The outbreak of novel coronavirus disease 2019 (COVID-19) due to severe acute respiratory syndrome (SARS-CoV-2) has been determined to be caused by a zoonotic organism, likely bats. The virus adapts to new host species by natural selection exerted on random mutations in the genome.
A new study examines how dogs and cats within households are likely to be infected with the virus from humans, and whether they act as reservoir species, posing a danger of transmitting the infection to unexposed humans or not immune.
This virus is found in the form of quasi-species, or variants with minor genomic variations between them, found in the same individual host. Variants in the same infected individual are selected by their higher tropism for the host, which can manifest itself in viral transmission between species.
This makes surveillance essential for non-human species, to understand how the virus adapts, changes and spreads among a wider range of species. Indeed, cats and dogs raised in a human household are at risk of being infected with the virus. Virus and antibodies have been detected in pet feces and/or respiratory secretions.
The current serological study, published in the journal Microorganismsexplored the epidemiology of this infection in pet cats and dogs.
Previous studies have shown that cats are easily infected with the virus, shed the virus at higher levels, develop respiratory disease, and transmit the virus to other cats more efficiently than dogs, which are refractory to infection. by this virus. In the current study, one in ten pets, cats and dogs, were seropositive for SARS-CoV-2.
This included more than a fifth of cats screened but less than one in twenty dogs. Nearly nine out of ten pets were HIV-negative. Nearly half of HIV-positive cats came from COVID-19 positive families, with less than one in seven cats coming into contact with COVID-19 positive neighbours’ cats. Cats from neighboring families were not tested for the virus, however.
Interestingly, no cats or dogs under the age of one were HIV positive. Among cats, the greatest risk was in those aged 1 to 3 years, compared to 8 years and older in dogs. No sex difference was found in the two species, or with access to the outside.
The animals in the shelters were always HIV-negative.
Clinical signs were present in 60% of the seropositive cats, including respiratory signs in about 45%, neurological signs or lethargy with lack of appetite in more than a fifth each, intestinal symptoms in more than a third and fever in just over one in ten. Two out of three HIV-positive cats recovered from their symptoms, the rest died or had to be put down. Only one in seven dogs were symptomatic, showing intestinal symptoms.
Swabs for viral polymerase chain reaction (PCR) testing were only present in four dogs and one cat, and only one dog, without any symptoms, showed the presence of viral genetic material. This happened a week after the owner tested positive. Canine viral shedding was sustained for the next six days, in nose, throat, and rectal swabs.
While this dog was HIV-negative in the first sample, at the same time the swabs were taken, the second sample was positive, 16 days from the first.
Among the seropositive animals, one cat became seronegative at three months, one dog after one month and one after nine months. Results of a dog’s Spike gene sequencing have shown what appears to be the Alpha variant of the virus.
The results of this study extend previous findings that showed SARS-CoV-2 infection may be present in domestic cats, dogs, and ferrets, as well as farmed mink, lions, and ferrets. tigers in captivity and gorillas in zoos. All of these are believed to have been infected through human contact, showing a higher rate of spread from humans to animals than previously thought.
The spillover in the opposite direction has only occurred in mink farms and is attributed to the rapid emergence of mink-adapted strains. Wild mink and white-tailed deer have been found to have been exposed to the virus, which may indicate the creation of a reservoir from which future outbreaks could be expected.
Domestic cats, dogs, ferrets, transgenic mice and many other species have been experimentally infected with the virus, while cattle, chickens, ducks and pigs appear to be resistant to it.
In this study, cats and dogs were infected at high rates under household conditions. They may be more susceptible to coronavirus infections as a whole, have been experimentally infected with this virus, and are also more intensively exposed to infected humans. Cats also effectively transmit the virus to other cats.
These animals can be tested, both to keep them healthy and prevent the virus from adapting or evolving further to its new host, but also to understand the risk of creating animal reservoirs or contamination of the environment by the virus via these pets.
Cats seem to be more susceptible, even in a study done at a time when the incidence was lower than it is now. Additionally, even cats in COVID-19 negative families appear to have been infected from outside the home, indicating easy transmission of the virus.
The highest risk, however, was when pets came from a family infected with human SARS-CoV-2, even in households with multiple pets. Only adult pets have been infected, confirming previous studies.
While shelter dogs were universally negative, a previous study showed that shelter cats were HIV positive in 2% of cases, indicating a greater risk of infection in cats with such conditions. More than a quarter of HIV-positive cats and dogs in this study had access to the outdoors, indicating that they could have acquired the infection from other animals or environmental contamination.
While a third of HIV-positive cats died or had to be put down and 60% showed symptoms, only one in seven HIV-positive dogs were symptomatic. Cats may be more likely to develop symptomatic and severe disease as well as die after SARS-CoV-2 infection. This contradicts an earlier study where more than 80% of infected animals were asymptomatic.
Initial testing of potentially infected animals could be misleading and repeat testing should be done two weeks after contact. Antibodies declined to undetectable levels in as little as a month in a dog, indicating that humoral immunity is not long-lasting in pets. Further studies will help understand this crucial feature of immunity in pets.
Finally, the virus spike protein could adaptively mutate in pets due to positive selection, and many non-human mutations have been found in virus recovered from animals.
The data obtained point to the need for further epidemiological studies and the need to implement animal surveillance plans for SARS-Co-2 in animals in the future.“, explained the researchers.