“ The emergence of new viral diseases by animal-to-human host switching has been, and will likely continue to be, a major source of new human infectious diseases. A better understanding of the many complex variables that underlie such emergences is of utmost importance to public health.”
This concluding sentence from “Cross-Species Virus Transmission and the Emergence of New Epidemic Diseases” published in 2008 in Microbiology and Molecular Biology Reviews is eerily predictive of the emergence of Covid-19 in our world. This article, in particular, addresses the complexity of how a virus jumps from animals to humans. Here is what I have heard from other people about how Covid-19 found its way into the human population:
Fable #1: It came from an open air market where pigs, chickens and other animals are slaughtered and set out for customers to buy. As shown in the movie with Gwenyth Paltrow- Contagion- these markets are not clean and the raw meat is not handled with care. In the movie, it is the chef at a local restaurant that is responsible for handling raw pork before posing for a photo with Gwenyth Paltrow and holding her hand. She later dies from the resulting illness.
Fable #2 Someone ate a bat infected with Covid-19. The virus is thought to have originated in bats so of course it jumped to humans when someone ingested a bat.
Fable #3 It was released from a research lab in China. Someone deliberately injected it into the population to wreak havoc.
None of these ideas is the complete story. The truth is this: the way a virus jumps from the animal population to the human population is not completely understood and likely a lengthy and convoluted process. Three major factors seem to be at stake:
1. How well the receptors on the virus match with the receptors on the recipient animal. In the case of Covid-19, the receptors seem well adapted to enter a human cell which is why the disease has spread so rapidly.
2. Proximity of different animal species to each other. For animals in natural habitats, there is less transfer of viruses due to the lack of opportunity for the viruses from one animal to have any contact with other potentially infectious animals.
“Fruit bats (genus Pteropus) are the reservoirs of Nipah virus, and planting of fruit orchards around piggeries attracted these bats, allowing spillovers of viruses to pigs and a large-scale outbreak (17), showing how ecological changes brought about by humans can impact disease emergence.”
3. Once a virus has a newly infected host, controlling the spread amongst that new species. “Early detection of inefficiently spreading viruses in a new host would provide opportunities for epidemic control... How viruses gain the ability to spread efficiently is a key question in viral emergence, but the mechanisms involved are poorly understood.”
It seems that when the process is complex and there are multiple unknowns, the resulting information available becomes the notion of fables- wild tales of eating whole bats.