In field training procedure
The trainers and the method (operant conditioning with positive reinforcement and a food reward provided for correct behaviour) employed were the same as in laboratory testing except they did not use the apparatuses to hold the samples. Training for the field screening was carried out from September 2021 to mid-January 2022, and passed through two steps, as described below, during which the dogs learnt to identify COVID-19 patients by sniffing the volunteers directly. Step one training sessions were conducted at various drive-thru COVID-19 testing points after volunteers had received their negative antigen test result. This was necessary so that, adding a positive scent or not, could reliably allow for presenting the dogs with positive and negative scents on humans. Round-shaped metallic stainless-steel boxes with 6 holes (height: 1.2 cm; diameter: 3.6 cm), typically employed in nose-work, were used during the training. The boxes contained either a positive or negative sweat-stained cylinder. The boxes used for positive samples were not mixed with the boxes used for negative samples. In each training session, the dogs had to enter a room where a maximum of 5 volunteers lined up having a box hidden in their clothing, preferring areas where people are expected to sweat the most (e.g., sleeves, shoes, etc.). The location of the person holding the positive sample was always randomised in the line-up. The volunteers could either be sitting or standing in a queue, thus simulating a typical situation in a testing point, including hospitals and pharmacies, and dogs were required to indicate that they recognised the scent of COVID-19 by stopping in front of the person holding the box with the target odour (positive scent). As in the training phase, the scent was obtained from patients with a diagnosis confirmed by RT-PCR. Overall, the dogs were trained using 131 positive samples. None of the samples presented to Nala (who had participated in phase 1) came from volunteers who had participated in the laboratory testing phase. This training step lasted 2 months for each dog.In step 2 of the field study, 192 incoming clients at community pharmacies volunteered for the study. In each trial, the dogs could sniff any part of the body of one volunteer and they were allowed to touch with their noses the body of the volunteer. This training step lasted 2.5 months for each dog.The trials in step 1 were unblinded (e.g., the handlers knew the position and number of the sniffers containing the positive cylinders) to observe the spontaneous behavioural cues offered by each dog when alerting on the target odour (positive scent). In step 2, the trials were single-blinded, as the handlers were unaware of who had received a positive diagnosis. By contrast, the volunteers and the director of training, who was present during both phases, were always unblinded, and the director informed the dog-trainer dyad of every correct alert. The results of this training step are reported in Table Training of sniffer dogs for field work on volunteers.