Electroencephalogram (EEG) assessment of brain activity before and after electrical stunning in the Nile crocodile (Crocodylus niloticus)

Abstract

The purpose of the study was to assess and describe brain activity before (pre-stun) and after (post-stun) electrical stunning (e-stunning) in the Nile crocodile (Crocodylus niloticus) by using electroencephalogram (EEG) analysis to determine if stunned crocodiles demonstrate EEG signs of unconsciousness. Twenty-six cadaver C. niloticus heads were used to examine the topography of the brain in the cranium and skull thickness. Identification of external sites for placement of the EEG electrodes on the head of the crocodiles was established by magnetic resonance imaging (MRI) and anatomical measurements taken from sagittal and coronal dissections. Seven EEG electrode placement sites were identified from studying the brain topography and skull thickness which was limited by the size of the cranial plate. The type of EEG electrodes, portable EEG machine, electrical dose and e-stunner were established using 14 live captive-bred C. niloticus in two pilot trials. From the pilot trials, the EEG recording procedure was established which included determining the suitability of pre-selected EEG electrode placement sites and verifying EEG recording device settings. Fifteen live grower crocodiles were used to assess EEG brain activity before and after e-stunning, and behavioural observations during e-stunning were documented. An effective stun was identified by a loss of consciousness, observed in EEG as a decrease in alpha waves and increase in delta waves patterns, in crocodiles immediately after-stunning. The study confirmed that e-stunning at 171.4 ± 1.7 V and 50 Hz for 5 - 7 sec applied to the neck (behind the cranial plate) was able to immobilise all crocodiles and to achieve an effective stun in 6 out of 12 crocodiles with measurable EEG recordings. The delta power significantly increased (p= 0.010) and alpha power significantly decreased (p= 0.015) immediately after stunning in these crocodiles. In crocodiles that were not effectively stunned (n= 6) the delta power significantly decreased immediately after stunning (p = 0.037) whereas alpha power showed no significant change (p= 0.336) after stunning. A mean maximum electrical current of 1.1 ± 0.2 amps and a total electrical current of 5.5 ± 1.4 amps*sec were delivered to the crocodiles. Size and weight of crocodiles and the electrical current delivered to the crocodiles had no significant (p > 0.05) effect in achieving conscious and unconscious. The results of study indicate the importance of monitoring crocodiles e-stunned for clinical signs of consciousness. Identifying an unsuccessful stun is easier than identifying a successful stun (unconsciousness) using behavioural observations. Further studies are recommended to improve the efficiency of e-stunning to achieve immediate unconsciousness in C. niloticus, measured by the proportion of animals affected, and to examine the safety of repeated e-stunning of crocodiles for management purposes.