Electro-analgesia for sheep husbandry practices: a review

Abstract

Several sheep-husbandry practices such as mulesing, castration, ear-tagging and tail-docking are currently performed with no, or little, anaesthesia or analgesia. The potential for using electrotherapies to provide analgesia during and after these operations is examined in this review. The most common electrotherapy is transcutaneous electrical nerve stimulation (TENS). TENS is the application of an electrical current from electrodes placed on the skin. Analysis of a large number of trials in humans and in animal models indicates that TENS provides effective relief from acute and chronic pain, including pain associated with surgery. There is strong evidence now that TENS analgesia operates at the levels of the periphery, the spinal cord and in the brain. The mechanisms involve the autonomic nervous system, the opioid pathways and neurotransmitters involved in descending inhibitory pathways from the brain. Centrally operating pathways mean the current does not have to be applied near the injured site and there is evidence of sustained pain relief lasting hours, days or even weeks post-treatment, particularly after very high-frequency, randomly variable current applications. Treatment of sheep during painful operations with such a current has the potential to provide immediate and possibly sustained pain relief. Combining such a treatment with electro-immobilisation of the animal would be advantageous for sheep-husbandry operations, but there is considerable evidence that high-intensity currents producing tetanic contractions are aversive and probably painful for sheep. Investigations of the application and efficacy of electrotherapies for painful sheep operations should be undertaken.