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Q; How come we tend to get crossover viruses from bats and pigs, but not cats and dogs who we're much more in contact with?

While the evolutionary relatedness of the hosts may be a factor in host switching, the rate and intensity of contact may be even more critical. Viral host switches between closely related species (e.g., between species within genera) may also be limited by cross-immunity to related pathogens or by innate immune resistance to related viral groups.

A new viral strain from an animal that is not so close to human beings might not have to face innate immune resistance immediately after switch-over and therefore can cause havoc while a strain from close contact animals cannot have this advantage as the contact and exchange might build some resistance over the years.

The specifics of Inter species jumps differ, the mechanism relies on the same fundamental premise: access and ability.

There are several factors that influence the host switching. Like availability of suitable receptor cells, ability to adopt to the available receptor cells, overcoming intracellular mechanisms , whether a virus is a generalist or a specialist, viral fitness trade-offs, ability to recombine and reassort. Only when the conditions are highly favourable, a virus can switch to other hosts successfully. Therefore, it is difficult to form rules.

Pet animals like dogs and cats are abundantly available for viruses. There is no need for the viruses to switch frequently to other animals in order to survive. DESTRUCTION OF HABITAT AND LOSS OF BIODIVERSITY ARE CREATING THE PERFECT CONDITIONS FOR DISEASES LIKE COVID-19 TO EMERGE because viruses have to search for new hosts when wild animals are not available (2) . We cut the trees; we kill the animals or cage them and send them to markets. We disrupt ecosystems, and we shake viruses loose from their natural hosts. When that happens, they need a new host. Often, we are it.

To conclude there are no hard and fast rules. It is a survival battle and several factors dictate how and why a successful change-over takes place. Only when a virus can overcome several of the obstacles mentioned above, it can get established into a new host, whether more genetically related or not, whether in close contact or not.

Footnotes:

  1. Cross-Species Virus Transmission and the Emergence of New Epidemic ...
  2. We are creating conditions for diseases like COVID-19 to emerge | E...

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