Electroencephalography as a biomarker of brain function in domestic ruminants: a systematic review and critical appraisal.

BACKGROUND: Electroencephalography (EEG) has been extensively used to investigate cortical activity in ruminants under physiological, pathological, and experimental conditions. Its applications range from the assessment of consciousness and nociception to experimental models of hypoxia-ischemia, neurodevelopment, pain, anesthesia, and neurological disorders. However, considerable methodological heterogeneity has limited the integration and critical interpretation of findings across studies. OBJECTIVE: This systematic review aimed to evaluate the use of EEG as a biomarker of brain function in domestic ruminants, with an emphasis on consciousness, nociceptive processing, and neurophysiological alterations under experimental and pathological conditions, while providing a critical appraisal of its current limitations. METHODS: A systematic search was conducted across PubMed, Scopus, Web of Science, ScienceDirect, and CAB Abstracts using controlled vocabulary and free-text terms related to electroencephalography and ruminant species. After screening and eligibility assessment according to predefined inclusion criteria, 112 studies were included in the qualitative synthesis. Data regarding study characteristics, EEG methodologies, and electrophysiological findings were extracted and analyzed qualitatively. Methodological quality was assessed using the SYRCLE risk-of-bias tool. RESULTS: Across studies, EEG consistently reflected context-dependent alterations in cortical network activity; however, its interpretation as a direct surrogate of consciousness or nociception remains limited. In studies of consciousness and insensibility, transitions to unconsciousness were characterized by epileptiform activity followed by cortical suppression or isoelectric patterns. Nociception and anesthesia studies demonstrated that EEG responses were highly influenced by stimulus intensity, anesthetic depth, and experimental conditions, frequently showing a dissociation between cortical and autonomic responses. In hypoxia-ischemia models, particularly in fetal sheep, EEG proved to be a sensitive early biomarker of cerebral dysfunction, commonly demonstrating suppression, low-frequency dominance, and seizure activity. Neurodevelopmental studies identified maturation-dependent electrophysiological changes, while studies of neurological disorders highlighted abnormal cortical excitability and an increasing application of automated analytical approaches. CONCLUSIONS: EEG represents a sensitive functional biomarker of brain network activity in ruminants across diverse experimental settings. Nevertheless, substantial heterogeneity in acquisition methods, analytical approaches, and interpretation criteria limits direct comparison between studies. Greater methodological standardization, critical interpretation of signal generation, and integration with complementary physiological measures are necessary to improve the translational and practical applicability of EEG in ruminant research.