The effect of insect excretions/secretions and decomposition fluid on DNA quality and quantity in human blood samples

Forensic science is a multi-disciplinary field, and these specialities often overlap when examining evidence or a crime scene. It is critical to determine how different disciplines interact and affect one another to be able to produce the most forensically useful information from the evidence possible. In this study, the interactions between human DNA in blood samples, insect excretions/secretions (ES), and decomposition fluids were investigated. In this study, Lucilia sericata larvae ES and decomposition fluid from a foetal piglet were collected over a period of two weeks. During this period, insect ES alone, decomposition fluid alone, and a 1:1 ratio mixture of the fluids was added to dried human blood on cotton to simulate a bloodied item of clothing. By having three different treatment groups, different crime-scene situations were simulated. This study found that insect ES and a mixture of insect ES and decomposition fluid will have a negative effect on DNA in human blood over a period of two weeks. Both treatments had detectable DNA degradation and PCR inhibition, which increased over time. The insect ES and mixture samples produced partial DNA profiles within a week. The decomposition fluid samples did not have any detectable effect on the DNA in human blood. These results occurred because of the different purposes behind the fluids. Insect ES contains specialised proteolytic enzymes and bacteria that assist with the breaking down of tissues, including DNA. Decomposition fluid does not contain active components that assist in the breaking down of cells, only the waste products from cell death.These results indicate that human DNA that blood samples that have been in contact with L. sericata larvae alone and in a mixture with decomposition fluid are time-sensitive samples that must be stored in a freezer as quickly as possible after collection to suspend any DNA degradation that may be taking place, and additional steps may be required during DNA analysis to effectively clean the sample for DNA profile generation to produce a forensically useful profile.