Cattle slaughtering and BSE risks. Part I. Potential dissemination of CNS tissue during slaughtering.

The usual cattle slaughtering process involves a few critical stages that may result in widespread dissemination of specified risk materials (SRM) within and/or onto the carcass and within the abattoir environment. These include captive bolt stunning, head handling and harvesting head tissues and sawing the spine lengthways. During stunning the entrance of the bolt into the cranial cavity results in massive damage to brain tissue. There is a risk of brain tissue particles being transferred via the blood flow in the minor blood circulation system. This can lead to a contamination of blood, lungs and heart with the BSE agent. Brain particles small enough to pass through the lung capillaries can be distributed throughout the entire carcass by the major body circulation. Furthermore, the transfer of prions to healthy animals via a contaminated bolt gun and the release of central nervous system (CNS) proteins through the bolthole onto the slaughter floor, result in direct and indirect dissemination of brain tissue, during and immediately after stunning. There is an abundance of critical hygiene points in connection with the handling of the head and the harvesting of head tissues. If a wide range of precautions is not taken, then the danger of cross-contamination of carcasses, abattoir workers, environment, and equipment, and even wastewater is unavoidable. The most critical process stage, in terms of edible meat contamination with SRM is the current common practice of the longitudinal splitting of the carcass. The carcass splitting saw poses a particular risk in terms of cross-contamination of carcasses, equipment, surfaces and operatives. Tissues of the CNS carry almost all of the infectivity in cattle sub-clinically and clinically affected by the BSE. The approved rapid post mortem tests cannot identify BSE infected animals early in the incubation period. Should animals suffering from sub-clinical BSE enter the usual commercial slaughter procedure, it is likely that the BSE prion will be disseminated onto the processed carcasses. If such carcasses are consumed by humans this may lead to the development of vCJD.