SARS-CoV-2 at the human-pet interface: transmission and pre-existing immunity in pets within infected households

This exploratory study investigated SARS-CoV-2 transmission dynamics and risk factors in companion animals from COVID-19-positive households in Madrid, Spain. The research was conducted during the early pandemic lockdown, providing a unique scenario of intensified human-animal cohabitation within domestic "transmission units."

A total of 87 pets (18 cats and 69 dogs) were enrolled through voluntary recruitment in veterinary clinics in Madrid. Exposure was assessed using RT-qPCR from nasopharyngeal swabs and the detection of specific IgM/IgG antibodies against SARS-CoV-2. Humoral immunity against common canine pathogens (CAV, CPV, and CDV) was also evaluated. Owners completed epidemiological questionnaires regarding pet habits, environment, and owner-reported clinical signs to assess potential risk factors.

While all animals were negative by RT-qPCR, specific antibodies were detected in 27.8% (5/18; 95% CI: 12.5-50.9%) of cats and 30.4% (21/69; 95% CI: 20.8-42.1%) of dogs. Seroprevalence was comparable between households of healthcare and non-healthcare workers (36.4% vs. 33.3% in dogs). No significant associations were found between individual risk factors and SARS-CoV-2 seropositivity. However, in dogs, reported symptoms were significantly associated (p < 0.05) with vaccination status, social contact, and diet. Notably, a strong and highly significant association (p < 0.001; V > 0.6) was observed between SARS-CoV-2 humoral immunity and antibodies against common canine pathogens (CAV, CPV, and CDV).

These exploratory findings suggest that pets are integral members of domestic transmission units, showing high exposure levels regardless of the owner's healthcare professional status. The correlation observed with endemic canine pathogens may be associated with the pet's immune background, although this requires further mechanistic validation through longitudinal studies. Given the limited sample size, particularly in the feline cohort, these results offer preliminary insights into the multifactorial nature of infection at the human-animal interface and support the value of integrated One Health surveillance to enhance global preparedness against zoonotic threats.