Researchers at Colorado State University have made important discoveries about domestic cats infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that could address concerns about human-to-animal transmission and the role domestic pets might play in spreading the virus.
The team also provides evidence that cat models of SARS-CoV-2 infection could help researchers understand viral spread in humans and further the development of vaccines for both humans and animals.
Richard Bowen and colleagues from the university’s College of Veterinary Medicine and Biomedical Sciences found that, while cats were susceptible to SARS-CoV-2 infection, they did not exhibit clinical symptoms, cleared the infection quickly, and developed an immune response that prevented them from being re-infected.
The team says this suggests that if a symptomatic human stayed indoors with a potentially exposed cat, that cat would be unlikely to infect another human.
A pre-print version of the paper is available in bioRxiv*, while the article undergoes peer review.
Novel Coronavirus SARS-CoV-2 This scanning electron microscope image shows SARS-CoV-2 (round magenta objects) emerging from the surface of cells cultured in the lab. SARS-CoV-2, also known as 2019-nCoV, is the virus that causes COVID-19. The virus shown was isolated from a patient in the U.S. Image captured and colorized at NIAID's Rocky Mountain Laboratories (RML) in Hamilton, Montana. Credit: NIAID
Transmission between different types of hosts
The high human-to-human transmission rate of SARS-CoV-2 has meant the coronavirus disease 2019 (COVID-19) pandemic that began in Wuhan, China late last year has now reached almost every country in the world.
Research has suggested that the virus probably originated in animals (possibly bats) before infecting people via an intermediate animal host (zoonotic transmission) and then adapting to humans for efficient human-to-human transmission.
However, since the virus is genetically similar to SARS-CoV-1, which infects many animal species, concerns have arisen about reverse zoonotic (human to animal) transmission and the role that domestic animals may play in the spread of COVID-19.
“While animals, including domestic animals and pets, are frequently implicated as the source of emerging pathogens, reverse zoonosis of SARS2 is more probable, as human cases are far more prevalent than domestic animals,” said Bowen and colleagues.
They add that there have been various reports of pets becoming infected following exposure to infected humans in Hong Kong, Belgium, New York, Spain, Germany, and France.
What did the current study involve?
Now, Bowen and colleagues have investigated the susceptibility of domestic cats and dogs to SARS-CoV-2 and analyzed viral shedding kinetics, virus neutralization, seroconversion, and transmission to uninfected animals.
Seven adult cats (aged 5 to 8 years) and three adult dogs (aged 5 to 6 years) that tested negative for coronavirus antibodies were obtained from a pathogen-free breeding colony at Colorado State University.
Three of the seven cats and all of the dogs were infected with SARS-CoV-2 and then had oropharyngeal and nasal samples taken up to 14 days post-infection, and blood samples were taken up to 42 days post-infection.
Two of the remaining four cats were also challenged with SARS-CoV-2, and 48 hours later, the remaining two naïve cats (contact cats) were introduced into the same room as these two infected cats.
All the cats were euthanized at the end of the study, and their tissues collected for histopathological analysis.
None of the animals exhibited symptoms of disease
None of the animals exhibited any clinical signs of disease such as fever, coughing, sneezing, or difficulty breathing at any point during the study.
The first three cats shed the virus both orally and nasally for up to five days following infection, with peak titers for nasal shedding observed on day three.
“Importantly, infected cats shed for no more than 5 days following exposure, suggesting that cats, if exposed to infected humans, will develop and clear infection rapidly,” said Bowen.
“Thus, if symptomatic humans follow appropriate quarantine procedures and stay home with their pets, there is minimal risk of a potentially exposed cat infecting another human,” they add.
At 28 days following infection, the first three cats were re-challenged with SARS-CoV-2, and no viral shedding was detected at any point after that.
The authors say that to their knowledge, “this is the first report of protective immunity against SARS2 following repeated exposure.”
The next two cats also shed the virus orally and nasally for up to five days, and the contact cats started shedding the virus orally 24 hours post-exposure.
Viral shedding was not observed among any of the dogs at any point following infection. The dogs were not re-challenged, and none of them were euthanized.
“This is not altogether different from human infections”
Histopathologic analysis of all sacrificed cats showed pathological changes in the lungs, suggesting that subclinical disease did occur, but did not present with recognizable symptoms.
“This is not altogether different from human infections, where the majority of cases are relatively mild but more severe disease tends to occur in older patients with significant comorbidities,” writes the team.
The researchers say the study suggests that cats may serve as a useful model for studying SARS-CoV-2 infection and for research into vaccines that could be used for both humans and animals.
“Resistance to re-exposure holds a promise that a vaccine strategy may protect cats, and by extension humans, to disease susceptibility,” they conclude.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.