What are the implications of tigers being susceptible to SARS-CoV-2?

Question

It was recently reported that a handful of tigers and lions tested positive at the Bronx Zoo for SARS-CoV-2 (see here and here). Does this imply that the SARS-CoV-2 strain in humans mutated into a form that can infect these other species? I thought these sorts of mutations required a long time and prolonged contact between the species (which I wouldn't have thought would be present with such large cats).

Alternatively, is it possible that a single strain of the virus is capable of infecting multiple species?

Answer 1

A partial answer, to the question of how the virus might spread to animals I found several references

• The HongKong dogs, the New York tiger, the Netherlands mink farm and the French cat have been sequenced and can be seen on nextstrain. Note the low amount of mutations in each case, which is intriguing because we expect for example that different ACE2 genes will induce some adaptation at in the viral spike. Those animals had an infection and detectable RNA in the nose for several days. For the dogs the viral load was low and in one case the sequence was identical to the owner. The minks and tiger had noticeable symptoms. A Wuhan cat partial sequence has been uploaded too, as well as a full-length French cat sequence.

• A lab paper said "SARS-CoV-2 replicates poorly in dogs, pigs, chickens, and ducks, but efficiently in ferrets and cats", in particular dogs can't transmit the disease efficiently. According to them subadult cats are susceptible while adult cats are not, and the disease is upper respiratory tract.

  There is a new preprint on a 4 month cat who developed severe disease, dyspnea, and had to be euthanized.

• A paper found antibodies in several Hubei cats from animal shettlers and vet clinics, some of the positive cats are "stray cats", others are sick cats, and a few were cats owned by covid positive people, in those the antibodies were found to be neutralizing the virus much better than in others.

• Another one found that ferrets are efficiently infected in lab and transmit the disease to each other, even if it is mainly upper respiratory tract (no severe pneumonia).

• Rabbits are susceptible though less than ferrets, and asymptomatic.

• Mice are not succeptible but human-ACE2 transgenic mice are and develop a mild pneumonia. Note that SARS-CoV and OC43 produced instead a deadly encephalitis.

  However, through 6 ($10^{5.8}$PFU) passages into mice lungs the researchers could adapt SARS-CoV-2 through acquisition of a single mutation in the RBD allowing the virus to produce a moderate pneumonia into both young and aged mice.

  The same researchers after 30 more passages obtained a super-deadly mouse adapted strain effective even at $10^2$PFU with 3 mutations in the RBD and 9 others mainly in nsp3-4-6 and N. Of note, two of the RBD mutations are in the South African variant N501Y.V2.

  Something similar had been done with SARS-CoV.

• The Baric lab has compared the RBD from SARS-CoV-2 and previous sarbecoviruses, the human and mouse ACE2 protein, and through computer simulations they could find two amino acids to change in order to switch the virus from humans to mice. With 10 more passages they obtained a deadly strain in aged mice.

• There are several mink farms in Netherlands where the animals got infected, this is possibly the most promising species for inter-species transmission. Edit : as predicted (first reported case) of animal to human transmission is believed to have occurred in such a mink farm. A detailed story including severe pneumonia is described in a paper.

• In lab, SARS-CoV-2 is provoking a severe disease in minks as well as in Roborovski dwarf hamsters.

• The Pangolin outbreak(s) : not SARS-CoV-2 but the newly discovered Pangolin-CoV in the same lineage, it killed 16 of 21 Malayan pangolins in March 2019. The virus was identified only in February 2020. The Pangolin-CoV whose RBD is very similar to SARS-CoV-2 was in this outbreak.

• Cats can transmit the virus to each other. In the nose the peak was $10^{4.5}$ PFU / ml which is a medium viral load.

Similar for hamsters and macaques, the latter are used for vaccine development.

Racoon dogs show high shedding and can transmit the virus through present only minor clinical signs.

This kind of result is not surprising, most human viruses can transmit to some species and not to others, often producing a different and milder disease, which is a problem for vaccines and antibodies development.

Answer 2

Your question may be based on a misconception. Mutations occur all the time. In almost all cases, it leads to decrease in reproductive viability. For a virus with one or more established host species, it has already evolved to be quite efficient at reproducing within those species. For it to jump to a new host species and spread in that other species, it must not only be able to infect the new host species, but must also have the opportunity to cross over. If it cannot normally infect the new host species, then it has to acquire some mutations. But mutations occur all the time, whereas the opportunities are not always there. So this is a matter of relative likelihoods. The more contact between wild animals and humans, the greater the likelihood of such inter-species infection happening, because of low prior contact between humans and wild animals.

Since we now have probably 10 million or more humans infected, we have provided a great number of opportunities for the virus to jump to other species, many orders of magnitude more than the original opportunities for the virus to jump to humans. This is why such large-scale events can have knock-on effects of much bigger scale than the events themselves.

While zoo visitors may not go close to large cats, they may sneeze or cough and thereby transmit the coronavirus in mucus droplets that can last very long even if alcohol-based disinfectants are applied. The zoo keepers themselves may be infected and have close contact with the animals. It is difficult to trace the source of infection when we have so many cases. Although the Bronx Zoo was supposed to be closed from 16 Mar onwards, it is not impossible that some animals were already infected prior to that, so any hypothesis is going to be guesswork.

Furthermore, viruses that already regularly infect more species tend to be able to infect more new species than other viruses. So once the coronavirus managed to jump to humans, it is reasonable to expect it to be able to infect other new species more easily than the strain in the original host species. So the fact that tigers are susceptible to the strain that infects humans does not imply anything about whether that strain has acquired some new mutations or not.

Answer 3

A single strain of virus (such as rabies) can infect multiple species of hosts.

The big question is whether domestic cats also get infected? If they do, two main problems. There would be a reservoir of virus circulating in cats allowing reinfection of humans. And worse, the virus may acquire mutations in the cat population that make it more virulent in humans.

Personally, I recommend keeping a good distance from all big cats to be safe.