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You are here: HomePublicationsElectrical Waterbath Stunning of PoultryIndicators of the effectiveness of stunning

Indicators of the effectiveness of stunning

A waterbath stunner should not be used until a person is available to ascertain whether it has been effective in stunning the birds. Immediately after application of an electric current, and before neck cutting, animals must be checked to ensure they are unconscious. When a bird has not been effectively stunned first-time, that person must stun and kill any such birds without delay. Depending on the electrical parameters used, even in a system thought to stun-kill the majority of birds, some may have retained a normal heart rhythm. Consequently, the design of the equipment, its layout and the line speed must allow adequate checks for effective stunning, whilst ensuring these factors do not significantly delay the application of neck cutting.

Assessing the effectiveness of stunning is a very important part of the entire slaughter process. Operators must be trained to identify signs of ineffective stunning and must understand the appropriate action necessary, to immediately protect birds from avoidable suffering. Ineffectively stunned birds must not be re-shackled for waterbath stunning a second time. Instead, a humane back-up stunner should be applied immediately, eg a captive-bolt device designed for poultry.

Assessment using a single animal behaviour may be misleading. Multiple reflexes and behaviours must be assessed in order to reach a reliable conclusion. Ideally, at any time after application of an electric current, birds should not display behaviours that might be associated with consciousness (eg rhythmic breathing).


Practicalities of assessment

It is preferable to assess a bird for the effectiveness of stunning, prior to neck cutting. If a bird is examined only after neck cutting, then:

  • If the bird’s spinal cord is damaged by the cut, it may not be possible to properly assess its state of consciousness.
  • Potentially, an automated neck cutter may occasionally cut a conscious bird, eg one that avoided the electrified water. This will most likely cause severe pain and suffering and is unacceptable.


Birds can be assessed for effective stunning in-situ in two ways. Whilst a single bird travels along a section of a shackle line, an assessor can follow it and perform a series of checks for indications of the effectiveness of stunning. Alternatively, or in addition, an assessor can stand still at a single point along a shackle line and perform certain checks on consecutive birds that pass by; however, at fast line speeds this typically only allows time to perform one type of assessment per bird and it may be difficult to assess whether a bird is rhythmically breathing.


Key indicators - rhythmic breathing and eye reflexes

Recovery of spontaneous breathing is considered to be the earliest indication of recovery of consciousness. The presence of rhythmic breathing indicates an animal is alive, but not necessarily conscious, but it remains a useful assessment tool because if a bird is breathing, then it has the potential to recover consciousness. EFSA suggests the presence of regular gagging (a brainstem reflex of forced/laboured breathing through the mouth) may gradually lead to resumption of rhythmic breathing, so any bird displaying gagging behaviour should continue to be observed and action taken if necessary. A humane back-up stunning method should be immediately applied to any breathing birds.

Death can be ascertained by testing for the absence of a nictitating membrane (third eyelid) reflex or by testing for the absence of a corneal reflex, as shown in Figure 15B. Although a positive reflex indicates a bird is alive, it does not necessarily indicate that the bird is conscious. However, the proportion of birds displaying eye reflexes (eg nictitating membrane reflex and palpebral reflex), either at certain times or over a specified time, can be useful for monitoring the effectiveness of an electrical stunning system. For example, a stun may be ineffective if the corneal reflex can be repeatedly elicited immediately after a bird exits the electrified water, or if a large proportion of birds display a positive reflex, or if, as the time increases since exiting the water, there is an increase in the proportion of tested birds displaying a positive reflex.


Indications that a bird has not been stunned or that it may be recovering from a stun (ineffective stunning):

  • Presence/return of rhythmic breathing - examine a bird’s abdomen (Figure 15Ai) for evenly-spaced rise-and-fall movements indicating inspiration and expiration. (Do not confuse with localised, rhythmic contractions specifically of the cloaca (Figure 15Aii).)
  • Presence of a corneal reflex (Figure 15Bv) or a nictitating membrane reflex (Figure 15Bi-iv), particularly if a positive result is highly repeatable.
  • Presence of a palpebral, or blink, reflex (the upper and lower eyelids meet to close the eye (Figure 15Bv) when the corner of the eye nearest the beak (medial/inner palpebral commissure/canthus) is gently touched).
  • Presence of a pupillary light reflex (the pupil constricts in response to a bright light shone close to the eye).
  • Presence of regular spontaneous eye blinking (ie blinking without human stimulation), particularly if the frequency increases with time. (Not to be confused with very rapid blinking that may terminate abruptly after a few seconds in a bird that is not breathing. These may be muscular fibrillations of the eyelid, not an indicator of recovery.)
  • Presence/return of muscle tone, eg a bird regains voluntary control of its neck and head. (Note: some electrical parameters may cause an involuntarily arched neck, which can be an indicator of effective stunning (Figure 15Aiii). The difference can be ascertained by placing a hand under the bird’s upper neck and head, and gently and repeatedly lifting them; if the bird holds its head away from the hand, or if the neck feels tense, it is likely to be recovering. Alternatively, an assessor can grasp the head of a shackled bird and gently pull it downwards; if the bird recoils it is probably conscious.)
  • Presence of voluntarily-controlled vocalisations.


Indications that a bird may be effectively stunned (but not killed):

  • No rhythmic breathing (examine the bird’s abdomen).
  • Absence of a corneal reflex or absence of a nictitating membrane reflex (note: the presence of these reflexes indicates a bird is alive but not necessarily that it is conscious - additional checks for consciousness should be performed immediately).
  • Absence of spontaneous eye blinking of the nictitating membrane or outer eyelids (may suggest a deep level of unconsciousness).
  • A lack of intrinsic (voluntary) control of muscles, eg a relaxed jaw with no muscular tension controlling movement of the beak; a relaxed neck with no self-controlled movement of the head.
  • Constant rapid body tremors.
  • Wings held tightly against body.


Indications that a bird may be dying, or has died, as a result of the electrical stun:

  • No return of rhythmic breathing; no gagging.
  • Absence of a corneal reflex and absence of a nictitating membrane reflex.
  • Absence of spontaneous eye blinking.
  • Pupils dilated and centrally-fixed.
  • Relaxed, limp body with no pulse, no muscle tone, no movement; wings drooping.



Figure 15. Assessing poultry behaviour to determine the effectiveness of stunning.






Please refer to the HSA Guidance Notes No.7 on ‘Electrical Waterbath Stunning of Poultry’ if additional detail is required.



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