.