Journal of American Veterinary Medical Association, 2001, Vol. 219:608-611
Untrained plant personnel have had problems misinterpreting eye reflexes. Indiscriminant poking at the eyes of electrically stunned pigs may result in reactions that resemble a comeal reflex, although a true reflex is not elicited. Eyelids that are stuck together with mucus may open suddenly when touched, and pressing above the eye can force the eyelid closed.
Practical experience gained from McDonald's Corporation audits during a 2-year period indicates that simply watching for natural spontaneous blinking is a better approach for evaluating eye reflexes in electrically stunned pigs. Vibrating nystagmus or rapid fluttering of the eyelids is not a natural spontaneous blink. The author has instructed plant personnel and restaurant welfare auditors to look at pigs in the holding pens to see what a natural blink looks like.
The author is aware that eye reflexes and blinking in properly stunned pigs may occur before the pig has regained sensibility to pain. The order of retum to sensibility is eye reflexes, response to a needle prick on the nose, and return of the righting reflex. In head-only stunned pigs, complete return to full sensibility and consciousness occurs within 15 to 20 seconds after eye reflexes return2.
Eye blinking is a welfare concern, because the animal has either started the process of retum to sensibility or was stunned with insufficient amperage. A slaughter plant is not a laboratory and standards need to be conservative. The author recommends checking stunned pigs for blinking 60 to 90 seconds after stunning in plants in which correct stunning amperages have been verified and checking for blinking at 5 and 60 to 90 seconds after stunning in plants in which stunning current and settings have not been verified. Plant personnel, veterinary inspectors, and welfare auditors should look at the head and ignore kicking unless it is being induced by contact with hot water in the scalding tank. lf more than 1 pig kicks when its nose touches the scalding water, a careful audit of stunning should be conducted.
Some scientists have argued that blinking is not of much concern, because stunned pigs are in a state of surgical anesthesia. Judging the depth of surgical anesthesia is not a precise science. ln the human literature, Stanski3 cited 33 joumal articles in which it was reported that humans had awareness and remembered events during surgery3. There appears to be no distinct division between conscious and unconscious states; therefore, the criteria for properly stunned pigs should be quite conservative. Continuous attention and auditing by management personnel is required to maintain high standards. The author has observed that practices tend to deteriorate unless they are continually managed and audited4,5. Quiet handling of pigs and minimizing the use of electric prods make it easier to stun pigs correctly because calm animals are less likely to move when the electrodes are applied.
The purpose of the study reported here was to determine causes and solutions for return to sensibility problems after electrical stunning in pigs.
Mean incidence of blinking after stunning in the 6 plants was 2.58% (range, 0.5 to 7%). Mean incidence of wrong stunner placement was 3.8% (range, 0 to 11%). Mean incidence of squealing when the stunner was applied was 4.3% (range, 1 to 10%). In the 5 market weight pig plants, 0.5, 1, 1, 1, and 5% of the pigs, respectively had eye reflexes (spontaneous blinking) after stunning and bleeding. In the sow slaughter plant, 7% of the sows were blinking within 5 seconds after stunning. In all 6 plants, righting reflexes and kicking in response to stimuli were absent, and all signs of possible return to sensibility disappeared before the pigs and sows reached the scalding tub. None of the pigs kicked in response to contacting the scalding water.
Improvements in stunning or bleeding procedures eliminated spontaneous blinking throughout the entire bleeding process. In all plants, a reaudit by the McDonald’s Corporation HACCP team indicated that 100% of the pigs were rendered completely insensible, and all spontaneous blinking stopped.
In the first plant, incorrect placement of the electrode wand was the cause of eye reflexes in 0.5% of the pigs. The operator mistakenly placed the head electrode of the head-to-body cardiac arrest stunner on the jowl below the pig’s ear on 2% of the pigs, and 5% squealed when the wand was applied. The jowl position will work with tong-type stunners with 2 head electrodes, but it is not effective when it is used with a head-to-body stunner that has only 1 head electrode. Spontaneous blinking disappeared when the head electrode was placed on the top of the head in the hollow behind the pigs ear; this position is closer to the pigs brain. The employee in this plant did not place the head electrode correctly because the ergonomic design of the electrode wand and the work station was poor. Modification of the equipment, which made it easier for the employee to place the head electrode behind the ear, eliminated the eye reflexes.
In the second and third plants, 1% of the pigs had blinking. In the second plant, a tired operator working at the end of the shift placed the stunner in the wrong position on 1% of the pigs, and 1% squealed when the stunner was applied. Some pigs may not have received the full stunning time.
In the third plant, the wand was incorrectly designed and was quite difficult for the operator to place in a manner that would cause the current to flow through the brain. The head electrode was placed on the neck of 9% of the pigs, and 4% squealed when the wand was applied. Redesigning the wand eliminated the eye reflexes, after which the plant scored 100% for correct wand placement.
In the fourth plant, internal audits revealed some problems with eye reflexes. This plant eliminated blinking by slowing the bleed line from > 1,200 pigs/h to 1,080 pigs/h. At this speed, workers were still able to achieve their maximum in-the—cooler speed of 1,000 pigs/h. They also added a second person for bleeding.
In the fifth plant, poor bleeding techniques caused 5% of the pigs to have signs of return to sensibility; although workers placed the stunner in the correct position on 100% of the pigs. The problems in this plant clearly illustrated the need for good bleeding. One hundred percent insensibility was achieved when bleeding methods were improved. This resulted in increasing the diameter of the stream of blood from approximately 2 to 3.5 cm.
In the sixth plant, problems with retum to sensibility were much more serious, and it was likely that instantaneous insensibility was not induced. This plant slaughtered sows, and the stunner was set at 1.25 A; this setting, which would be correct for market-weight pigs, was probably too low to induce instantaneous insensibility in old sows. In the first 5 plants, in which the correct amperage was used, eye reflexes and blinking were not observed until ≥ 60 seconds after stunning. In plant 6, spontaneous blinking occurred within 5 seconds after stunning in 7% of the sows, although the stunner was placed in the correct position on 100% of the sows. Blinking ceased within 60 seconds after stunning. Vocalization and the righting reflex were absent. Because this plant was using a cardiac arrest stunner and the amperage was insufficient to induce a grand mal seizure, it is likely that the sows were sensible immediately after stunning; the combination of cardiac arrest and bleeding eliminated the blinking shortly after stunning. This problem was corrected by purchasing a new stunner and using a wide flat electrode on the head to reduce electrical resistance.
In 5 plants, the stunner amperage was set correctly at ≥ 1.25 A for market-weight pigs. In 100-kg market-weight pigs, a setting of 1.25 A is required to reliably induce the seizure activity required to induce instantaneous insensibility8-10. Although some pigs blinked after stunning in these 5 plants, they were probably still in a state of surgical anesthesia. Eye reflexes may occur when a pig is still unresponsive to a painful stimulus2. A setting of 1.25 A was too low to induce instantaneous insensibility in the single sow plant. Sows may have higher resistance and be more difficult to stun than market weight pigs.
Results of research studies clearly indicate the electrical specifications for stunning that reliably induce instantaneous insensibility9-11. Evaluation of new electrical specifications must be performed in a laboratory in which an EEG or other measurements of brain activity may be monitored. In a slaughter plant, the first step in quality control for electrical stunning is to set the stunner at the correct amperage and place the electrodes in the correct position on the animals head. Electrode placement and stunner settings should be monitored on a regular basis. Placement should be evaluated by audits of ≥ 100 pigs. Scoring should be performed for placement and squealing (also known as hot wanding). Hot wanding occurs when the electrode is energized before it is in firm contact with the pig. Hot wanding is a welfare concern, because the pig feels the shock before it is rendered insensible. In high-speed plants hot-wanded pigs may not get sufficient stunning time to induce complete insensibility. The use of electronic monitoring of poor initial contact of the electrodes and interrupted contact of the electrodes during stunning is also recommended, as is the use of more sophisticated stunning circuits to prevent hot wanding.
Results of research by Neville Gregory in Australia with electrically stunned sheep and cattle indicated that poor initial contact of the stunning electrode was the most common cause of poor electric stuns12. Computer records obtained from a plant with an electrical stunner that recorded the incidence of interrupted current flow during the stun indicated that an operator’s performance deteriorates after 2 hours.
Electrically stunned pigs are more difficult to assess for insensibility than are cattle that have been stunned by use of a captive—bolt gun. In captive bolt-stunned cattle, a rolled-back eye or nystagmus (vibrating eye) are signs of a poorly stunned animal. In pigs that have been properly electrically stunned by use of > 50 to 60 Hz, nystagmus or rapidly fluttering eyelids may appear. A small percentage of pigs stunned correctly with 60 Hz have nystagmus 60 to 90 seconds after stunning, generally with the eyelids half open or completely open, but natural spontaneous blinking does not occur. In a plant using an 800 Hz current passed through the head to stun pigs, the author observed that a high percentage of pigs had nystagmus, but none had a spontaneous natural blink.
2. Anil M.H., McKinstry, J.L. The effectiveness of high frequency electrical stunning in pigs. Meat Science. 1992. 31:481—491.
3. Stanski D.R. Monitoring the depth of anesthesia. In: Miller R., ed. Anesthesia. 4th ed. New York: Churchill Livingston, 1994. 1127-1159.
4. Grandin T. Euthanasia and slaughter of livestock. J Am Vet Med Assoc. 1994. 204:l354—1360.
5. Grandin T. Livestock handling and transport. Wallingford, UK: CABI Publishing, 2000.
6. Grandin T. Guidelines for handling and stunning. Washington, DC: American Meat Institute, 1997.
7. Warrington P.D. Electrical stunning: a review of literature. Vet Bull. 1974. 44:617—633.
8. Gregory N.G. Animal welfare and meat science. Wallingford, UK: CABI Publishing. 1998.
9. Council of Europe. Council directive of 18 November on stunning of animals before slaughter (74/577/EEC). Off J Eur Communties. 199l. L316:26.
10. Hoenderken R. Electrical and carbon dioxide stunning of pigs for slaughter. In: Eikelenboon G., ed. Stunning of animals for slaughter Boston: Martinus Nijhoff Publishers, 1983. 59-63.
11. Berghaus A., Troeger K. Electrical stunning of pigs: minimum current flow time required to induce epilepsy at various frequencies. Int Congr Meat Sci Technol. 1998. 44:l070-1073.
12. Gregory N.G. Current profiles during electrical stunning. Int Congr Meat Sci Technol 2000;46:366-367.
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