Bat populations harbour hundreds of different types of coronavirus, including the SARS-Co-V-2 that causes Covid-19. As individual bats become infected with multiple strains, they create new types, which they then spread. Occasionally a new type jumps to another animal species, for example a civet (SARS), camel (MERS), or a pangolin (possibly the intermediate host for SARS-CoV-2). From there the novel coronavirus jumps again to the human population. It is, therefore, a “zoonosis,” a disease that is naturally transmissible from vertebrate animals to man.

It is also known that diseases can jump from humans to other animals, including domestic pets. “Reverse zoonotic” diseases are far less well studied and understood than zoonoses, although a survey of literature to 2014 identified cases of diseases transmitted from people to “wildlife (n = 28, 50%), livestock (n = 24, 43%), companion animals (n = 13, 23%), and various other animals or animals not explicitly mentioned (n = 2, 4%). Published reports of reverse zoonoses transmission occurred in every continent except Antarctica therefore indicating a worldwide disease threat.”

The latest research suggests Covid-19 is also a reverse zoonosis.

Until now it has been assumed that cats and dogs are not likely to be infected by Covid-19. The relevant gov.uk page advises that, “there is no evidence of coronavirus circulating in pets or other animals in the UK and there is nothing to suggest animals may transmit the disease to humans.” It suggests that an infected person asks someone else to walk the dog. Cat owners are encouraged to wash their hands. The Centres for Disease Control and Prevention (CDC) in the United States has been somewhat more circumspect, advising that infected humans should limit interaction with their pets, as they do other members of their households.

Three brand new studies suggest we must be very cautious, because both dogs and cats not only act as hosts for SARS-CoV-2, it appears they can also contract Covid-19.

A study pre-published on 30 March (not yet peer-reviewed), “Susceptibility of ferrets, cats, dogs, and different domestic animals to SARS-coronavirus-2” concludes that “SARS-CoV-2 replicates poorly in dogs, pigs, chickens, and ducks, but efficiently in ferrets and cats”. Although this study found that dogs have a lower susceptibility to the novel coronavirus, a Pomeranian dog in Hong Kong has now died of Covid-19 and authorities are quarantining all pets of humans confirmed to have contracted the disease. A second dog in Hong Kong has tested positive. Neither dog showed any symptoms.

We cannot assume that because pets become infected that they are likely to spread the virus. Children, more very recent research suggests contract the novel coronavirus as readily as adults, but are not so effective at spreading it within households. However, a second study, pre-published on 31 March, suggests we can both pass the virus to our pets and receive it from them.

“Atlas of ACE2 gene expressions in mammals reveals novel insights in transmission of SARS-CoV-2” focused on the transmembrane protein ACE2 (angiotensin-converting enzyme 2). Scientists have previously shown ACE2 to be the “SARS-CoV-2 receptor”, meaning that the novel coronavirus enters cells by binding itself to ACE2. The greater the presence, or “expression”, of ACE2 in a cell, the greater the chance that the novel coronavirus can enter that cell.

In humans, ACE2 is abundantly expressed in tissue lining the airway – the mouth, nose, and throat – and in the lungs, which is why the SARS-CoV-2 manifests as a respiratory disease. A correlation between the amount of ACE2 in the lungs and the severity of Covid-19 has been observed. Those exposed to lung irritants, including particulates from air pollution and inhaled smoke, appear to produce more ACE2, which is intended to protect the lungs. However, more ACE2 facilitates entry of coronavirus and may result in more severe infection and inflammatory lung disease.

ACE2 is not limited to cells in the respiratory system. It is highly expressed in the intestines of humans, which means ingesting meat from mammals harbouring SARS-CoV-2 (for example, pangolin) can lead to infection. ACE2 is also present in the eyes (corneal epithelial cells), suggesting they are vulnerable to Covid-19 infection.

ACE2 was highly expressed in many other mammals, including bats and pangolins examined by the Chinese authors of the “Atlas of ACE2 gene expressions in mammals.” Most worryingly for most of us, (I have improved the English), the authors conclude that “our analyses suggest a high probability that cats and dogs can host SARS-CoV-2. Furthermore, in both dogs and cats, skin had a high and abundant expression of ACE2. So did the ear tips of cats and the eyes of dogs.” This makes it possible that the novel coronavirus can be passed between humans and their dogs and cats simply by stroking and petting.

A third study pre-published on 2 April, confirms that the cat population of Wuhan was infected during the novel coronavirus outbreak. Of 39 samples taken from cats before the outbreak (a fortuitous fact presumably related to a different study), none tested positive. Of 102 samples taken after the outbreak, 15 (14.7%) tested positive. The authors of the study recommend that (again, English is improved), “More studies are needed to investigate the transmission route of SARS-CoV-2 from humans to cats. Importantly, immediate action should be taken to keep a suitable distance between humans and companion animals such as cats and dogs, and strict hygiene quarantine measures should also be carried out for these animals”.

This has enormous potential ramifications. Social distancing has increased human-animal interaction. People are spending far more time at home, petting and playing with their dogs and cats, stroking their ferrets, and exercising their gerbils (probably the vector for the Antonine Plague).

People are spending far more time at home, petting and playing with their dogs and cats

There has been a well-documented spike in the number of people adopting and fostering from animal shelters. Children have brought their class pets home and are looking after them for the duration of school closures. Millennials are buying puppies now that they find they have enough time at home to attempt toilet training. Animals offer comfort and companionship to those self-isolating. The may also become reservoirs of the virus or may die from the disease it causes.

As we seek answers for why some regions seem to have higher rates of infection and transmission, all factors might be considered before they are discounted, including prevalence of stray and abandoned dogs and cats, rates of pet ownership, types of pets kept in infected households, and the age and vulnerability of pet owners.

For example, Italy has one of the highest rates of cat ownership in the world. In 2017, Dalia research surveyed 43,034 people in 52 countries to conclude that 59% of Russians own a cat. The largest numbers of Europe cat owners lived in Belgium (42%) and Italy (40%). The UK (32%) lagged behind France (39%), Poland (39%), the Netherlands (35%), Sweden (33%), and Switzerland (33%). Only 26% of Germans own a cat.

In the USA, pet ownership was already at record highs before the recent spike. Data varies by source, and a survey by the US National Pet Health Insurance Association, estimated that Americans have 94.2 million cats and 89.7 million dogs in their homes. According to a different study, Pet Population and Ownership Trends in the US, in 2017, 55% of US households owned a pet. Millennials had surpassed Baby Boomers as the largest demographic, but pet ownership among those aged 70 or older, the most vulnerable to Covid-19, also grew 10% in the decade to 2017. Now, 40% of the Boomer generation own pets.

These are all pre-published studies, and peer review will challenge and may undermine their findings. However, we cannot now fully discount the role of domestic and stray animals in the transmission of novel coronavirus. We must also be aware that they may act as disease reservoirs, storing the virus in an around our own homes. Most will be asymptomatic.