Last Updated on: April 3, 2020 by Ingrid King


Guest post by Ingrid R. Niesman, MS PhD

Yesterday we ran ran my piece titled One Cat Out of Million is Not a Trend: A Science-Based Look at the Unlikely Risk of COVID-19 Infection in Cats, which explained my analysis on ACE2 and cats, which supported the current belief that it is highly unlikely that cats could get infected by the novel coronavirus. The same day, Biorxiv, an open source platform for researchers to put up early and tentative results, published a paper by researchers in China that claimed that cats and ferrets can be infected. Papers published on Biorxiv are not peer-reviewed and represent data that might never eventually become published works. “Junk science” was a common response I received as I reached out to colleagues.

Taking a closer look at the study

Caveat number one: For this study, researchers directly infected five 8-9 month old cats deep into their nasal passages with 100,000 infectious virus particles. We still do not have data showing what a human viral dose would be, but 100,000 particles may be significant overkill for small cats and not close to what cats might experience in households with humans who have COVID-19.

Dr. Elizabeth Colleran, DVM, MS, DABVP and former President of the American Association of Feline Practitioner (AAFP) and owner of Chico Hospital for Cats  believes “that this study is a disservice to the public. It is important to understand that inoculating young cats with immature immune systems with an overwhelming viral dose and directly into their nasal passages is an artificial and unnatural model.” Further, she explains, “Cats are prone to respiratory infections in general, so having these cats infected in this manner, clearly will lead to some unusual results.”

After six days, pathology results on two of the cats both revealed viral RNA and infectious viral particles in the cats’ nasal passages, tonsils, soft palates and windpipes. “Not surprising,” according to Dr. Colleran.

The three remaining cats were then placed in cages next to three uninfected cats. After three days, the researchers detected viral RNA, not viral particles or viral DNA, in the feces of one of the originally uninfected cats. Pathology results of the newly infected cat also showed viral RNA in its nose, tonsils, soft palate and windpipe.

Based on these clinical indications, the researchers concluded, “that respiratory droplet transmission had occurred in this pair of cats.”

Caveat number two: Two of the three inoculated and previously uninfected cat pairs did not acquire transmissible disease even with the cages adjacent for prolonged periods (up to 10 – 11 days). It is known that unfortunately, cage cleaning in shelters is remarkably effective at spreading viruses, so one cannot rule out inadvertent infection in the cat that did test positive. I would need to see more stringent controls to draw the conclusion of respiratory transmission.

Viral particles capable of infection are found in the upper respiratory system of infected cats, but not in the lungs or small intestine. No infectious particles were determined to be present in feces.

You can still feel safe cleaning your litter boxes.

Caveat number three: All inoculated or infected cats displayed robust antibody responses against SARS-CoV-2, indicating healthy immune systems. No young adult cat died from acquired disease in the study. Antibodies are our natural protectors, and this level of antibody generation within a few days indicates that even small subclinical exposures for cats may be protective in the long run for feral or household pets. Humans should be so lucky.

Sadly, the same cannot be said for the kittens. Using the same viral dose to infect kittens intranasally was bound to cause disease; they weigh half as much and are still developing immune systems. Kittens succumbed to the disease within three days. “The degree of viral load is overwhelming here, completely artificial,” says Dr. Colleran. Also, she adds, “Kittens would have some maternal protection in natural settings, at least up to 6-8 weeks of age.”

Although troubling on face value, this particular study is based on a total of eight young adult cats and fewer than five kittens. Results from studies with those low subject numbers are not considered statistically valid. This is not to say that apparent colonization of the upper respiratory system by virulent virus particles and one transmission to a previously uninfected cat out of three is not concerning, but it shouldn’t be reason for panic.

What should you do with this information?

This information shouldn’t change your attitude or response to this crisis with regards to your cats. We should be keeping a watchful eye on our cats, just like we always do. If practical, keep your cats indoors to prevent accidental exposure to ill people outside of your home. Play with your cats, snuggle with them and enjoy them, just wash your hands.

We have nearly a million people infected currently. We had to forcibly infect cats. They are your comfort in this time, not your enemy.

And don’t sneeze on feral cats.

A note from Ingrid (King): It goes without saying that I consider the way this study was conducted cruel and unnecessary. I made the decision to provide this information  in hopes that it would reduce your worries about your cats. In order to do that, we had to explain how the study was conducted. Comments have been disabled for this post.

Ingrid R. Niesman MS PhD is the Director of the SDSU Electron Microscope Imaging Facility at San Diego State University. She graduated from Utah State University and received her MS from the University of Illinois-Urbana-Champaign. After 30 years of technical electron microscopy, cell biology, neuroscience and infectious disease research, Dr. Niesman completed her PhD in the UK at the University of Sunderland. Her work experience includes time at LSU Medical School, Washington University, UAMS in Little Rock, UCSD, TSRI and a postdoctoral year at CALIBR in La Jolla, CA. She has worked for at least two National Academy of Science members and is credited with over 50 publications. She can be reached at [email protected]