COVID-19 had intermediate animal hosts before beginning to infect humans. It is not uncommon for viruses and illnesses to have animal hosts that have the ability to transmit it to other organisms, one such example being mosquitoes and various illnesses such as malaria and the West Nile virus. It has been proven that bats are carrier of SARS-CoV-2, but scientists have been trying to discover how exactly it got from bats to humans. This knowledge matters because understanding where the virus originated and how it came to infect humans could prove crucial to future treatment and control.
It was originally suggested that snakes were the intermediate host due to a genome study; however, it had a lot of scientific criticism for a few reasons, one such being the fact that the coronavirus has only been known to infect mammals and birds. Meanwhile, another unrelated study comparing the spike proteins to that of HIV-1, discovered a few unexpected similarities. Due to the rise of conspiracy theories and rumors, a scientist by the name of Yang Zhang, along with some colleagues, decided to conduct a more in-depth study on SARS-CoV-2 sequences.
Yang Zhang and his colleagues uncovered the error in the analysis that claimed snakes were the intermediate host. Additionally, they analyzed and compared DNA and protein sequences from pangolin tissues in order to try to find those similar to SARS-CoV-2. They were able to identify protein sequences that were 91% identical to those found in the human virus’ proteins. The spike protein found in pangolins only had 5 differences in the amino acids as compared to the 19 differences in the bat viral proteins which is further evidence that pangolins are the intermediate host. However, researchers say that it is possible for other intermediate hosts to exist.
So how does an illness transfer from animals to humans? One belief is that rapid mutation is the main factor that allows viruses to adapt to overcome a new hosts barriers and immunological defenses. Another proposed theory is host similarity as explained by Gary McCracken, a professor at the University of Tennessee. They tested this theory by analyzing hundreds of rabies viruses in various species of bats. They found that the more genetically similar species of bats had these cross-species viruses.
As more tests have been conducted, it has been found that some animals, other than bats and pangolins, are also able to be infected with the coronavirus. João Rodriguez from Stanford University and some colleagues used computers to simulate and investigate how the spike protein interacts with different animal cells ACE2 receptors. The better the viral “key” fit into the receptor “lock”, the more susceptible that species was to SARS-CoV-2. Therefore, by applying this knowledge (as well as McCracken’s theory) to other situations, it would make sense that humans could be infected by a cross-species virus, particularly from other mammals.
This connects to what we learned in class about receptors and cell signaling. ACE2 receptors are found on cells throughout the body, most notably in this case, in the lungs. ACE2 receptors help regulate blood pressure, wound healing, and inflammation. Once a message (a signaling molecule or such) is received by the receptor, cell signaling moves into the next stage of transduction which ultimately produces a response. Therefore when something (in this case COVID-19) interferes, the proper signaling becomes changed or altered leading to the symptoms we have come to recognize as COVID-19.