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SARS-COV-2: Why cats and not dogs?

“There is little to no evidence that domestic animals are easily infected with SARS-CoV-2 under natural conditions and no evidence to date that they transmit the virus to people.” – American Veterinary Medical Association (AVMA) on May 15, 2020

“To date, there is no evidence that animals play an epidemiologically relevant role in spread the human disease. Therefore, there is nojustification in taking measures directed at animals, particularly companion animals, which may compromise their welfare.” – World Organization for Animal Health (OIE)

Over the next few months, you will either hear or read about SARS-CoV-2 experiments on cats and ferrets. These are experiments conducted in laboratories. The cats and ferrets used in experiments usually receive large doses of the virus. The conditions are nothing like a home environment or how natural transmission would occur; under such conditions we only have a handful of companion animals testing positive around the world at this time. As well, we have no evidence of companion pet-to-human transmission.

With that being said, we want to tell you about a small research experiment from the University of Wisconsin that looked at whether or not cats could infect other cats with SARS-CoV-2. The researchers inoculated three kittens with the SARS-CoV-2 coronavirus and put them in separate cages. The next day, they introduced a kitten to each of the three occupied cages. After five days, virus was detected in the three uninoculated kittens. It is important to note that none of the kittens in this study showed any symptoms, including abnormal body temperature, substantial weight loss or conjunctivitis. Mild symptoms were reported in some cats in other studies, and the handful of cats that tested positive for coronavirus SARS-CoV-2 and live in home environments.

Bear in mind, this study only demonstrated that a heavily infected kitten can infect another kitten in a 22”x31”x42” inch cage. This experiment did not prove cat-to-cat transmission in a natural setting and certainly did not prove cat-to-human transmission.

Also, bear in mind that we have several questions about the experiment that was fairly abbreviated in a Letter to the Editor of the New England Journal of Medicine. Setting those questions aside, we need to discuss the research focus on cats rather than dogs.

SO, WHY CATS AND NOT DOGS?

Many animals have ACE2 receptors on the surface of their cells. By the way, ACE2 is short for angiotensin-converting enzyme 2. ACE2 receptors in humans help to stabilize blood pressure. So, they’re not necessarily bad and do serve a valuable health purpose.

Putting aside the cell for the moment, we need to talk about the word “corona”, which is Latin for “crown”. Coronaviruses have spike proteins on their surfaces that reminded the research discovery teams of monarchical crowns.

Now, we  need to put the two together. Think of the SARS-CoV-2 spike protein as a key and the ACE2 receptor as its lock. The SARS-CoV-2 spike protein locks on – or binds to – the ACE2 receptor very well in humans.

So, why does it bind to the ACE2 receptor in humans and not all animals with ACE2 receptors?

ACE2 receptors have different configurations in different species. Think of it this way, your house key won’t work on your car door.

So, why cats, ferrets, Syrian hamsters, and not dogs?

ACE2 receptors in cats and ferrets are very similar to those in humans, but ACE2 receptors in dogs are less structurally similar. Thus, making cats, ferrets and hamsters more susceptible.

Of course, the virus situation is more complicated than binding. After a spike protein binds onto an ACE2 receptor and gains entry into a cell, it needs to replicate efficiently to survive.

AVMA GUIDANCE

Remember, all of the naturally infected and experimentally infected cats have had no or relatively mild symptoms.

This is fair warning that you might be hearing about the outcomes of more experiments. We wanted you to know why they are being conducted so you can be powered by knowledge.

Indeed, with all of the experiments or natural infection news stories you may hear or read, please visit the AVMA or OIE websites for guidance as to how to protect yourself and your companion pets. We are posting the AVMA guidance below, which stems from an abundance of caution:

  • Make sure you have an emergency kit prepared, with at least two weeks’ worth of your pet’s food and any needed medications. Usually we think about emergency kits like this in terms of what might be needed for an evacuation, but it’s also good to have one prepared in the case of quarantine or self-isolation when you cannot leave your home.
  • Animal owners without symptoms of COVID-19 should continue to practice good hygiene during interactions with animals. This includes washing hands before and after such interactions and when handling animal food, waste, or supplies.
  • Do not let pets interact with people or other animals outside the household.
  • Keep cats indoors, when possible, to prevent them from interacting with other animals or people.
  • Walk dogs on a leash, maintaining at least 6 feet from other people and animals. Avoid dog parks or public places where a large number of people and dogs gather.
  • Until more is known about the virus, those ill with COVID-19 should restrict contact with pets and other animals, just as you would restrict your contact with other people. Have another member of your household or business take care of feeding and otherwise caring for any animals, including pets.  If you have a service animal or you must care for your animals, including pets, then wear a cloth face covering; don’t share food, kiss, or hug them, and wash your hands before and after any contact with them.
  • At this point in time, there is no evidence to suggest that domestic animals, including pets and livestock, that may be incidentally infected by humans play a role in the spread of COVID-19.
  • Routine testing of animals for SARS-CoV-2 is NOT recommended. Veterinarians are strongly encouraged to rule out other, more common causes of illness in animals before considering testing for SARS-CoV-2.

Human outbreaks are driven by person-to-person transmission. Accordingly, we see no reason to remove pets from homes even if COVID-19 has been identified in members of the household, unless there is risk that the pet itself is not able to be cared for appropriately.