#16,728
While COVID-19 is primarily a pandemic for humans - the SARS virus is believed to have originated in bats - and it has been detected in numerous, non-human hosts (see WHO/FAO/OIE Joint Statement On Monitoring SARS-CoV-2 In Wildlife & Preventing Formation of Reservoirs), raising concerns that new, unpredictable variants could develop, and jump back into humans at some future date.
It's not such a farfetched concern. In fact, we've already seen it happen (several times).
In the fall of 2020 SARS-CoV-2 jumped from humans to farmed mink in Denmark, and began to mutate into new mink-variants (see Denmark Orders Culling Of All Mink Following Discovery Of Mutated Coronavirus).
Several mutated viruses jumped back into humans, and began to spread in the community (see WHO 2nd Update: SARS-CoV-2 mink-associated variant strain – Denmark), forcing North Denmark To Lockdown Over Mutated Coronavirus Concerns.
This outbreak was relatively short-lived, as a more biologically `fit' Alpha variant emerged and quickly overwhelmed this `mink variant'.
Last July, in EID Journal: Peridomestic Mammal Susceptibility to SARS-CoV-2 Infection, we looked at the admittedly low, but worrisome possibility that COVID could establish itself in a non-human species, either in farmed livestock or in the wild.
Companion animals - primarily dogs and cats - are a special concern since they have so much direct contact with humans.
While we've seen no evidence that dogs and cats play a significant role in the spread of the virus, in recent weeks we've seen Hong Kong authorities taking a hardline stance on quarantining pets (see Hong Kong ACFD `Clarifies' Policy On COVID-Positive Pets).
The CDC's Animals and COVID-19 webpage (updated April 27th,2022) has this to say:
Risk of animals spreading SARS-CoV-2 to peopleWe are obviously still learning about how the SARS-CoV-2 virus interacts with other hosts. To that end we have a new research article in the CDC's EID Journal that looks at the rate of infection among dogs and cats, how long they remain infected, and their potential for spreading the virus.
Based on the available information to date, the risk of animals spreading COVID-19 to people is considered to be low.
At this time, there is no evidence that animals play a significant role in spreading SARS-CoV-2, the virus that causes COVID-19, to people. However, there have been reports of infected animals (mink, hamsters, and deer) spreading the virus to people during close contact, but this is rare. More studies and surveillance are needed to understand how SARS-CoV-2 is spread between people and animals.
Some coronaviruses that infect animals can be spread to people and then spread between people, but this is rare. This is what happened with SARS-CoV-2, which likely originated in bats.
While infection rates (via seropositivity) were high for both dogs and cats, both seem to clear the infection fairly quickly, which probably reduces their ability to pass it on. Dogs are considered less likely to do so than cats, but ". . . the actual risk for cat–human transmission is unknown (44).'
This is a long and detailed report, and I've only posted some excerpts. You'll want to follow the link to read it in its entirety.
Research
Risk Factors for SARS-CoV-2 Infection and Illness in Cats and Dogs
Dorothee Bienzle, Joyce Rousseau, David Marom, Jennifer MacNicol, Linda Jacobson, Stephanie Sparling, Natalie Prystajecky, Erin Fraser, and J. Scott Weese
Abstract
We tested swab specimens from pets in households in Ontario, Canada, with human COVID-19 cases by quantitative PCR for SARS-CoV-2 and surveyed pet owners for risk factors associated with infection and seropositivity.
We tested serum samples for to spike protein IgG and IgM in household pets and also in animals from shelters and low-cost neuter clinics. Among household pets, 2% (1/49) of swab specimens from dogs and 7.7% (5/65) from cats were PCR positive, but 41% of dog serum samples and 52% of cat serum samples were positive for SARS-CoV-2 IgG or IgM.
The likelihood of SARS-CoV-2 seropositivity in pet samples was higher for cats but not dogs that slept on owners’ beds and for dogs and cats that contracted a new illness. Seropositivity in neuter-clinic samples was 16% (35/221); in shelter samples, 9.3% (7/75). Our findings indicate a high likelihood for pets in households of humans with COVID-19 to seroconvert and become ill.
SARS-CoV-2 originated in horseshoe bats and probably reached humans through an unidentified intermediary host (1). The virus is aerosolized and highly transmissible among humans; new variants have arisen and spread in successive waves across the world since late 2019. Since a report of SARS-CoV-2 infection in a dog in March 2020 (2), an ever-increasing range of species has been shown to be susceptible to infection, including household cats, dogs, ferrets, and hamsters (3–10).
Companion animals have closest contact with humans, creating ample opportunity for exposure. Experimental infections have suggested that most companion animals are infected only transiently, as indicated by PCR positivity or virus isolation (11,12). Conversely, detection of antibodies by ELISA or neutralizing antibody assay suggests infection rates of 0.2%–43.9% related to factors such as the likelihood and frequency of interaction with infected humans (13–16). Infections in animals are typically subclinical or associated with transient respiratory or gastrointestinal disease (17,18).
In rare cases, death has been attributed to SARS-CoV-2 infection; however, defining the contribution of SARS-CoV-2 to death in animals with underlying conditions such as cancer, bacterial pneumonia, or obesity is challenging.
(SNIP)
Discussion
Our findings suggest that transmission of SARS-CoV-2 from infected humans to their pets as indicated by seroconversion is common. PCR-based detection of SARS-CoV-2 in pets was uncommon within 3 weeks from owners being symptomatic or having a diagnosis of COVID-19, which may reflect genuine brevity of infection in pets, as noted experimentally in cats (12).
Other factors are variations in time intervals between owner infection and pet sampling and the challenge of obtaining representative samples from the nose in cats (12). Other studies of infection of cats from households of persons with COVID-19 had similarly low PCR-based prevalence (16,25–28). The timeframe required for owners to be diagnosed, contact the study team, and arrange a household visit likely resulted in false negative PCR results from samples being collected too late relative to onset of infection. The definition of COVID-19 symptoms and access to PCR testing for sick persons was limited early in the pandemic, and it is possible that pets in this study were infected concurrently or immediately after their owners but swabbed only after they had eliminated infection.
Kittens 4–5 months old experimentally infected with 1 × 106 TCID50 of SARS-CoV-2 intranasally and orally had detectable viral RNA for 10 days in nasopharyngeal swabs, 7 days in oropharyngeal swabs, and 14 days in rectal swabs, but such high viral challenge may not simulate typical human–cat household interactions (12). Subtle pulmonary lesions and viral RNA detectable until 6 days postinfection in experimentally infected cats suggest that, even with high viral inoculates, cats rarely get sick and can clear infection relatively quickly (29).
Longitudinal samples were rarely available; however, serial sampling for 1 cat revealed prolonged PCR positivity. That cat had chronic upper respiratory disease; whether the condition played a role in the prolonged PCR positivity is unclear. Despite the duration of PCR positivity, it is unlikely that the cat was infectious because the relatively high PCR Ct values would be consistent with low-level shedding of viral nucleic acids. Similar prolonged PCR positivity has been reported for a cat exposed in a retirement home (30) and for tigers and lions in zoos (31). More data regarding the duration of positivity in naturally infected dogs and cats, and whether infectious virus is shed, are needed.
Seroprevalence was much higher than PCR positivity. We expected this finding because serologic data represent historical exposure and there is not a need to sample animals within a narrow infection window. Seroprevalence detected in other studies was 0.4%–30% or higher; in most instances such variability could be attributed to the extent of pets’ exposure to infected humans (6,9,32–34).
(SNIP)
Our study’s first limitation was sample size; enrollment was hampered by low human COVID-19 infection rates in the study region throughout the main sampling times and by difficulties identifying exposed households in an appropriate timeframe. Lack of a coordinated One Health approach concurrently investigating human and animal exposures was a problem; local or provincial public health agencies had little interest in leading research or performing a joint study. The timing of sampling also affected PCR results. More complete validation of the specificity of serologic assays with a samples from animals with diverse other infectious and inflammatory conditions remains to be done. Ideally, the timeframe for sampling would have been more condensed to focus testing on animals whose owners were more recently infected (e.g., 1–2 weeks after the onset of the owner’s infection).
These data indicate relatively common transmission of SARS-CoV-2 from humans to animals and that certain human–animal contacts (e.g., kissing the pet, pet sleeping on the bed) appear to increase the risk. We inferred that infections in dogs and cats reflect direct transmission from humans to animals, given the pandemic nature of this virus in humans and limited contact of most household pets with other animals (41). Intra-household transmission cannot be ruled out as a cause of some infections; however, multiple seropositive animals were only identified in 3/19 (16%) households where multiple animals were tested. We did not specifically investigate whether this relates to differences in individual animal susceptibility or animal–owner contact.
The relevance of human–pet transmission of SARS-CoV-2 needs further study. We observed an association between infection and clinical disease in both dogs and cats; in most cases, disease was very mild and self-limiting. Clinical data from this study are consistent with other studies indicating limited overall health risk to otherwise healthy dogs and cats (17,18,42).
The zoonotic risk posed by dogs is probably low based on the lower infection rate and lack of evidence of transmission experimentally (43). Risk is probably higher for cats; cat–cat transmission has been identified, but the actual risk for cat–human transmission is unknown (44). Our findings support the occurrence of human–dog and human–cat transmission and highlight the need for further study of the animal and human health consequences of spillback of this zoonotic pathogen into animals.
Dr. Bienzle is a professor of veterinary pathology at Ontario Veterinary College. Her research interests include infectious diseases of companion animals.
While there may be a theoretical risk of zoonotic transmission from a companion animal, or for the emergence of a canine or feline COVID `variant' - for now - your pets are at greater risk of being infected by you, than you are by them.
