“The search for a scapegoat is the easiest of all hunting expeditions.” - Dwight D. Eisenhower
“Science will eventually figure it out. “ - Robert Redfield, Jr., American Virologist, Ex-Director, U.S. Center for Disease Control and Prevention (CDC)
Identifying the source of the devastating COVID-19 pandemic is regarded as a critical step in preventing the next worldwide spread of yet another emerging pathogen. While the source of SARS-CoV-2 has yet to be established conclusively, we’ve moved closer. This Germ Gems post like two of my others, “Preventing Animal Spillovers of Microbes to Humans” (November 11, 2020) and “COVID-19: What Animal Is to Blame?” (April 15, 2020), deals with the putative role of bats as the source of a spillover to humans of SARS-CoV-2, the coronavirus that causes COVID-19. The plot, however, has thickened. Is the bat the source or merely a scapegoat?
What’s special about bats? A review. Bats have some remarkable features. Like humans, bats are vertebrates belonging to the Class Mammalia. But unlike our singular species, Homo sapiens, in the Order Primates, bats belong to the Order Chiroptera that includes 1,240 known bat species worldwide or about 20% of all 5,416 mammal species.
Bats harbor a group of viruses called the bat virome. Bats carry viruses belonging to at least 28 Families that are single or double-stranded DNA or RNA viruses. The greatest share of bat-associated viruses belongs to the Family Coronaviridae. SARS-CoV-2 is a member of this viral family, as is SARS-CoV-1, the cause of the severe acute respiratory syndrome (SARS) pandemic in 2002-2003, and MERS-CoV, the etiology of Middle East respiratory syndrome (MERS), an ongoing pandemic that emerged in Saudi Arabia in 2012.
A vast majority of the members of the bat virome lack zoonotic potential, that is, they don’t cause disease in humans. But those that do warrant our attention. These pathogenic zoonotic viruses include rabies virus, SARS-CoV-1, MERS-CoV, Nipah virus, Marburg virus, and Hendra virus. Of these, SARS-CoV-1 is most relevant to our understanding of whether its very closely related relative, SARS-CoV-2, spilled over from bats to humans.
A spillover? SARS-CoV-1 was discovered in 2003 in Asia in humans suffering from SARS. When the virus was found in palm civets, racoon-like mammals sold in wild animal food markets in China, the civets were quickly eradicated from these markets. At that time, they were considered the secondary or reservoir host of SARS-CoV-1 that hypothetically had spilled over from bats to civets and then on to humans. In retrospect, the eradication of civets from the wild animal food markets appears to be the reason that the SARS pandemic was so short-lived—the World Health Organization (WHO) declared it contained on July 5, 2003.
Eventually, an intensive hunt in bat caves in China resulted in the discovery of SARS-CoV-1 in Chinese horseshoe bats by a team of Chinese researchers including the virologist She Zhenglii, fondly referred to in China as the “Bat Woman.” These Chinese researchers’ findings were ultimately reported in Nature in an article entitled “A pneumonia outbreak associated with a new coronavirus of probable bat origin.”
The hunt for the origin of SARS-CoV-2. When SARS-CoV-2 was discovered as the cause of an outbreak of pneumonia in Wuhan, China in December 2019, the horseshoe bat was immediately incriminated as the prime suspect of harboring this coronavirus. Then the search for a reservoir host of SARS-CoV-2 began and at first focused on wet markets in Wuhan, where wildlife is sold for food and medicinal purposes. Early on pangolins appeared to be the most likely reservoir host for SARS-CoV-2. To date, however, a reservoir host has not been definitively established. And, recently some researchers have suggested that a reservoir host isn’t necessary to explain the spillover to humans because infected bats could manage this feat directly all by themselves.
A conspiracy? The plot thickens. She Zhenglii (the Bat Woman) is the scientific director of the Center for Emerging Infectious Diseases at the Wuhan Institute of Virology where she and her colleagues carry out extensive research on bat coronaviruses. In the early months of the COVID-19 pandemic, it was suggested that one of her laboratory staff carried a coronavirus that was created in the laboratory, namely, SARS-CoV-2, into the Wuhan community in late 2019. Moreover, from the outset the Chinese government hasn’t been transparent about COVID-19, and many credible sources suggest that it has covered up as much as possible about this virus. By so doing, the Chinese government has played into the hands of people labeled “conspiracy theorists” who claimed SARS-CoV-2 didn’t jump from one animal species to another but rather was transmitted by an infected laboratory worker to its own species, Homo sapiens.
WHO to the rescue? After a great deal of resistance by the Chinese government, in early 2021 a WHO-led team of more than a dozen international scientists gained entry into China for a one-month-long search for the origin of SARS-CoV-2. The full report of their investigation was released on March 30. It discredits the theory that SARS-CoV-2 originated in the Wuhan Institute of Virology. Also, it points to an animal spillover, probably from a wet market or wild animal farm in the vicinity of Wuhan.
The continued lack of transparency of the Chinese government, however, clouds our interpretation of the results of the WHO-led investigation. As expressed by U.S. Secretary of State Antony Blinken in a recent CNN interview, “We’ve got real concerns about the methodology and the process that went into that [WHO] report, including the fact that the government in Beijing apparently helped to write it.” And to stir up the controversy further, the former head of the United States CDC, Dr. Robert Redfield, Jr., recently stated his opinion that SARS-CoV-2 came from a Wuhan lab, which he believes is more likely than the scenario that it jumped from a bat or another animal to humans in a wet market in Wuhan. WHO Director-General Tedros Adhanom Ghebreyesus commented: “[A]s I have said, all hypotheses are on the table and warrant complete and further studies from what I have seen so far.”
So, we are still left in a quandary as to whether this virus originated as a spillover or not. I am waiting for science to deliver a clear answer, and I hope that it will be soon and put this issue to rest. Until then I agree with the WHO Director-General “all hypotheses are on the table.”
The enormous value of bats. Bats are commonly associated with vampires and horror stories which cause fear and misunderstanding of these flying mammals. They’ve always had a bad rap. For centuries, humans have scapegoated bats; COVID-19 is just the latest example. As we reflect on whether bats are the primary source of SARS-CoV-2, or are a scapegoat taking our minds off human complicity, we must recognize the huge importance of the Order Chiroptera and the fact that the world is far better off with bats than without them.
Bats are essential to the world’s ecosystems. They eat insects and are estimated to save farmers in the United States alone at least $3.7 billion per year in pest control services. They also help control the number of insects, like mosquitoes and ticks, that carry infectious agents that threaten us. And nectar-drinking (fruit) bats play a vital role in pollinating plants. Yet, largely due to loss of habitat spurred by human activities, more than 200 bat species in 60 countries around the world are considered threatened (Critically Endangered, Endangered, or Vulnerable) by the International Union for the Conservation of Nature.
Another piece of good news about bats is that they are receiving increased scientific attention because of their astonishing ability to support a mindboggling virome composed of scads of viruses that, with rare exception, don’t sicken them. How do they accomplish this? At one time it was thought that the heat generated by their increased metabolism during nightly flights kept their virome in check, much as a fever serves us as a defense for humans against microbes that can’t stand the heat. Increased evidence, however, suggests that the bat’s immune system evolved to be more tolerant of stressors, including the many viruses they carry.
In his weekly newsletter on March 18, Francis Collins, Director of the NIH, featured David Weesler, a French biochemist and an assistant professor in the Department of Biochemistry at the University of Washington. (Weesler was involved in the landmark discovery that ACE2 receptors on cells serve as the docking site for the spike protein of SARS-CoV-2 that permits its entry into cells.) His research group now focuses on the structure of bat antibodies that make them different from human antibodies. Dr. Collins believes bat antibodies will serve as blueprints for promising new treatments to combat many potentially deadly viruses, including our most recent enemy, SARS-CoV-2. Rather than causing us harm, lessons from the bat’s immune system may ultimately lead to improved prevention and treatment of our viral infections. How’s that for irony?
