You may be asking yourself: Where did this pandemic come from? You probably know that the early cases came from Wuhan, the capital city of China's Hubei providence. But how did this virus "just suddenly appear" in humans?
Well, coronaviruses have been known for many years to infect humans. Some types of coronaviruses infect humans every winter and cause cold-like symptoms. It has been reported that every person on the planet will likely get infected at some point in their life by some type of coronavirus (Coronavirus "Germ Profile", Intermountain Healthcare). Here's a U.S. CDC chart that shows the normal seasonal coronavirus cases (as indicated by coronavirus tests) in the Midwestern U.S. from March 2018 to March 2020.
Here's another chart of season coronavirus cases, this time in the Salt Lake City, Utah region.
In 2003, a novel and much more deadly coronavirus appeared in the Guangdong Province of China. This virus, called SARS-associated coronavirus (SARS-CoV), caused a Severe Acute Respiratory syndrome (SARs), and infected 1000s of people, starting in 2003. SARS0Cov 2003, is a highly related coronavirus to the current coronavirus SARS-2 CoV (2019) (See e.g., Wu et al 2020). Thus, we can use SARS 2003 to learn more about SARS-CoV-2 2020.
So how did SARs-CoV suddenly infect humans in 2003? Xu et al. 2004 reported that a high percentage (39%) of early cases were from food industry workers. On further investigation, they concluded that "Early-onset patients were more likely to live within walking distance of a produce market (an agricultural market where live animals are sold, killed, and butchered in situ, also known as a "wet market") than late-onset patients (OR undefined, lower 95% CI 2.39)." Bats are carriers of coronaviruses, and such bat coronaviruses are believed to jump species (Menachery, Nature Med. 2015). Wild animals such as civet cats, and apparently much more rarely bats, are sold at these wild animal markets (Young et al. 2020).
Wang et al. 2005 looked more specifically at 4 individuals who were infected with SARs-CoV from the winter of 2003/2004, the year after the larger outbreak in 2003. The SARS-CoV from that year was less harmful and less able to spread in humans. Their epidemiologic findings showed that 2 of the 4 SARS patients from that year were linked to a restaurant serving palm civets as food. One patient was a 20-year-old waitress who had palm civets in her work area and she often passed or stood a short distance from the animal cages. The other patient was a 40-year-old physician who ate at the restaurant at a dining table within 5 m of civet cages.
Civet cat at market photo from "The Conversation".
The article went on to describe the restaurant serving these wild cats… "The restaurant is in a 2-story building in downtown Guangzhou. Eight animal cages containing 6 palm civets (Paguma larvata) were stacked (2 cages per stack) at the front door of the restaurant. The cages were approximately 1 m from the sidewalk and 2 m from the first row of dining tables on the ground floor of the restaurant. Pedestrians walking in the street and customers dining on the ground floor could easily see the animals in the cages." Wang et al. The authors then tested the pam civets and found SARS-CoV (2003) in 5 of the 6 civet cats in the restaurant. Furthermore, the nucleotide sequence of the genomes of the SARS-CoV in the cats was very similar to that of the 2 infected people, but different than SARS-CoV in other animals in a nearby wild animal market from the prior year, and different than publicly available SARS-CoV sequences from 2003 as well.
These authors concluded that "We provide the first direct evidence that SARS-CoV was transmitted from palm civets to humans, and that a restaurant serving palm civets positive for this virus was the source of infection for 2 of 4 confirmed SARS patients during the resurgence of SARS in the winter of 2003–2004." Wang et al. They further concluded that " Clearly, SARS cases contracted at the restaurant were the result of recent interspecies transfer from a putative palm civet virus reservoir, rather than the result of circulation of SARS-CoV in the human population. This was similar to the conclusion of another study from that year with another SARS infected man showing SARS-CoV genomic sequences very closely matched to those in 16 of 21 infected civet cats sold at a wild animal market. Parry 2004.
A much more recent paper by Menachery et al. 2015 performed work with bat CoV viruses and human cells to conclude that there are a number of possible routes that a coronavirus similar to SARS-CoV in bats could have led to a human epidemic. These authors found that bat CoVs similar to SARS-CoV could infect human airway cells directly in cell cultures. Thus, they concluded that bat CoVs similar to SARS-CoV likely crossed species into human either through an intermediary animal host, such as a civet cat, or directly without an animal intermediary. However, Peter Daszak, an expert in species switching of viruses between animals and humans indicated that bats are rarely found in the Chinese wet markets, and concludes that an animal at the market was the likely intermediate between infected bats and infected humans in our current Covid-19 pandemic (Young et al. 2020 and Morens et al. NEJM 2020).
What is a civet cat? As an American unfamiliar with civet cats, as I learn about their ability to adopt to various habitats but primarily live in forests, and nocturnal habits, they remind me of racoons. There are other animals called raccoon dogs that are also sold in the wild animal markets in China and have been reported to carry coronavirus. However, racoon dogs are not racoons and civet cats are very different too. The important point is likely that coronaviruses originate in bats as the reservoir in nature. Then bats pass on the viruses, likely through their feces to other animals, like civet cats. Here's an explanation from a National Geographic article "Coronaviruses not only spread via the air and the respiratory tract, but also if fecal matter comes in contact with another creature’s mouth. Bats aren’t exactly clean, so if one nibbles on a fruit, the food may get contaminated with fecal matter. If the fruit drops to the ground, then it can serve as a viral crossover point for farmed animals like civets."
This article claims that the actual mammal that transmitted the bat novel coronavirus-SARS2 to human was the pangolin. The article cites a prior article. that indicates that two researchers in China, Shen Yongyi and Xiao Lihua, identified the pangolin as the potential source of nCoV-2019 because they identified a 99% identical coronavirus in pangolins. However, the work has to be considered preliminary unless someone finds a publication in a peer-reviewed journal and analyzes the results in such an article. It could be true that it was the pangolin. However, we'll need to see the published work.
Regardless of the exact mechanism of transmission of SARS-CoV-2 (2019) to humans, like SARS-CoV (2003 and 2004), it seems highly likely that SARS-CoV-2 (2019) originated in a bat, and was transmitted from a wild mammal in a wet market in China to humans at that market. Apparently most or all of the early Covid-19 cases, which is the disease caused by SARS-CoV-2, were in individuals who had contact with wet animal markets in China (Singer 2020). Whether a civet cat was the intermediate that led to the highly virulent CoV strain, or another wild animal at the market, or less likely a bat directly transmitted the virus to a human, is not known. Regardless, it is frustrating after we learned that the coronaviruses that caused SARS 2003 outbreak, and 2004 infections, originated from wild animals at outdoor markets and/or restaurants, that those markets, and likely restaurants serving wild animals, continued to operate over a decade later, likely causing our 2019 pandemic.