Saliva, and saliva derived DNA, can transfer between individuals during everyday interactions, often without their awareness. This may be relevant to a range of different crime types, including sexual assaults and homicides, where saliva transfers to the skin of a victim or an offender from actions such as biting, kissing, licking or sucking. The detection of DNA and saliva in such instances, and an awareness of the presence of saliva and non-self-DNA without such activities occurring, can provide valuable information to forensic investigators. However, examinations of a suspect or victim often do not occur immediately after the offence. Therefore, a better understanding of the transfer of saliva derived DNA to skin immediately after an action of biting, kissing, licking or sucking, and its persistence on skin after an extended time period, is necessary for the accurate interpretation of this evidence type. 

This study investigated the transfer of saliva and non-self-DNA on human skin and its persistence following actions of biting, kissing, licking and sucking. Seven male-female couples were asked to shower and then bite, kiss, lick and suck separate defined areas of both of their partner’s arms, then one arm was sampled immediately and the other, 6-hours later[RV1.1]. Background (prior to shower) and control (post shower but before deposition) samples were also obtained. For participants who wore long sleeve clothing post activity and prior to sample collection, the inner sleeves were also sampled along with the tested skin areas.

Presumptive testing for saliva was completed on all background, control, 6-hour experimental and clothing samples collected, using the RSID-Saliva immunological test kits. DNA extraction and quantification was conducted on all samples, and all background, 6-hour and clothing samples underwent DNA profiling. The results [RV2.1]were considered together with the questionnaire data provided by participants, to assess what factors may have influenced the presence or absence of any saliva, partner or foreign DNA. Comparisons of the DNA quantities recovered from the immediate samples [RV3.1]and 6-hour samples were made. Furthermore, saliva detection rates and DNA profile data was compared between all samples generated from the four different actions which resulted in saliva transfer to skin. The findings from this research will assist forensic investigators when interpreting biological evidence recovered from victims or suspects, particularly in cases involving alleged activities that result in saliva transfer to skin.

Authors

• Cara Woollacott (Flinders University College of Science and Engineering, Australia)
• Mariya Goray (Flinders University College of Science and Engineering, Australia)
• Roland van Oorschot (Victoria Police Forensic Services Department, Australia)
• Duncan Taylor (Forensic Science South Australia, Australia)