Three days ago, there was a report saying a Malayan tiger at the Bronx Zoo in New York city has tested positive for the SARS-CoV-2.1 It is suggested she and the other big cats (two other tigers and three African lions) were infected by an asymptomatic zoo keeper. They started to show COVID-19 symptoms like dry cough and decrease of appetite since late March this year, but they are all expected to make a full recovery soon.
That was the first report of humans transmitting the disease to animals and causing sickness to the animals. Other than that, there had been a couple of instances of pet dogs testing positive for the coronavirus elsewhere in the world.2,3 It is believed that they were being transmitted by their owners, but none of the dogs got sick or transmitted the disease to other species.
In my first article about coronavirus, I had mentioned that there is a chance animals can be infected, as ACE2, the receptor for SARS-CoV-2, is also present in dogs and cats. This indicates the potential of animals contracting the disease. However, we can expect that the number of coronavirus transmissions from humans to animals is not high, as transfer of a virus to a new host species is not a simple random process. For a virus to replicate and spread, it must be able to 1) bind to a cell-surface molecule; 2) carry out membrane fusion; 3) deliver critical components into the cytoplasm of the host; 4) avoid triggering apoptosis and highly suppressive type I interferon response; 5) interact successfully with cellular cofactors to replicate its genome and structural proteins; and 6) carry out virion assembly and exit the cells.
The infected animals in the Zoo have so far developed very mild symptoms and they are expected to recover soon. It is believed that they are infected with only a "fairly low dose" of the virus as the animals "did not have continual close contact with the asymptomatic zoo keeper", according to Dr Sarah Caddy, Veterinarian and Clinical Research Fellow at the University of Cambridge. However, as I said in my article written earlier, the other reason for very mild to no symptoms developed in the infected animals is their innate defences to suppress the infection. Additionally, their adaptive responses may eliminate the illness even if the infection from the virus overcomes their innate defenses. Therefore, the most common result of the virus transmission from humans to animals is the complete absence of disease or very mild symptoms.*
Although there is no need to fear the transmission of the virus back to humans from the infected animals, there is a possibility that the virus can linger on cats' fur and be "transmitted through touch in the same way it can be picked up from surfaces like tables and doorknobs." Therefore, according to Daniella Dos Santos, president of the British Veterinary Association (BVA), "cats should be kept inside if they live in a household where someone is suffering with the new coronavirus".4 Accordingly, the same principle should also apply to all households with animal pets.
*Although the first dog tested positive died few days after being released from mandatory quarantine, it did not show clinical symptoms when it was tested, and it was 17 years old, quite old by the breed's standards. Moreover, it was suffering from other underlying illnesses even before its 'weak positive' test. "It is very unlikely the virus had any contribution to the death of the dog."5
References
1. "Coronavirus: Tiger at Bronx Zoo tests positive for Covid-19" BBC News, 6th April, 2020. https://www.bbc.co.uk/news/world-us-canada-52177586
2. "Coronavirus: pet dog belonging to Covid-19 patient infected, Hong Kong health authorities confirm" South China Morning Post, 4thMarch,2020. https://www.scmp.com/news/hong-kong/health-environment/article/3065016/coronavirus-pet-dog-belonging-covid-19-patient
3. "Second Dog in Hong Kong Tests Positive for Covid-19 Virus" Bloomberg, 19th March,2020. https://www.bloomberg.com/news/articles/2020-03-19/second-dog-in-hong-kong-tests-positive-for-covid-19-virus
4. "Cats should be kept inside if owners show coronavirus symptoms, veterinary scientists say" CNBC, 8th April, 2020. https://www.cnbc.com/2020/04/08/cats-should-stay-inside-if-owners-show-coronavirus-symptoms-vets-say.html
5. "Coronavirus: 'very unlikely' Hong Kong dog that tested positive died from Covid-19, source says, citing old age, underlying illnesses" South China Morning Post, 4thMarch, 2020. https://www.scmp.com/news/hong-kong/health-environment/article/3075770/coronavirus-very-unlikely-hong-kong-dog-tested
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