Study which claims cats can “easily” transmit SARS-CoV-2, the virus which causes COVID-19, to each other and expresses concern about transmission to people shows no such thing.
Late last week, CBS News published a somewhat alarmist article called, “Study shows cats can easily spread coronavirus to each other”. The article also expressed concern about cat-to-human transmission of SARS-CoV-2, the virus that causes COVID-19. Similar articles appeared in The New York Times, CNN, USA Today, Fox News, and other news outlets, all citing a new “study in the New England Journal of Medicine”.
The study showed no such thing, as will be discussed below.
The misreading of the study aside, the reporters appear to have been misled by the study authors themselves, who drew conclusions and made claims not justified by their own data. For example, the lead author concluded in the study that “there is a public health need to recognize and further investigate the potential chain of human–cat–human transmission” and further claimed that, “a better understanding of the role cats may play in the transmission of SARS-CoV-2 to humans is needed” if we are going “to stop the coronavirus disease 2019 pandemic.”
Does stopping the pandemic hinge on study of cat-to-human transmission of the virus? No.
First and foremost, the study did not look at whether cats can transmit the virus to people so there is no basis for such a dramatic conclusion. It is a non sequitur.
Second, no other study has shown that cats have the potential to transmit the virus to people.
Third, the Centers for Disease Control, the American Veterinary Medical Association, the U.S. Department of Agriculture, and the World Health Organization have all stated that no evidence suggests that they do.
Fourth, there are no real world cases of any cats having done so.
Sixth, the “study in the New England Journal of Medicine” was not a study in the New England Journal of Medicine. It was a “Letter to the Editor” by the study authors to the New England Journal of Medicine. That indicates the study was neither peer reviewed, nor accepted as a study for publication. It was less than one page in length (excluding images), begins “To the editor,” and is published under “Correspondence.”
Finally, the lead author admitted to a potential conflict-of-interest which can be found in supplemental materials provided separately. Specifically, he receives funding from several sources to do these kinds of studies and his urgent call for additional studies does not ethically play well in terms of his alarmist claims, especially given its potential to negatively impact the welfare of cats.
So what does the study actually prove?
In analyzing the study, I will (once again) focus on the methodology. What I won’t dwell on — though it should go without saying — are the ethical issues inherent in any study that would deliberately attempt to inflict on sentient, non-human animals a disease so grave in terms of potential suffering and death that we have all collectively adopted extreme measures, including shutting down the economies of the world, to avoid contracting it ourselves.
Researchers took three cats and intentionally injected them with a large dose of SARS-CoV-2 in the nose, throat, mouth, and eyes. These cats were then individually placed in a cage with another cat who had no previous infection, to see if the other cats contracted the virus.
What does the study show?
In a laboratory setting, when massive amounts of the virus was introduced directly into the nose, throat, mouth, and eyes, cats were possibly capable of transmitting it to other cats. I say possibly for several reasons. First, none of the six total cats (the three who were injected with the virus and the three other cats placed in cages with them) showed any symptoms.
Second, while the three cats with no prior infection tested positive in a nasal swab, all three tested negative with a rectal swab. As the study authors admit, “No virus was detected in any of the rectal swabs tested.” As such, cats being “dead end hosts” — in which “The virus may be able to infect tissues or cells in a host, say, the respiratory tract, but they’re not able to complete the life cycle in terms of transmitting to a new host” — cannot be ruled out.
Third, none of the cats were positive when tested again by nasal swab a week later.
What does the study not show?
The study does not show that cats, infected or not, can transmit COVID-19 to humans.
This study also does not show that infection will readily occur naturally outside a laboratory environment. For example, a typical human cough can spray roughly 3,000 large particles. A sneeze, by contrast, can result in upwards of 40,000. A human who is in the near presence of a sneezing human will inhale only a fraction of that. But the study in question injected the cats with 100,000 particles directly up their noses, in their mouths, in their throats, and in their eyes, a dose several orders of magnitude of what would be found in a community impacted by COVID-19. In short, the study was done in extreme conditions.
The study does not show that transmission to other cats will naturally occur outside a laboratory environment or even that it can “easily” be done so. Thankfully for the sake of the cats, the sample sizes were so low we have no information about what the transmission rate may be.
What should we do?
The lead author’s call and desire for more research grant funding aside, nothing in the study shows that we should fear cats, round up community cats, require surveillance of cats, quarantine cats, or stay away from our cats. To date, only two U.S. cats have tested positive, they did not infect other cats in their household, and they readily recovered. There is no evidence whatsoever they can transmit it to people. To the extent cats can get it from people, it appears to be extremely rare, mild, and resolves on its own.
The “Letter to the Editor” describing the study and entitled “Transmission of SARS-CoV-2 in Domestic Cats,” is here.
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