The Feasibility of Animal-Based Indicators of Consciousness and Unconsciousness for Stunning in Sheep: A Systematic Review

Simple Summary Animal welfare at slaughter is one of the most critical points raising moral questions of ethical sustainability. One way to reach the animal welfare requirements demanded by law would be to monitor the loss of consciousness in animals at the time of the killing to avoid unnecessary pain, stress, and fear. Animal-based indicators (ABMs), directly observed on the animal, are used to assess states of consciousness and unconsciousness, and thus the effectiveness of the stunning method. This is also true for sheep; however, there is still a lack of information on their application under the different conditions in which sheep are slaughtered. This study aimed to critically evaluate the aspects which can strongly affect the possibility of evaluating ABMs, beginning with a literature review of the scientific information available. The results outlined the need to deepen and cover information gaps on the feasibility of ABMs, since different physical aspects can affect the possibility of evaluating the indicators, and thus hamper the protection of sheep welfare at slaughter. Background: According to EU legislation, animal-based indicators (ABMs) are used to evaluate the efficacy of stunning methods to ensure that animals do not regain consciousness. EFSA has provided a list of ABMs for electrical and mechanical stunning in sheep; however, there is still a lack of information on their feasibility. We aimed to identify and evaluate the feasibility constraints of ABMs commonly applied in slaughterhouses to assess proper stunning in sheep. Method: For this systematic review, we searched the Scopus and Web of Science databases from 2000 to 8 August 2022, including full peer-reviewed papers written in English on the welfare of sheep at the stunning and restraint phases. We excluded studies using a gas stunning method or without prior stunning, as well as manuscripts in which indicators were applied after sticking. Results: Of 1289 records identified, only 8 papers were eligible for the critical evaluation of physical aspects that affect the feasibility of ABMs. These aspects were defined as a given definition of the feasibility of ABMs, and information was summarized and critically evaluated. The results highlighted a lack of information on the feasibility of ABMs which should be considered in the various conditions of commercial slaughterhouses.