How to Determine Insensibility in Cattle, Pigs, and Sheep in Slaughter Plants

(Revised July 2012)

by Temple Grandin
Dept. of Animal Science
Colorado State University


In both captive bolt and electrically stunned animals kicking will occur. Ignore the kicking and look at the head. To put it simply, THE HEAD MUST BE DEAD. When cattle are shot with a captive bolt, it is normal to have a spasm for 5 to 15 seconds. After the animal is rolled out of the box or hung up its eyes should relax and be wide open. After electrical stunning, a properly stunned animal should have a rigid (tonic) phase followed by a clonic (paddling of the legs) phase. This is an indication of a grand mal epileptic seizure. The seizure induces insensibility. Very long application of the current may weaken or eliminate these reactions in a properly stunned animal.

Below are the signs of a properly stunned animal:

  1. The legs may kick, but the head and neck must be loose and floppy like a rag. After captive bolt or electrical stunning, a normal spasm may cause some neck flexing, but the neck should relax and the head should flop within about 20 seconds. Check for natural blinking that looks like a live animal in the yards if flexing continues. Animals stunned with gas stunning equipment should be limp and floppy though they may exhibit slow limb movement.

  2. The tongue should hang out and be straight and limp. A stiff curled tongue is a sign of possible return to sensibility. If the tongue goes in and out, this may be a sign of partial sensibility.

  3. When cattle and pigs are hung on the rail, its head should hang straight down and the back must be straight. In sheep, the head should be limp and floppy but due to anatomical differences, the neck may be on an angle. It must NOT have an arched back righting reflex. When a partially sensible animal is hung on the rail it will attempt to lift up its head. It will be stiff. Momentary flopping of the head is not a righting reflex. Compared to cattle and pigs, sheep necks may not hang completely straight due to differences in anatomy.

  4. When captive bolt is used the eyes should be wide open with a blank stare. There must be no eye movements. Immediately after electrical stunning the animal will clamp its eyes shut, but they should relax into a blank stare. It is normal for animals stunned properly with electricity to have nystagmus (vibrating eye).

  5. When captive bolt is used the animal must NEVER blink or have corneal reflex in response to touch. In electrically stunned pigs eye movements can be misinterpreted when untrained people indiscriminantly poke at the eyes. It is often best to observe without touching the eye. If a pig blinks with a natural blink where the eye closes and then re-opens it is not properly stunned. If you are not sure what a natural blink looks like, go and look at live pigs in the yards (lairage) before assessing insensibility. Natural blinking must be absent after all stunning methods.

  6. Rhythmic breathing must be absent after ALL stunning methods. Count as rhythmic breathing if the animal's ribcage moves two or more times. Gasping is a sign of a dying brain and is acceptable after gas or electric stunning. It will look like a fish out of water. A twitching nose (like a rabbit) may be a sign of partial sensibility. Gasping must be absent after captive bolt.

  7. In captive bolt stunned animals, insensibility may be questionable if the eyes are rolled back or they are vibrating (nystagmus). Nystagmus is permissible in electrically stunned animals, especially those stunned with frequencies higher than 50 or 60 cycles. Nystagmus must not be confused with natural blinking.

  8. Shortly after being hung on the rail, the tail should relax and hang down.

  9. No response to a nose pinch or pinprick on the nose. The painful stimulus should be applied only to the nose. Animals entering a scauld tub must not make a movement that is in direct response to contact with the hot water. For all types of stunning this is an indicator of possible return to sensibility.

  10. No vocalization (moo, bellow or squeal) after all methods of stunning.

The above methods can be used for determining insensibility for all types of stunning and for ritual slaughter which is done without stunning. Just remember, kicking reflexes are normal in captive bolt stunned animals, electrically stunned animals and after ritual slaughter. They should be absent or very feeble for C02. Captive bolt stunning induces instant insensibility by both concussion and physical destruction of the brain. Stunner maintenance is essential to maintain maximum hitting power.

Electrical stunning, renders an animal instantly insensible by inducing a grand mal epileptic seizure. Scientific research has shown, that in order to induce the seizure the electric stunner must be set at a minimum of 1.25 amps for market weight pigs and l amp for sheep. Large sows will require 2 or more amps. If' lower amperages are used the stunner may induce cardiac arrest but the animal will feel the shock because the seizure was not induced. Electrical frequencies up to 800 hz (cycles) can be used. Frequencies over 800 hz should not be used. Research has shown that 1500 cycles failed to induce instant insensibility. Animals that are dehydrated may have high electrical resistance and be difficult to stun.

In some plants, cattle or sheep are immobilized after electric stunning with a small electric current to stop kicking. This immobilizer current completely masks signs of return to sensibility. To assess return to sensibility the immobilizer current MUST be turned off. Electric immobilization is highly distressful to animals and it must never be confused with electric stunning, which induces instantaneous insensibility by passing a high amperage current through the brain.

If an electrically stunned animal blinks within 5 seconds after stunning this is a sign that the amperage is too low. In electrically stunned animals, blinking should be checked within 5 seconds and after 60 seconds. In most plants blinking will not be found immediately after stunning, because the plant is using the correct amperage. After it has been verified that the amperage is set correctly, the most important point to observe for signs of return to sensibility is 60 seconds after electrical stunning. This provides time for the eyes to relax after the epileptic seizure. Checking for signs of return to sensibility after bleeding insures that the animal will not recover.

When stunned animals of all species are viewed from a distance, the most important signs to look for in a properly stunned animal are:

  1. A floppy head. The head should flop like a wet rag when a hind leg kicks due to reflexes.

  2. Tongue hangs straight out and is limp. There are some animals that are stunned properly and the tongue may be trapped in the mouth.

  3. In cattle and pigs, the back and head hang straight down. There is no arched back righting reflex.

  4. Ignore limb movement for all methods of stunning. Look at the head.

Animals that show all three of the above signs will be insensible and blinking and other eye reflexes will be absent.

Order of the events which indicate Return to Sensibility:

  1. Single feeble eye (corneal) reflex in response to touch (probably still insensible and not conscious).

  2. Return of rhythmic breathing, where the ribcage moves in and out two or more times. This is a primary indicator of poor stunning and it may occur before the corneal reflexes.

  3. Spontaneous natural blinking without touching (recommended sign for determining return to sensibility for regulatory purposes). In large plants this is easier to assess than rhythmic breathing.

  4. Response to a painful stimulus such as pricking the nose with a pin. The stimulus must be applied to the head to avoid confusion with spinal reflexes.

  5. Righting reflex and raises it's head.

  6. Fully conscious and sensible. Complete return to sensibility can occur within 15 to 20 seconds after eye blinks appear if an electrically stunned animal is not bled.

The American Meat Institute guidelines require that ALL of the signs of return to sensibility MUST be absent to pass an audit. Even thought an animal is probably insensible if it shows a weak corneal reflex or tongue movement, it is starting the process of return to sensibility. Weak indicators of return to sensibility can be abolished by improved stunning practices. Slaughter plants are not research laboratories where conditions are carefully controlled. Therefore a much greater margin of safety is required to ensure that the animal remains insensible.

An animal showing any of the above signs must be immediately re-stunned before any slaughter procedures are started.

Research findings on Insensibility (Updated November 2011)

Due to increased USDA enforcement of the Humane Slaughter Act, thousands of animals have been observed and data collected by USDA inspectors and plant personnel. A tiny percentage of animals still have a weak corneal reflex with no other indicators of return to sensibility present. These animals MUST be immediately re-stunnned before scalding or any invasive dressing procedure such as leg removal or skinning is performed. Scientific research shows that an animal with a weak corneal reflex in response to touching with a pen is insensible if no other indicators of return to sensibility are present (Anil 1991, Knudsen 2005, and Rumpl et al 1982). Vogel et al (2010) found that in pigs stunned properly by a cardiac arrest method, in some pigs a weak corneal reflex was present. Other signs of return to sensibility such as spontaneous natural blinking, righting reflex, vocalization, rythmic breathing, and visual tracking were all absent. A corneal reflex must NEVER be confused with natural spontaneous blinking that can be observed in live animals in the yards. Animals with spontaneous natural blinking are sensible. To insure that animals remain completely insensible after stunning and bleeding, good bleeding is essential (Grandin 2001). Increased sampling and measurement is probably detecting faint corneal reflexes that were not detected previously. My previous statement that weak corneal reflexes can be completely abolished in CO2 and electrically stunned animals may be wrong. Since corneal reflexes are the beginning of the process of return to sensibility, effort must be made to reduce them to a very low level and insure that all other signs of return to sensibility are absent. All animals showing a corneal reflex MUST be immediately re-stunned. There is a need to collect data on thousands of animals to determine the incidence of weak corneal reflexes when no other signs of return to sensibility are present in animals stunned with CO2 or electricity. In animals stunned with captive bolt corneal reflexes can be abolished.

There is also a possibility that some stunning failures in one of several thousand animals may be due to biological variability or abnormal nervous system development. In one strange case, 1 of 8000 electrically stunned pigs had a stiff arched back righting reflex 45 to 60 seconds after bleeding and there was no corneal reflex, no blinking in response to light, and no response to a nose prick. Most of these pigs were large, 275 lb (124 kg), and extremely heavy muscled. Another possibility is that their physiology was so overloaded by genetic selection or ractopamine (Paylean) that they went into rigor. The plant employees immediately re-stunned the pigs. The righting reflex was not due to poor bleeding because the effectiveness of bleeding was verified with dissection of the throat area. To insure that animals do not return to sensibility while hanging on the line, a plant employee MUST re-stun all animals that have a corneal reflex or any of the signs of return to sensibility listed in this paper. On a 100 animal audit, ALL animals must have no signs of return to sensibility.

Common mistakes and how to avoid them

  1. Mixing up gasping like a fish out of water with rythmic breathing. Gasping may be present after proper electrical or CO2 stunning. Gasping must be absent after captive bolt. Rythmic breathing is a sign of return to sensibility after ALL methods of stunning.

  2. Mixing up nystagmus (vibrating eye) with natural blinking. Natural blinking is a sign of return to sensibility after ALL methods of stunning. Look at live animals in the yards to help you differentiate between nystagmus and natural blinking.

  3. Mixing up a momentary flop of the head of a properly stunned animal with a true righting reflex. A true righting reflex is a sign of return to sensibility after ALL methods of stunning. When a righting reflex occurs, the neck will be stiff. During a momentary flop, the head will be floppy like a wet rag.

  4. When the tongue is extended, soft, and flacid, the animal will be properly stunned and insensible after all methods of stunning. However some properly stunned animals may have the tongue stuck inside the mouth.

Signs of a Properly Stunned Animal by Stunning Method: From the 2010 Edition of Recommended Animal Handling Guidelines and Audit Guide
Head Tongue Back Eyes Limbs Vocalization Respiration Tail Response to pain
Cattle - Captive Bolt Must appear dead, hang straight and floppy Straight and limp Hanging straight, no righting reflex No natural blinking; Wide open, blank stare; no response to touch; nystagmus absent Uncoordinated kicking of hind legs acceptable; No righting reflex present None Rythmic breathing (ribs moving in and out at least twice) is absent; Agonal gasping not acceptable Relaxes shortly after being on the rail A pinch or pinprick may be applied to nose only and NO response should be observed
Cattle - Electric Must appear dead, hang straight and floppy Straight and limp Hanging straight, no righting reflex Eyes may vibrate (Nystagmus), but no natural blinking Uncoordinated kicking of hind legs acceptable; No righting reflex present None Agonal gasping like a fish out of water normal; Rhythmic breating (ribs moving in and out at least twice) is absent Relaxes shortly after being on the rail A pinch or pinprick may be applied to nose only and NO response should be observed
Pigs - CO2 Must appear dead, hang straight and floppy Straight and limp Hanging straight, no righting reflex No natural blinking Uncoordinated kicking of hind legs acceptable; No righting reflex present None Agonal gasping like a fish out of water normal; Rhythmic breating (ribs moving in and out at least twice) is absent Relaxes shortly after being on the rail A pinch or pinprick may be applied to nose only and NO response should be observed
Pigs - Electric Must appear dead, hang straight and floppy Straight and limp Hanging straight, no righting reflex Eyes may vibrate (Nystagmus), but no natural blinking Uncoordinated kicking of hind legs acceptable; No righting reflex present None Agonal gasping like a fish out of water normal; Rhythmic breating (ribs moving in and out at least twice) is absent Relaxes shortly after being on the rail A pinch or pinprick may be applied to nose only and NO response should be observed
Pigs - Captive Bolt Must appear dead, hang straight and floppy Straight and limp Hanging straight, no righting reflex No natural blinking; Wide open, blank stare; no response to touch; nystagmus absent Uncoordinated kicking of hind legs acceptable; No righting reflex present None Rythmic breathing (ribs moving in and out at least twice) is absent; Agonal gasping not acceptable Relaxes shortly after being on the rail A pinch or pinprick may be applied to nose only and NO response should be observed
Sheep - Electric Must appear dead; neck hangs on an angle with limp and floppy head Straight and limp Due to anatomical differences in sheep, neck may not hang completely straight; No righting reflex Eyes may vibrate (Nystagmus), but no natural blinking Uncoordinated kicking of hind legs acceptable; No righting reflex present None Agonal gasping like a fish out of water normal; Rhythmic breating (ribs moving in and out at least twice) is absent Relaxes shortly after being on the rail A pinch or pinprick may be applied to nose only and NO response should be observed

References

Anil, M.H., 1991. Studies on the return of physical reflexes in pigs following electrical stunning. Meat Science. 40:13-21.

Grandin, T., 2001. Solving return to sensibility problems after electric stunning in commercial pork slaughter plants. Journal of the American Veterinary Medical Association. 221:1258-1261.

Knudsen, S.K., 2005. A review of the criteria used to assess insensibility and death in hunted whales compared to other species. Veterinary Journal. 169:42-59.

Rumpl, E., Gerstenbrand, F., Hackl, J.M., and Prugger, M., 1982. Some obervations on the blink reflex in post traumatic coma. Electroencephalography and Clinical Neurophysiology. 54:406-417.

Vogel, K.C., Badtram, G., Claus, J.R., Grandin, T., Turpin, S., Wegker, R.E., and Voogel, E. 2010. Head only followed by cardiac arrest electrical stunning is an effective alternative to head only electrical stunning in pigs. Meat Science. 89:1412-1418.


Preventing return to sensibility.

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