Coronaviruses infect many animal species as well as humans. COVID-19 is what’s called a ‘zoonosis’, which means that this disease jumped species into humans, probably through close contact with animals.
The causative agent of the disease COVID-19 is a coronavirus, (SARS-CoV-2). COVID-19 is a zoonosis, which means that it can be transmitted to humans from animals. Zoonoses caused by cross-species transmission represent 60% of the emerging infectious diseases worldwide, and are often life-threatening. We all remember the influenza pandemics caused by viruses that crossed the barrier from birds or swine to humans. Cross-species transmission often occurs when animals and humans are in close contact, such as large animal farms, or crowded food markets where live animals are sold. But how do these viruses make the jump to a new species? Ultimately, to infect a new host, the viral proteins involved in transmission must be able to bind to receptors present on the surface of cells in the new species. The changes usually arise from error-prone replication of the viral genome. As an example, the gene coding for the Spike protein, which binds to the receptor of SARS-CoV-2 in humans, is a mutational hotspot. Coronaviruses, like influenza viruses, can also exchange parts of their genome with other viruses during a coinfection. When such major genomic changes happen in the right region, they can generate new chimeric viruses capable of infecting a new host.
Zoonoses become really threatening when the viruses are able to efficiently spread from human to human, usually by droplet infection, as is the case for SARS-CoV-2 and related viruses – our species doesn’t “know” the virus and is therefore not protected. Identifying the species of origin and possible intermediate hosts is extremely important to be able to control future transmission events. In the case of COVID-19, full-length genome sequences of the SARS-CoV-2 virus obtained from patients (including a worker in the now infamous food market in Wuhan) are almost identical to that of coronaviruses found in bats.
However, Coronaviruses are in many animal species besides bats, and SARS-CoV-2 is not the only one that made the jump to ours. Of the 7 coronavirus isolated from humans, 4 cause common cold symptoms; the remaining 3 cause severe respiratory disease. The most similar to SARS-CoV-2 is SARS-CoV-1, responsible for the SARS (Severe Acute Respiratory Syndrome) outbreak originating in Guangdong, China in 2002, presumably originating from civet cats sold for food in a live-animal market, which were possibly infected by contact with bats. SARS spread rapidly to more than 30 countries, but luckily there have been no SARS cases since 2004. MERS-CoV was responsible for a 2012 outbreak with a clear epicenter in the Arabian Peninsula, and was traced to dromedary camels. Both viruses took a heavy toll on human health, but COVID-19 spreads with unprecedented speed and threatens to overwhelm health systems around the world.