Agriculture Reference
In-Depth Information
haemorrhages are said to be avoided, the number of
bone fractures is reduced to zero and the amount of pale
soft exudative (PSE) may be reduced.
brine and the skin of the head is kept clean but dry.
The presence of wool on the sheep's head can increase
electrical resistance significantly. The practice of wet-
ting the pig's head should be discouraged since it may
result in current tracking over the surface of the pig's
skin rather than through the brain.
Electrical stunning
There are many different types of electrical stunning
systems in use, most being manually operated while
some are automatic in operation, especially for pigs
and poultry.
This method consists of passing an alternating cur-
rent (AC) through the brain and, with some techniques,
also the heart of the animal, the instrument most com-
monly employed being one which resembles a pair of
tongs
. The current causes massive depolarisation of neu-
rons in the brain, resulting in EEG patterns similar to
that recorded in humans with a generalised epileptic sei-
zure and the associated tonic-clonic muscle contrac-
tions. This epileptiform activity is analogous to a loss of
consciousness and appears to be associated with loss of
sensory awareness.
The epileptiform activity in the brain is induced by a
release of neurotransmitters, glutamate and aspartate,
into the extracellular space. This is a reversible process
with the animal starting to regain consciousness about
40 seconds after the stun.
The conditions necessary to produce an effective elec-
trical stun are as follows:
1
The strength of the electric current should be suffi-
cient to ensure that the animal is killed outright by
cardiac arrest or remains insensible until death occurs
by exsanguination.
2
Provided sufficient current is applied, a genuine
elec-
troplectic shock
will be induced. The mains voltage
may fluctuate considerably and at times fall to a dan-
gerously low level in the stunning apparatus, and it is
therefore required that every electrical stunning appa-
ratus is fitted with indicators visible to the operator
that provide a warning if the current or voltage drops,
or the time of application falls short of 7 seconds for
low-voltage systems and below 3 seconds for high-
voltage equipment. Many systems apply a minimal
test current immediately before the stun current is
applied to gauge the resistance. If the resistance is too
high for an effective stun, the stun current will not
be applied.
3
The electrodes should be correctly positioned so that
the current will pass through the thalamus and cortex,
the chief sensory centres in the forebrain. This relates
to the space between the eyes and the base of the ears
on most species. The electrical resistance of contact
with the hair and skin may be lowered by ensuring
that the electrodes are kept moist by immersion in
There is little doubt that the failure of operators to
observe these criteria has been the cause of much of the
criticism of electrical stunning methods, firstly on the
grounds that the method was not always humane and
secondly because
haemorrhages
were often observed in
the muscular tissue of animals stunned by electrical
means.
It is necessary for an adequate amount of electrical
current to pass through the brain in a sufficiently short
period of time. This depends on the voltage applied and
the resistance, or more correctly impedance, present. If
too high a voltage is employed, carcase quality may be
compromised by the production of
muscle haemorrhages
and broken bones
; if too low a voltage is used, the animal
may be paralysed but still conscious of pain. Operator
safety also plays a part in determining voltage levels; lev-
els under 150 V are not generally effective, especially
allowing for fluctuations in mains voltage.
Whatever type of electrical stunning is used, a
backup
stunner
, in the form of a portable captive bolt device
should be available for use, not only on incorrectly
stunned animals, but also for casualty animals in their
transport vehicle or in the lairage. It may be advisable to
have an additional set of stunning tongs for use in several
electrical sockets positioned throughout the lairage and
casualty accommodation. This is particularly useful if
sows and boars, which are difficult to stun effectively
with a captive bolt, are to be slaughtered.
Since the brains of animals are relatively small, it is
important that electrodes are accurately and firmly
placed high up on the sides of the head. The irregular
anatomy of the head makes this difficult, especially if the
animal is moving. The electrodes are easier to position if
the tongs are relatively heavy and if the operator is able to
apply them to the animal downwards rather than
upwards or horizontally. Placement of tongs is also
achieved with less effort in efficient restrainer-conveyor
systems.
Head-to-body
equipment should be correctly
counterbalanced. The electrodes must be in good repair
and not be corroded or coated with carbon. It is essential
that the equipment is earthed properly for operator
safety (Fig. 7.6).
The passage of electric current through the brain
results in a rapid rise of blood pressure due to vasocon-
striction and increased heart rate, hence the need for
immediate bleeding in order to avoid blood splashing.
