Animals
Dogs were provided by local dog owners and screened for inclusion in the study. The initial screening phase included the assessment of the effectiveness of food as a reinforcer for each dog and the dog's suitability for work in a laboratory environment in the presence of non-familiar people. Out of the 6 dogs that met the inclusion criteria, 3 dogs underwent the entire training session, while the others were exempted, due to either health or management problems, or low motivation for working in the scent line-up. One 8 years old Belgian Malinois female dog (Nala), an 8-year-old female mixed-breed dog (Helix), and a 5-year Dachshund male dog (Otto) underwent a training period (two weekly training sessions) to discriminate between the sweat of people with COVID-19 and the healthy controls. The training lasted 5 months for Helix and 7 months for Nala and Otto. All dogs had previous experience with bio-detection work (Helix: human lung cancerThe dogs were not used for other tasks during the study. All dogs lived at home with their owners, they were handled by professional dog trainers or behaviour scientists during the training and testing sessions, and were cared for by their owners between sessions.