Exploring multi-step human DNA transfer in semi-controlled scenarios following dog patting

Recent studies report detection of human DNA on companion animals, often attributed to the animal's owner(s). Human DNA from dogs can also be secondarily transferred and recovered from the hands of an individual after contact with an animal. Beyond secondary transfer, tertiary transfer has also been noted from the patter's hands to the subsequently handled surfaces. However, data on indirect transfer are limited, hindering a clear understanding of the dog's role as a vector for DNA transfer. This semi-controlled study aimed to evaluate indirect transfer of DNA initially deposited on a dog through contact by a foreign individual to the hands and handled items of the dog owner. In the experiments, one dog (non-shedding) was contacted by one of three different individuals, who did not regularly interact with the dog or its owner, on separate days, after which various scenarios ensued. These scenarios included the owner patting their dog, followed by pre-determined tasks such as using a knife (pre-used by dog owner) or an item of underpants (worn by the dog owner) at different time points, including immediately and 2-, 4- and 6- hours after the initial patting. Samples were collected for DNA profiling from the dog, knives underpants and owner's hand. DNA profiles were generated and analysed to determine the occurrences and extent of transfer of the one-off patter's DNA to the dog's owner and their items (knives and underpants), and persistence of this DNA on their hands and the dog. Direct transfer of the patter to the dog was frequently detected; in 67% of samples taken immediately after patting (6 of 9 samples; underpants experiment only) and in 44% of samples taken 6 h after patting (4 of 9 samples in underpants experiments and 4 of 9 samples in knife experiments). Tertiary indirect transfer of the patter via a dog owner to the owner items was also observed; in 11% of knife samples (always as minor contributors) but not in any of the underpants samples. The data generated in this study adds to the growing knowledge base and data availability regarding the transfer, prevalence, persistence and recovery of DNA through companion animals and will assist those collecting samples during investigations of criminal activity and those conducting activity level evaluations.