Effect of electrical stunning methods on petechial haemorrhages and on the blood pressure of lambs

Abstract

Electrical stunning of lambs to render them unconscious and insensible to pain can result in blood splash (spots of blood in muscles) or speckle (petechial haemorrhages in fat or connective tissue overlying the muscle). Stunning through electrodes applied to the head causes mainly blood splash with some speckle, while currents applied head-to-back or head-to-foreleg can produce speckle without blood splash. To identify the causes of speckle, animal movement and blood pressure changes that occur during stunning were measured. Three different stunning methods were employed: head-only, head-to-back and head-to-foreleg. Head-only stunning, which does not stop the heart and produces a moderate amount of speckling, was accompanied by elevated arterial blood pressures up to 300 mm Hg, and venous pressures up to 100 mm Hg. The carcass became rigid on stunning but reflex kicking occurred when the hind legs were shackled. Head-to-back stunning, which stops the heart, produced a momentary arterial and venous blood pressure rise during the stun, as a result of thoracic pressure changes, followed by a decline in pressure to 40–50 mm Hg. The muscles became markedly rigid due to direct stimulation of the loin and spontaneous kicking was absent following current switch off, although spontaneous coughing persisted. Stillness associated with head-to-back stunning was accompanied by the highest incidence and severity of speckling, whereas head-to-foreleg stunning, which also stops the heart, resulted in less muscle contraction and was accompanied by the least incidence and severity of speckling. Kicking was also eliminated if sufficiently high currents were used. Stopping the heart with KCl before a head-to-back stun lowered arterial blood pressure to 20–30 mm Hg and markedly reduced speckle. Abolition of nervously mediated muscle movement with curare before stunning resulted in no observable speckle in head-only stunned lambs but, in the head-to-back stunned lambs, there was a low incidence of speckle in the loin as a result of the muscle being directly stimulated to contract. The results from this study are consistent with the view that speckle is a result of vigorous movements between muscles during stunning, causing a shearing and rupture of blood vessels in the fat and connective tissues, thus allowing the muscles to squeeze out blood. The incidence of speckle does not appear to be related to blood pressure during stunning.