Insect colonisation of bodies brought into the Johannesburg Forensic Pathology Service: implications for accurate time of death estimations

Abstract

Forensic entomology involves the analysis of insects that colonise human bodies after death. These insects become associated with the body shortly after death, in a continual succession as decomposition progresses. This dissertation provides an analysis of the insect taxa associated with human bodies and factors that may affect the species and number of taxa on the remains. This dissertation also provides a simulated model of the post-mortem interval (PMI) estimation, using the Monte Carlo simulation. Published base temperature measures for six dipteran species collected after autopsy, were used to estimate variations in PMI. A total of 33 bodies, out of 3,427 (0.96%) received by the Johannesburg Forensic Pathology Service for post-mortem examination over a 16-month period, were colonised by insects. No cause of death was determined in 48.48% of the colonised bodies as a result of the degree of decomposition. Calliphora vicina was the most commonly occurring species followed by Chrysomya albiceps, Ch. chloropyga and Lucilia sericata. Bodies in the active decay stage were found to have the highest species richness with nine insect taxa found. Logistic regressions were performed to determine the factors affecting the number of taxa present and the species occurring on the remains, and both temperature and the stage of decomposition were found to be significant. The importance of temperature in this study is further reiterated in the Monte Carlo simulation using the modelled accumulated degree days from the time the body was removed from the death scene, refrigerated and finally autopsied, before insect collections were performed. Six species of Diptera were modelled, including Calliphora vicina, Chrysomya albiceps, Ch. chloropyga, Lucilia sericata, Musca domestica and Piophilia casei. Temperature measurements recorded over the duration of the first half of the study were used as input variables in the simulation. From the simulation it was found that temperature fluctuations could result in continued development of insect larvae during refrigeration. However, all species except Ca. vicina were found to have less than a 45% likelihood of continued development, while Ca. vicina was found to have 76% likelihood of continued development during refrigeration. This continued development was found to produce increased mean PMI estimations of between 11.45 and 13.37 accumulated degree-days (ADD) with standard deviations of 6.79 to 7.42 ADD depending on the species. These results highlight the importance of temperature for the taxa present, the number of taxa, and the ability to accurately determine the PMI estimation in forensic cases by forensic entomologists.