Canines are often used for tracking in police investigations, but forensic researchers at Flinders University (Australia) say dogs can provide even more crucial evidence—including DNA. The team’s new study sheds light on the presence and transfer of human DNA on dogs, possibility expanding opportunities for DNA to assist in criminal investigations.
For the study, published in Genes, researchers from Flinders University, the Victoria Police Forensic Services Department and Deakin University worked with 20 dogs, each from a separate household. One of the study authors attended the house of each participant, gathering a total of nine DNA samples directly from each of the 20 dogs.
Donning clean gloves, the researcher collected samples from six distinct areas of each dog to assess the general presence of human DNA. The six areas, in order of collection, were the chest, top of the head, back, left and right sides and stomach. The swabs applied to the left side were placed under the fur to make contact with the skin; all other samples were taken from the surface of the fur.
Overall, the study results show human DNA was retrieved from all areas of the dogs that were sampled—but caveats exist in terms of strength of DNA, number of contributors and sampling location.
For example, of the 120 samples analyzed, the minimum number of contributors could not be determined in 33% of profiles generated—16 provided no profile and 23 yielded limited profiling information. The majority of these samples with low DNA quantities came from the left side of the dog (14), followed by the chest and stomach (9 samples each).
Interestingly, the samples collected from the left side of the dog were taken from deeper in the fur and closer to the skin. Therefore, the study authors theorize that DNA collected by dogs is retained on the surface of the fur rather than being embedded under it. Overall, higher quantities of DNA were recovered from the back, head, and right side of the dogs.
In 15% of the 120 samples, a single contributor was observed. In these profiles, the main human was the most common contributor (9 DNA profiles), followed by an unknown source (6) or another person within the household (3). In a further 19% of profiles, a major contributor was observed. This component frequently corresponded to the main human (11), followed by another person from the household (9) or an unknown source (3). Six percent of samples were considered complex mixtures with 5 or more DNA profiles. Finally, the remaining samples consisted predominantly of two-person mixtures (49 profiles), single source (18) and three-person mixtures (7).
In a second part of the study, the researcher patted and scratched each participating dog under the neck/chin area with a single bare hand to determine if their DNA could be transferred and detected on a dog during a single contact event. The area was swabbed immediately after contact.
Of the 20 profiles generated, 10 contained DNA of the individual patting the dog. This included 2 profiles as a major contributor (along with an unknown minor), 7 as a near equal contribution (along with contributions from other people in the household or unknown sources), and 1 as a minor contributor. Thus, as the individual patting the dog was not detected as a contributor in half of the profiles obtained, the researchers say it is reasonable to suggest that the last person to touch the dog will not always be reflected in the profile.
"This information may assist those investigating criminal acts in which dogs are involved to consider situations [where] it may be useful to sample for human DNA from a dog,” the researchers conclude in their paper. "[The study] also showed that investigators may need to consider dogs as a vector for indirect transfer of human DNA within particular scenarios."