The effect of ivermectin on the duration of the decomposition stages of goats (Capra aegagrus hircus) and on the developmental rate of scavenger insects

The use of insects or insect remains, in order to identify drugs and toxins present in carcasses tissues is known as entomotoxicology. An important focus of entomotoxicology is the investigation of the effect of drugs and toxins on the developmental rate of arthropods. Studies show that the presence of drugs in animal tissues due to drug use or abuse, can affect the rates of larval development of insects that may be using these tissues as a food source, resulting in inaccurate estimates of the post-mortem interval (PMI) when based on larval development time. In Brazil, the antiparasitic drug, ivermectin has caused several cases of animal poisoning, especially in dogs and cattle, fact that increased the demand for forensic studies focusing on the interference of this drug in the post-mortem changes and on the colonization of these carcasses by insect larvae. Thus, this study aimed to evaluate changes in successional pattern  and decomposition rate of goat carcasses treated with ivermectin. Fieldwork was conducted in a wooded area located on the campus of the State University of Campinas (UNICAMP), Campinas, SP, Brazil. Four carcasses of goats (Capra aegagrus hircus) were exposed simultaneously, and divided into two groups (treated and control). In the treated group, ivermectin was inoculated subcutaneously at a concentration of 0.02ml/kg before death by concussion. In each group, one carcass was exposed on a shady spot, and the other under direct incidence of sunlight, to evaluate the interference of this abiotic factor in the decomposition process, thus named: IS (Treated with ivermectin on shady spot); ISun (Treated with ivermectina with sun light incidence); CS (Control on shady spot) and Csun (Control with sun light incidence). To avoid the attack of large predators, the carcasses were kept in metal cages with trays underneath, to collect the immatures leaving the carcasses to pupate. The results showed a significant difference in  decomposition period between IS, that took 9 to 11 more days to be completely decomposed, and the other carcasses. This delay in the process occurred basically during the decay stage, when the maggots were more active and numerous. Chrysomya albiceps, Hemilucilia segmentaria (Diptera: Calliphoridae) and Ophyra aenescens (Diptera: Muscidae) were the most abundant species breeding in the carcasses. The sunlight incidence influenced the colonizing species. Two species of Hemilucilia. H. segmentaria and H. semidiaphana were predominant  in shaded carcasses, contrasting with the carcasses exposed to sunlight, where few  individuals were collected. The difference observed between the carcasses IS and ISun can be explained by the higher incidence of sunlight in the ASun carcass, and the consequent increase in temperature, the  two major factors that accelerated the degradation of  ivermectin. As a consequence, the drug did not affect the development of immatures that fed on this carcass. Based on these results, we observed that several biotic and abiotic affected  the decomposition process of carcasses, and the interactions between them are still not clear. More studies on veterinary entomotoxicology are still needed, in view of the large number of domestic and wild species exposed to several drugs that, when used incorrectly, can cause animal death.