Although the incidence of bovine spongiform encephalopathy in cattle continues to decline in the United Kingdom, it remains important to maintain vigilance of all potential routes of transmission of infection to humans. Initial studies have demonstrated a potential risk of carcass contamination with brain tissue following the use of captive bolt gun stunning in cattle. The objective of this study was to further explore these initial findings particularly in regard to captive bolt guns currently in use in the United Kingdom. Brain tissue fragments or elevated levels of a marker protein for brain tissue were detected in venous blood samples from 4% (95% confidence interval, 1.6 to 9.8%) of cattle stunned by penetrating captive bolt gun and from 2% (95% confidence interval, 0.6 to 7%) of those stunned by nonpenetrating captive bolt gun.

The epidemic of bovine spongiform encephalopathy in the United Kingdom and the recognition of a variant of Creutzfeldt-Jakob disease prompted revision of the guidelines for slaughter of cattle and sheep to prevent contamination of the edible parts of the carcass with central nervous system tissue. We previously showed that captive bolt gun stunning, which is routinely used for the slaughter of cattle and sheep, causes entry of fragments of central nervous system tissue into the jugular vein. To determine whether such tissue can traverse pulmonary capillaries to enter the systemic circulation, we introduced small volumes of brain tissue that had been disrupted by stunning with a captive bolt gun into the jugular vein of sheep sent for slaughter. We examined aortic blood samples by immunocytochemistry for neurofilament and S100 proteins and by enzyme-linked immunosorbent assay for glial fibrillary acidic protein and found fragments of neurofilament- and S100-immunopositive central nervous system tissue in samples from 2 of 11 sheep and elevated glial fibrillary acidic protein in 6 sheep. Our findings suggest that central nervous system tissue that is dislodged during routine captive bolt gun stunning and slaughter of sheep can enter the systemic arterial circulation and that, in some cases, this method of slaughter of an animal infected with bovine spongiform encephalopathy would be likely to contaminate edible parts of the carcass with infective material.

In accordance with controls instituted to protect the consumer from meat potentially infected with bovine spongiform encephalopathy (BSE), brain tissue emboli caused by the use of captive bolt gun (CBG) stunning have been identified as a potential public health risk that requires further investigation. As the natural occurrence of BSE in sheep remains uncertain we have investigated the frequency of brain tissue embolism associated with stunning by two types of CBG that are in commercial use in the United Kingdom. Blood samples collected from sheep following stunning were analysed by ELISA as well as by a combination of microscopy and immunocytochemistry. The combined positive results from each method of sample analysis were used to determine the frequency of brain tissue embolism in the sample population. The frequency of brain tissue embolism was found to be 23% in sheep stunned with a cartridge-activated CBG (95% confidence interval of 15.8-32.2%) and 14% in those stunned with a pneumatically activated CBG (95% confidence interval of 8.5-22%). The frequency of brain tissue embolism associated with the use of CBG stunning may represent a significant source of carcass contamination with brain tissue.

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