In the EU the use of the pithing rod for the slaughter of animals was banned by Decision 2000/418/EC coming into force on the 1.1.2001. This ban was based on the observation that in cattle CNS fragments could be spread if conventional cartridge operated captive bolt gun were used in combination with pithing, while no spreading was observed following the use of the captive bolt gun alone (Anil et al., 1999).
These investigations are sometimes taken for evidence that only the additional use of the pithing rod might lead to dissemination of CNS fragments in cattle. However, the results do by no means exclude that the observed spread had already occurred by the captive bolt shot instead of being caused by the subsequent use of the pithing rod. Additional investigations of the working group at the University of Bristol in sheep clearly demonstrated that also conventional captive bolt guns without subsequent pithing were able to disseminate CNS fragments via the circulatory system. The pneumatically activated (but not gas injecting! Anil, personal confirmation) captive bolt gun (Cash Ramrod, Accles and Shelvoke) as well as the conventional cartridge-operated captive bolt gun (Temple Cox Mark X, Accles and Shelvoke) caused each in 2 of 15 sheep stunned with either gun notable spread of CNS tissue (Anil et al., 2001). There exist no anatomical or physiological differences between cattle and sheep which would suggest one should ignore the respective risk in bovines only because in cattle it has not yet been proven.
Moreover, in certain instances a mere rise in intracranial pressure seems to suffice for spreading of CNS fragments. Accordingly in the literature between 1956 and 1990 a total of 18 cases of lung embolism caused by brain tissue have been reported in neonates. These birth injuries were blamed on a severe increase in intracranial pressure, especially in breech deliveries. In one case the dislocations were so massive that practically all small elastic-type or larger muscular-type pulmonary arteries were blocked (Hauck et al., 1990). CNS fragments could enter the circulation via torn brain vessels, especially the wide-bodied dorsal and basal venous sinuses which collect the blood from the head to forward it via the jugular veins to the heart. In view of these case reports on birth injuries as well as published autopsy findings in humans having suffered from closed head injuries (Collins u. Davis, 1994) a dislocation of brain tissue can principally not be excluded even with concussion stunning (non-penetrating captive bolt shot). Also the TSE agents are not exclusively located within the cells of the brain and spinal column. Therefore not only innumerous microscopically small brain tissue fragments may be pried loose from their natural connections by the penetrating bolt. Also BSE agents dispersed in the extracellular space or cerebrospinal fluid might be carried off by the circulatory system and - due to their small size - easily pass the lung and enter the body circulation, thus contaminating musculature and organs of the slaughter animal (Heynkes, 2000). Investigations in which the bolt or the entrance wound had been contaminated with marker organisms clearly demonstrated that these could later be found in nearly all examined tissues (Daly et al., 2001; Mackey und Derrick, 1979).
Brain particles and indicators of the induced brain damage like Syntaxin 1B or Annexin V pass the jugular veins within 10 - 40 seconds after the shot (Anil et al., 1999 and 2001). How much time these fragments need in cattle to pass from the cranial cavity via the heart into the lungs and - provided their small size allows for this - to disseminate all over the body is not yet known. In all respective investigations the spread of brain tissue was only examined after the animal had been killed by bleeding. After captive bolt stunning the heart will continue to beat for several minutes until the blood loss initiated by sticking results in circulatory break down and cardiac arrest. In cattle circulation time between the left and right udder veins were found to be around 52 sec, in horses between the left and right jugular veins even only 31,5 sec were observed (Altman, P. Handbook of Circulation, 1959, quoted in Scheunert/Trautmann: Lehrbuch der Veterinärphysiologie, 7. Ed., 1987).
While the existing hazard of disseminating brain tissue by the captive bolt stun has been sufficiently demonstrated the amount and extent of such dislocation can presently only be guessed. The volume of brain tissue which the penetrating captive bolt disconnects from its natural surroundings should roughly correspond to the volume of the penetrating bolt. It was estimated by Anil et al. (2001c) to be about 10 g. The frequency of finding dislocated brain fragments in the jugular veins and the organs following conventional captive bolt shot in experimental investigations depends also on the extent and thoroughness of histological examinations of lung tissue. The time required for the histological examination of an entire bovine lung - cut into microscopically examinable slices - would be enormous. Accordingly, the number of animals which can be investigated within a given amount of time is rather limited. So far the observed instances of dislocated CNS fragments as well as the number of animals used in these experiments have been small (4 of 30 sheep, Anil et al., 2001 resp. 0 of 15 cattle, Anil et al., 1999). The prevalence of central nervous tissue emboli which may be derived from these numbers with 95% statistical probability thus lies somewhere between 4 and 30% in sheep and 0 - 31% in cattle (Snedecor und Cochran, 1967).
The results of practical investigations were recently published by Lücker et al. (2002). In 48 of 726 cattle stunned by captive bolt (part of them with additional pithing, Emboli-like particles were detected in the pulmonary arteries which had been cut longitudinally down to a diameter of 3-5 mm. Only in 2 of the 58 retrieved Emboli the presence of neuron specific enolase could be shown by anti-NSE-Western Blot. Both emboli were found in bovines which had undergone pithing (a method nowadays EU-wide prohibited). However, in this investigation only emboli visible to the naked eye were collected. Moreover, with the described anti-NSE-Western Blot it had not been possible to detect brain tissue in lungs experimentally perfused with brain homogenate although histological examination showed this tissue to be present in all areas of the lung. The publication does not give any information as to how captive bolt stunning or bleeding were carried out. Accordingly it can only show that with the stunning and slaughter process in the visited slaughter establishment apparently no major brain tissue fragments which might have been dislocated by the captive bolt shot had been filtered out by the lungs.
Horlacher et al. (2002) examined in the abattoir of Gießen 323 bovine lungs for macroscopically visible Emboli in their pulmonary arteries. In 194 of these lungs (60%) a total number of 358 emboli were found. 355 of these emboli were immunochemically examined in pooled samples each of which - with two exceptions - comprised of between 14 and 41 lungs for neuron specific enolase (NSE) and acid glial filament protein (GFAP). Two pooled samples which comprised of 40 respectively 28 emboli originating from 15 lungs per pooled sample gave only weak positive results with both detection methods. Without giving the slightest proof for this the authors assume that in both cases only one of the 15 lungs contained central nervous tissue. Additionally 3 yellowish emboli of ca. 1 cm were immunochemically and immunohistochemically examined for CNS with negative result. Because of their tiny size 3 other yellowish emboli measuring ca. 3mm were only immunhistologically investigated. 2 of them were NSE as well as GFAP positive. As brain cells could not be definitely identified in the histological preparation these two samples were categorized by the authors as questionable.
Also in this investigation only such Emboli were collected which could be spotted with the naked eye. The detection limit of the immunochemical method used was ca. 0.1% CNS. In view of the dilution obtained by pooling the samples the examined emboli would have had to contain several times this CNS content or consist entirely of CNS tissue to provide a detectable amount of NSE and GFAP.
A minimal infective dose for humans cannot be stated. According to the Prion hypothesis a single prion would suffice to start misfolding of the cellular prion proteins. However, the probability that a Prion actually arrives at its destiny increases with the inoculated amount of infective agent. Also, according to the results of experiments in hamsters, one has to assume a cumulative effect of repeatedly ingested small amounts of TSE infectivity (Diringer et al., 1998). One cannot yet decide whether only the apparently rare event of spreading macroscopically visible CNS tissue (potentially carrying the TSE-agent) presents a risk for the consumer which - in view of the present lack of practicable alternatives - would have to be tolerated in smaller abattoirs and during house slaughter. Or whether not also a regularly occurring dislocation of tiny macroscopically invisible CNS fragments could pose a non-negligible infection risk should the agents have already reached the brain at levels not yet detectable by a BSE-screening test. To provide a reliable risk assessment the experiments have to be repeated in a larger number of animals in spite of the enormous preparatory effort. Appropriate research projects to quantify the amount, distribution and frequency of CNS fragments with different stunning methods have already been projected in England for groups of 100 animals each (Anil, personal communication).
Following the precautionary principle all available measures by which this presently not sufficiently quantifiable risk could be minimized should be taken.
The onset of ventricular fibrillation establishes the loss of coordinated contraction of the heart muscle fibers. This leads to immediate circulatory arrest. Even if the electrical stun would cause a temporary increase in capillary permeability and consequently allow migration of infectivity into the vessels the distribution of the agent by the blood stream could be effectively prevented by causing circulatory arrest simultaneously with or subsequently to the head-only stun.
Commercially available automatic or semi-automatic equipment for electrical stunning of cattle can hardly be used in small abattoirs. Not only do they take up a lot of space, but their comparatively high cost would not pay off with small slaughter numbers. If concentration of cattle slaughter in only a few specialised large abattoirs is to be prevented alternatives will have to be developed which are applicable in small slaughter plants and humane from the view of animal welfare, but nevertheless reliable in preventing an inner contamination of the carcase and the organs with potentially BSE-infective central nervous tissue.
As long as measures could guarantee that BSE contamination would be limited to the right ventricle and the lungs - contamination of the left ventricle would require passage of the lungs - the risk could be limited by destroying heart and lungs, too, as specified risk material (SRM). The contamination risk cannot be totally excluded, however, as it would hardly be possible to eviscerate these organs without severing the vessels potentielly harbouring the agent. Severance of the vessels in any case means the theoretical risk of setting free potentially present BSE infectivity which would subsequently be able to contaminate the surface of the carcase.
To prevent distribution of CNS tissue fragments beyond the lung any circulatory arrest would have to come into effect within the time the fragments would need to reach the lung. This could be achieved by triggering ventricular fibrillation immediately after the captive bolt shot. First, the longest admissible duration for this measure to take effect would have to be determined. This could be done using simple and easily detectable marker substances.
For this the time required for severing the vessels and separating the head from the body would have to be determined and compared with the chronological spread of CNS tissue in the body (practicability test). Also investigations to determine the bacterial contamination of the meat surface laid open by the cut and of the inner organs would become necessary (hygiene test). Complete separation of the head (decapitation) would also guarantee that the animal does not regain consciousness. Thus with decapitation it would be possible to replace the penetrating captive bolt shot causing a lasting stun by other methods which are associated with only a short period of unconsciousness. It would be totally unacceptable, however, for reasons of animal welfare to cut the neck of the conscious animal or decapitate it without prior stunning. Halal slaughter is only acceptable to comply with religious needs. These have to be demonstrated before the competent authority may grant a derogation to carry out ritual slaughter, provided, additional criteria are fulfilled to meet the requirements of animal welfare. One of these requirements would be not to move or even touch the animal within the first 3 minutes of bleeding to allow it to die from blood loss without inflicting any additional pain or distress. This would substantially reduce the slaughter capacity.
As concussion stunning according to § 14 No. 1 of the German Animal Welfare at Slaughter Regulation (Tierschutz-Schlachtverordnung) may only temporarily be used for the purpose of testing and under derogation by the competent authority a second alternative would be electrical stunning using electric tongs placing the electrodes firmly on both temples or by using a fork-like hand piece which is applied to the top of the head. In over 6-week-old cattle it is required by Annex 3 Part II No. 3.3 of the German Animal Welfare at Slaughter Regulation (TierSchlV) to trigger ventricular fibrillation. To achieve this an electrical current has to be passed through the heart subsequent to the head-stun, and the hand piece would have to be removed from the head and to be pressed onto the chest of the animal. To ensure occupational safety electroimmobilisation is recommended. This can be done by passing a weak current along the spine via one electrode applied to the mouth and the other to the base of the tail or the anus (TVT 2001).
Alternatively the current for triggering ventricular fibrillation might well be passed via the electrodes used for electroimmobilisation. From the view point of occupational safety, however, this would require additional safety measures. An electrode with live tip which would have to be pushed into the anus, and a crocodile clip with only small uninsulated areas on its inner lining therefore seem an optimal solution (Boosen, personal communication). Strength and duration of the electrical current which under these conditions would trigger ventricular fibrillation in the stunned animal would have yet to be established.
Aspects of this suggested research program are already under investigation by the Federal Institute for Meat Research (Bundesanstalt für Fleischforschung - BAFF) Kulmbach within the frame of a common BAFF/BgVV research project granted by the BMVEL. Unfortunately my participation in this research project, although projected, could not be realised due to duties associated with the organisation of FVO inspections in Germany which had priority.
Remarks and critical comments are welcome anytime.