Solving return-to-sensibility problems after electrical stunning in commercial pork slaughter plants

Temple Grandin
Department of Animal Science
Colorado State University
Fort Collin, Colorado, USA

Journal of American Veterinary Medical Association, 2001, Vol. 219:608-611


To determine causes and solutions for return-to-sensibility problems after electrical stunning in pigs.


Case Studies

Sample Population

6 federally inspected pork slaughter plants


One hundred to 200 pigs were scored in each plant for stunner positioning, squealing when stunner was applied, and signs of insensibility. All pigs were held in a V-shaped restrainer conveyor and stunned with a manually applied head-to-body electrical stunner.


Percentage of pigs that had blinking after stunning ranged from 0.5 to 7. None of the pigs had a righting reflex or kicked in response to stimuli. All signs of possible return to sensibility disappeared before bleeding pigs reached the scalding tub. Spontaneous eye blinking was eliminated by improving bleeding practices to increase blood flow, ergonomically redesigning the stunner operator's work station to make correct placement of the stunner easier, redesigning the head electrode to facilitate correct placement, reducing line speed from 1,200 to 1,080 head/hr, correcting problems with poor initial contact of the stunner, and increasing amperage of a stunner that was set too low for sows. In 1 plant, a fatigued operator was the cause of stunner placement mistakes that resulted in signs of returning to sensibility.

Conclusions and Clinical Relevance

Problems with electrial stunning can be easily corrected, but effective stunning requires monitoring of correct electrode placement, amperage, and bleeding procedures. Observation of spontaneous natural eye blinking without touching the eye is recommended for use under field conditions, because it is less prone to misinterpretation than are other methods.

Full Publication

Since 1999, restaurant companies have been auditing the slaughter plants that supply them with pork and beef to ensure that they handle and stun animals properly because the public is becoming increasingly concerned about the welfare of livestock. To provide the best possible welfare, a stunning method must instantly render an animal completely insensible to pain, and bleeding must be done correctly to ensure that the animal does not return to sensibility. Audits by McDonald’s Corporation have already greatly improved handling and stunning in beef plants1. Many plants now perform self-audits to ensure that pigs and cattle are rendered completely insensible and that they remain insensible during slaughter procedures. Close scrutiny of electrically stunned pigs has resulted in locating and correcting electrical stunning problems that were previously undetected.

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.

Materials and Methods

Thirty-eight animal welfare audits were conducted by McDonalds Corporation Hazard Analysis Critical Control Points (HACCP) team during 1999 and 2000. Data for this study were collected from 6 slaughter plants that failed a McDonalds Corporation audit or an internal audit, because 1 or more pigs had signs of possible return to sensibility. The author was consulted to correct problems. In each plant, 100 to 200 pigs were scored on 3 variables: correct positioning of the electrode on the head to ensure that the current went through the brain, squealing when the electrode was applied, and signs of retum to sensibility6. Results of research indicate that the current must pass through the brain to induce instantaneous insensibility7,8. Squealing will occur when the electrode is applied if the operator mistakenly energizes it before it is in firm contact with the pig. When this occurs the pig will feel the shock and, in plants with high line speeds, may not receive sufficient stun time for reliable loss of sensibility. A plant failed the audit if any of the following signs of retum to sensibility were observed in a stunned pig: spontaneous blinking in which the eyelids closed completely and then opened without being touched, righting reflex in which the pig attempted to lift its head, vocalization (squealing), rhythmic breathing, or response to painful stimulus (reacting when the scalding water was contacted). Gasping and kicking of the limbs were ignored unless the kicking was a reaction to contact with scalding water. To determine that the return-to-sensibility problems were corrected, a McDonalds Corporation HACCP auditor returned to each plant and evaluated a minimum of 100 pigs after corrections were made. One plant that failed its own internal audit was also reaudited by a McDonald’s Corporation HACCP auditor.


In 1999, 17 plants passed the insensibility audit, and 2 failed. ln 2000, 16 plants passed, and 3 failed. One plant failed its own internal insensibility audit and passed the McDonald’s Corporation audit, because it had corrected its problems. Among the 6 plants that failed the insensibility audit during 1999 or 2000, only 2 had 100% correct stunner placement. Audits performed during 2000 revealed that 13 of the 19 plants had 100% correct stunner placement and 100% complete insensibility. Squealing during stunning was also high in plants with return—to-sensibility problems. During 2000, approximately half of the plants (n = 10) had 0% squealing when the stunning wand was applied and also had 100% completely insensible pigs. However, none of the 6 plants that failed the insensibility audit had 0% squealing during stunner application. None of the 19 plants used a tong-type head-only stunner with 2 head electrodes; all used manual cardiac arrest stunning in which 1 electrode was placed on the head in the hollow behind the ear, and the second electrode was placed on the side of the body. Each plant used a stunning wand that was designed and built in their maintenance shop, which is a common practice in large US plants. All stunning devices operated at the standard US electrical current of 60 Hz and were set at ≥ 1.25 A. In all 19 plants, the pigs were held in a V-shaped restrainer conveyor during stunning. Line speeds ranged from 625 to 1,250 head/h for plants slaughtering market-weight pigs; 1 sow slaughter plant had a line speed of 70 head/h. The cause of return-to-sensibility problems was located and corrected in all 6 plants.

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.


The plants that had return-to-sensibility problems had much worse stunner positioning than did the plants that passed the audit with 100% insensibility. This indicates that correct stunner wand application is essential for inducing instantaneous insensibility.

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.


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