The outbreak of novel coronavirus disease 2019 (COVID-19) due to the severe acute respiratory syndrome (SARS-CoV-2) was determined to be caused by a zoonotic organism, probably from bats. The virus adapts to new host species via natural selection exercised upon random mutations in the genome.
A new study looks at how dogs and cats within households are susceptible to infection with the virus from humans, and whether they act as reservoir species, posing a danger of transmitting the infection back to unexposed or non-immune humans.
This virus is found in the form of quasispecies, or variants with minor genomic variations between themselves, found in the same host individual. Variants in a single infected individual are selected by their higher tropism for the host, which may be shown by viral transmission between species.
This makes surveillance a must for non-human species, so as to understand how the virus adapts, changes and spreads among a wider range of species. Indeed, cats and dogs brought up in a human home are at risk of infection with the virus. Both the virus and antibodies have been detected in pet animal feces and/or respiratory secretions.
The current serologic study, published in the journal Microorganisms, explored the epidemiology of this infection in pet cats and dogs.
Earlier studies demonstrated that cats are readily infected by the virus, shed the virus at higher levels, develop respiratory disease and spread the virus to other cats more efficiently, compared to dogs, who are refractory to infection by this virus. In the present study, one in ten pets, both cats and dogs, were seropositive to SARS-CoV-2.
This included over a fifth of the screened cats but less than one in twenty of the dogs. Almost nine out of ten pets were seronegative. Almost half the seropositive cats came from families that were positive for COVID-19, with less than one in seven being in contact with cats from neighbors who were COVID-19-positive. The cats from the neighboring families were not tested for the virus, however.
Similarly, <7% lived with other cats from the same COVID-19-positive household, but which had not been tested. For dogs, too, 4/7 seropositive dogs were from COVID-19-positive families. In both cases, living with virus-positive humans increased the risk of being seropositive.
Interestingly, no cats or dogs less than a year old were seropositive. Among the cats, the greatest risk was among those aged 1-3 years, vs 8+ years in dogs. No sex differences were found in either species, or with access to the outdoors.
Pets from shelters were always seronegative.
Clinical signs were present in 60% of seropositive cats, including respiratory signs in ~45%, neurologic signs, or lethargy with poor appetite in over a fifth each, gut symptoms in over a third, and fever in just over one in ten. Two out of three seropositive cats recovered from their symptoms, the rest died or had to be put down. Only one in seven dogs was symptomatic, showing gut symptoms.
Swabs for viral testing by polymerase chain reaction (PCR) were present in only four dogs and one cat, and only one dog, without any symptoms, showed the presence of viral genetic matter. This occurred a week after the owner became test-positive. Canine viral shedding was prolonged for the next six days, in the nose, throat and rectal swabs.
While this dog was seronegative in the first sample, at the same time the swabs were collected, the second sample was positive, at 16 days from the first.
Of the seropositive animals, one cat became seronegative at three months, one dog after one month and one after nine months. Spike gene sequencing results from one dog showed what appears to be the Alpha variant of the virus.
The results of this study extend earlier findings that showed that SARS-CoV-2 infection could be present in domestic cats, dogs, and ferrets, and in farmed mink, captive lions and tigers, and gorillas in zoos. All of these are suspected to have been infected by human contact, showing a higher rate of spillover from humans to animals than previously thought.
Spillover in the reverse direction occurred only on mink farms and is attributed to the rapid emergence of mink-adapted strains. Both wild mink and white-tailed deer have been found to have been exposed to the virus, which could indicate the creation of a reservoir from where future outbreaks could be expected.
Domestic cats, dogs, ferrets, transgenic mice, and many other species have been experimentally infected with the virus, while cattle, chicken, ducks and pigs seem to resist it.
In this study, both cats and dogs were infected at high rates in domestic conditions. They may be more susceptible to coronavirus infections as a whole, have been experimentally infected with this virus, and are also exposed to infected humans more intensively. Cats also spread the virus to other cats efficiently.
Such animals may be tested, both to keep them healthy and keep the virus from adapting or evolving further to its new host, but also to understand the risk of creation of animal reservoirs or contamination of the environment with the virus through these domestic animals.
Cats seem to be more susceptible, even in a study carried out at a time when the incidence was lower than at present. Moreover, even cats in COVID-19-negative families seem to have been infected from outside the house, indicating ready transmission of the virus.
The highest risk was, however, when the pets came from a family with human SARS-CoV-2 infection, even in households with multiple pets. Only adult pets were infected, confirming earlier studies.
While dogs from shelters were universally negative, an earlier study showed shelter cats to be seropositive in 2%, indicating a greater risk of infection among cats under such conditions. Over a quarter of seropositive cats and dogs in this study had outdoors access, indicating that they could feasibly have contracted the infection from other animals or from environmental contamination.
While a third of seropositive cats died or had to be put down, and 60% showed symptoms, only one of seven seropositive dogs was symptomatic. Cats may be more likely to develop the symptomatic and severe disease as well as to die following SARS-CoV-2 infection. This contradicts an earlier study where over 80% of infected pets were asymptomatic.
The initial testing of potentially infected animals could be misleading, and repeated testing should be done two weeks from contact. Antibodies waned to undetectable levels in as little as one month in one dog, which indicates that humoral immunity is not durable in pets. More studies will help understand this crucial feature of immunity in pets.
Finally, the spike protein of the virus could undergo adaptive mutation in pets due to positive selection, and many non-human mutations have been found in the virus recovered from animals.
The data obtained emphasize the need for further epidemiologic studies, and the need to implement animal surveillance plans for SARS-Co-2 in animals in the future”, explained the researchers.