Are intermediate hosts essential to facilitate the bat-originated coronavirus hop to humans?

It was considered that most of the viruses in nature are species-dependent and only hijack their susceptible hosts to generate off-springs. To comply with the dependence, viruses make minor mutation adjustments along with their hosts throughout their evolution and therefore, the cross-species transmission is not common and seems fortunate. However, recent severe outbreaks such as bird flu, Ebola fever, SARS, and COVID-19, made researchers ask whether cross-species transmission is more frequent and important than what we previously realized.

The ongoing outbreak of COVID-19 strongly drives researchers to find out the intermediate hosts of SARS-CoV-2, the cause of COVID19. Last month, a group of scientists said that snakes, particularly the Chinese krait and the Chinese cobra, may be the source of the new virus. Later on, less than one month, another research team in China claimed that pangolins were mostly likely the intermediate host of the virus. Unfortunately, the official statement from WHO said that there was “no conclusive evidence”, and make that information like this is just an FYI.

Besides the struggling to determine the intermediate host for SARS-CoV-2, the other voice that bat viruses can directly infect humans draws people’s attention. Ecologist Kevin Olival, the Vice President of EcoHealth Alliance at Columbia University, said that bat viruses do not need to mutate in another animal to infect humans. Dr. Olival has been working on the bat and its associated diseases for over 10 years. Their findings showed that SARS-like coronavirus in bat populations successfully infected human cells in a petri dish, indicating that bat viruses have the potential to enter human cells directly without the need to infect another host to cause additional mutations. This conclusion is further confirmed in another paper published in the 2015 "Nature Medicine" magazine. The studies were led by Dr. Zhengli Shi, an investigator of Wuhan Institute of Virology, and revealed that a bat coronavirus with SHC014 spikes has the ability to directly infect human respiratory cells but attenuates the risk of potential cross-species propagation.

For the current outbreak of COVID-19, sequence comparisons were conducted between SARS-VoC-2 and bat viruses collected before the outbreak. Outcomes showed that SARS-VoC-2 is 96.2% identical at the whole-genome level to RaTG13, a coronavirus isolated from the Chinese chrysanthemum bat. The entire academic community was shocked by this report because the findings indicate that RaTG13, the bat-originated coronavirus, is an ancestor of SARS-VoC-2. "If bat coronavirus is a close ancestor of Novel Coronavirus, it is easy to jump from bat to human without an intermediate host; even if bat coronavirus is a distant ancestor of Novel Coronavirus, then it can also jump directly to people without an intermediate host." Professor Richard H. Ebright said. Dr. Ebright is the Professor and Director of Waksman Institute of Microbiology at Rutgers University.

Although the direct evidence that bat coronavirus may cross the barrier between species and directly infect humans still lacks, the possible risk is highly concerned. What did the mysterious bat coronavirus go through and how was it brought into the Wuhan population? Scientists all over the world are looking for answers.

References:

1. Geoghegan JL, Duchêne S, Holmes EC (2017) Comparative analysis estimates the relative frequencies of co-divergence and cross-species transmission within viral families. PLoS Pathog 13(2): e1006215.
2. Ji W., Wang W., Zhao X. et al. (2020) Cross‐species transmission of the newly identified coronavirus 2019‐ J Med Virol. https://doi.org/10.1002/jmv.25682
3. Liu, Jiang J., Wan X. et al. (2020) Are pangolins the intermediate host of the 2019 novel coronavirus (2019-nCoV)? bioRxiv 2020.02.18.954628; doi:https://doi.org/10.1101/2020.02.18.954628
4. Menachery V., Yount B., Debbink K. et al. (2015) A SARS-like cluster of circulating bat coronaviruses shows potential for human emergence. Nat Med 21, 1508–1513. https://doi.org/10.1038/nm.3985
5. Zhou P., Yang X., Wang X.et al. (2020) A pneumonia outbreak associated with a new coronavirus of probable bat origin. Nature. https://doi.org/10.1038/s41586-020-2012-7