It can be difficult these days to follow the news and find reliable information about the novel coronavirus that has profoundly changed our daily lives. Information about how the virus affects our pets is no exception. New information is coming out all the time and will be incorporated as it becomes available.
First and most importantly there is no evidence that domestic animals can transmit COVID-19 to humans or of any risk to humans from their pets, in fact all evidence is to the contrary. There is however an increasing body of evidence pointing to the ability of humans to infect domestic animals with COVID-19 in certain rare conditions. This appears to result in a weak and asymptomatic infection that does not produce community spread.
With over 1 million confirmed cases (representing a small percentage of total infections) of COVID-19 since it first appeared in December of 2019 there have only been three reported instances of "natural" transmission of the virus from humans to their pets. Animals such as cats and dogs which have direct contact with large numbers of people on a daily basis are extremely unlikely to play any role in the transmission of COVID-19 because extensive evidence would already exist that this was taking place. In other words, the strongest evidence that there is no risk to our household pets is the continued observable lack of evidence otherwise. The species that we should be concerned about are those that have infrequent contact with a small numbers of people such as certain animals that contribute to our food supply or exotic animals such as big cats.
Active COVID-19 Infections
Globally, millions of infected human to cat interactions take place daily. Despite the frequency there are currently only two reports of domestic cats testing positive for COVID-19. Both were tested after being in continual contact with their infected owners.
The first was in Belgium and was showing potential signs of a gastrointestinal illness but no clinical signs of COVID-19. The other cat tested positive on March 31st in Hong Kong after its owner was confirmed positive for the virus and has since remained asymptomatic. The Belgium case is now testing negative after having been removed from the source of the infection.
COVID-19 Antibodies Found in Wuhan Cats
A study out of China appeared on biorxiv.org last week claiming cats can be infected and develop antibodies to COVID-19. BioRxiv is a "preprint" server which makes preliminary findings available before a study has been published and peer reviewed. Under normal circumstances this would not be considered a reliable source as the methods and conclusions may be found fundamentally flawed after community evaluation. For this reason the findings must be viewed with caution. Because of the speed at which information is currently evolving it has become necessary to include a broader range of sources.
The study evaluates serum samples from 39 cats in the Wuhan Province before the outbreak and 102 individuals after the outbreak had moved through the region. The cats in the former group showed no signs of COVID-19 antigens while 15 out of the 102 (14.7%) tested post outbreak were positive for viral antigens as well as several individuals that were positive for neutralizing antibodies against the virus. None of the cats showed clinical symptoms and no shedding of the virus was detected.
Assuming the study methods are determined to be sound there are many reasons why the results are not overly concerning to the veterinary community. Viral tropism is the ability of a virus to infect specific types of cells. This can refer to a virus's ability to infect a specific host but also refers to which type of cells within that host are compatible. It has been shown that COVID-19 requires ACE2, a membrane-bound aminopeptidase as its functional receptor to bind to a cell. Unfortunately, we don't have a study showing the ACE2 distribution in felines but we do know it has been identified as structurally compatible with COVID-19 in a study published on January 29th, 2020. The distribution (amount and location) of ACE2 within an individual or compatible species is directly correlated with intensity of the illness and its ability to replicate. Another study from 2013 found a correlation between chronic kidney disease and increased ACE2 in dogs and potentially cats. Studies have shown the same correlation in human cardiovascular disease, pulmonary disease, and other chronic conditions which is why individuals with these conditions are disproportionately affected by the virus.
Given the low infection rate reported, the lack of clinical symptoms, and the absence of shedding it is likely that ACE2 distribution in cats is too low for the virus to effectively replicate, that other cellular features interfere with binding, or that individuals with chronic health conditions such as kidney or heart disease are the ones showing the ability harbor the virus due to increased ACE2 availability.
A study posted March 31st to bioRxiv claims that COVID-19 replicates effectively in cats and ferrets. The main caution here, beyond remembering this study has not been peer reviewed, is that the exact same findings were published for SARS in 2008. Neither species has ever been shown to have played a role in the spread of SARS or have developed communal spread and the virus was never identified in pet populations by the veterinary community. This is significant because the coronavirus colloquially referred to as SARS uses the same ACE2 receptor as COVID-19 to bind with cells. Feline Coronavirus, which is highly contagious among cats does not use this receptor.
Big cats are a different story. The USDA issued a press release confirming a Tiger at the NY Zoo has tested positive for COVID-19. It further describes what appears to be community spread between the big cats at the zoo, affecting both tigers and lions. These cats are also said to be symptomatic, showing signs of respiratory disease. This is the type of scenario that would have developed within domestic cat and dog communities if those species were compatible hosts for the virus. Owners/caretakers of big cats should take precautions when interacting with the animals, preparing food, and cleaning their enclosures and isolate sick animals from humans and from other cats.
There have been millions of interactions between humans actively shedding COVID-19 and their companion animals since the virus was first identified in December. Despite many pets living in virus saturated conditions there have been no significant reports of illness occurring in pets and absolutely no evidence of pets shedding the virus. Companion animals were found to be similarly susceptible to the 2003 SARS virus under laboratory conditions but the virus has never been successful in replicating within the canine or feline communities. There is no reason to believe that COVID-19 will begin to affect species that have close and frequent contact with humans, evidence would already be apparent after months of exposure. Concern should lie with species that have infrequent human contact and have potentially not yet been exposed to the virus.
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