Household transmission of SARS-CoV-2 from humans to dogs in Washington and Idaho: burden and risk factors

Background: SARS-CoV-2 is believed to have emerged from an animal reservoir as a zoonotic pathogen. Over the course of the current pandemic, evidence has mounted that infected humans can transmit the virus to animals including household pets, however the frequency of and risk factors for this transmission remain unclear. We carried out a community-based study of pets in households with one or more confirmed SARS-CoV-2 case among the human residents, and report here on interim findings from sampling of dogs. Methods: Data collection included a survey of human and animal demographic and clinical variables, features of their shared environment, and human-animal contact; blood collection from animals for serology for anti-SARS-CoV-2 antibodies; and nasopharyngeal sampling for PCR testing for SARS-CoV-2. Results: Sampling consisted of 67 dogs from 46 households. Nasopharyngeal PCR testing results were available for 58 dogs, and serological testing results were available for 51. Clinical signs consistent with COVID-19 were reported in 14 dogs (23.7%, 95% CI 0.13, 0.35), and SARS-CoV-2 antibody testing using viral receptor binding domain ELISA was positive in 22 dogs (43.1%, 95% CI 0.30, 0.57). All PCR tests of nasopharyngeal swabs were negative. Survey respondents commonly reported close human-animal contact, and the majority of households were aware of and adopted measures to mitigate human-to-animal transmission of SARS-CoV-2 following diagnosis. While no statistically significant associations were detected between human-animal contact variables and either seropositivity or COVID-19 like illness in dogs, positive trends were found for sharing beds with humans and the number of SARS-CoV-2 positive humans in the corresponding household. Reported measures taken by the household to mitigate transmission showed a protective trend, and COVID-19 like illness in a dog was positively associated with seropositivity in that dog. Discussion: These data indicate that human-to-animal transmission of SARS-CoV-2 in households is common, in a study population characterized by close human-animal contact. They also indicate that infected pets often manifest signs of COVID-like illness. While nasopoharyngeal sampling of dogs in this study has not to date demonstrated positive PCR results, this could be due to delays in sampling. Household members reported taking precautions to protect pets from SARS-CoV-2 infection, indicating an opportunity for further measures to reduce transmission of SARS-CoV-2 between people and animals sharing households.