SARS-CoV-2 - there may be multiple hosts

Deeper research shows how dangerous the Covid-19 virus can be, given the range of animal hosts it can occupy.

SARS-CoV-2 - there may be multiple hosts

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• What is a spillover: In the language of epidemiology, a “spillover” is a virus that has made the leap from one host species to another. The spillovers of most concern to people are those from other animals to Homo sapiens. These may then go on to create “zoonotic” human diseases — of which covid-19 is believed to be one (having come from some bat species originally, in Wuhan, China).
• Two way traffic: Such traffic can run in two directions.

1. In 2020 the World Health Organisation (WHO) reported that SARS-CoV-2 had spilled over in Denmark from human beings into farmed mink, and was thereafter transmitted from animal to animal to create a separate veterinary epidemic.
2. In Nov 2021, a paper on BioRxiv presented proof that the virus is also circulating in white-tailed deer in North America, having presumably spilled over from people there.
3. There were already reports suggesting that domestic pets, especially cats and dogs, can also pick up SARS-CoV-2—and in the case of cats at least, can then pass it on to others of their kind.

• Serious spillovers: Such spillover are serious, for two reasons.

1. They create viral reservoirs beyond the easy reach of medical science and monitoring. Even if there are no human cases of covid in an area, such animal reservoirs mean that SARS-CoV-2 may still be lurking, waiting to spill back into people.
2. Exposure to the immune systems of novel hosts may drive the evolution of new and (if they then do spill back) potentially threatening viral strains. The mink incident therefore led the governments of Denmark, the Netherlands and Spain to cull 18 million animals and place strict lockdowns in regions around the fur farms concerned. Treating a wild population, such as white-tailed deer, in a similar way would be harder. But precautions are still possible.

• Reactive and proactive approaches: These are reactive approaches. A proactive one would try to establish which species are at greatest risk of becoming reservoirs for SARS-CoV-2 before they actually do so. That would permit the monitoring of threats before they got out of hand.

1. ACE2 the entry point - Just after covid emerged in January 2020, scientists focused their attention on ACE2, a cell-membrane protein that had been identified as the virus’s point of entry. ACE2’s job is to help regulate blood pressure, and most vertebrates have it in one form or another.
2. Now the researchers wanted to know in which other vertebrate species SARS-CoV-2 might be expected to bind as strongly to the local ACE2 receptors as it does to those in human beings. These would be candidates for the role of reservoirs.
3. So they gathered molecular information about every version of ACE2 that they could get their hands on. These were from mammals — 142 species of them. They then used computer modelling of the interatomic forces involved to work out the strengths of the bonds likely to develop between SARS-CoV-2’s spike protein and each version of ACE2.
4. They found the bond with mink ACE2 strong. They also found a strong affinity with ACE2 from white-tailed deer! Cats and dogs also showed up as being at risk—which reports then confirmed that they were. And gorillas and macaques, which have suffered a few cases in zoos, looked susceptible as well.

• Deeper research: Scientists then went beyond these 142 species, and built a comprehensive database of evolutionary traits shared by the species with the most vulnerable ACE2 receptors. This is a technique which has been used successfully in the past on rodents and bats, to assess their likelihood of acting as reservoirs for viruses including Ebola and Zika. It is based on the idea that particular proteins of species with similar physiologies and ways of life might be expected to evolve in similar ways.

1. The result was the revelation of 540 species which seemed likely to have vulnerable ACE2 receptors and thus the potential to function as covid reservoirs.
2. Most primates were on this list—which, considering that people are primates too, was expected. Nor, given suspicions about SARS-CoV-2’s origins, was the inclusion of 35 types of bat a surprise.
3. Though the common house mouse does not look to be a risk, which is good news, two of its fellow rodents, the ricefield rat and the Malayan field rat, both do. Since these species are often preyed on by domestic cats, themselves now known to be covid-susceptible, that provides a route by which people might become infected.
4. Dozens of other species were also flagged up as potential reservoirs. These included red foxes and raccoon dogs—two creatures which, like mink, are sometimes farmed for fur—and white-lipped peccaries (pig-like creatures found in South and Central America) and nilgai (a large Asian antelope), both of which are farmed occasionally, and also hunted and eaten.

• Water buffalo at risk: Among the more widespread livestock, the species of most concern is the water buffalo. There are over 200 million of these around the world, acting as both beasts of burden and sources of milk. And other frequently hunted animals, such as the duiker (another antelope), the warty pig and the mule deer were also reckoned vulnerable, together with some rarities, including two critically endangered antelopes, the addax and the scimitar-horned oryx.
• Very wide range: The sheer range of species involved is staggering. No one imagined to ever see a virus with such a high cross-species infection potential. It appears that there are at least an order of magnitude more species that are susceptible to SARS-CoV-2 infection than any other zoonotic virus.
• EXAM QUESTIONS: (1) Explain the scientific method to determine which animal species may end up hosting Covid-19 virus, and how risky could that be. (2) Explain the role of ACE2 in the Covid-19 spread. (3) Prepare a plan to tackle a new pandemic that may arise in 2030. List ten steps.

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