Humane slaughter - Gracey's Meat Hygiene - page 139

was detected in jugular blood of 2% of animals, Coore
et al. (2005).
Pithing, described later, has been demonstrated to
increase the risk of neural emboli.

cost of installation and operation. The technique was
banned for a period during the 1980s in the Netherlands
because it was thought to lead to unconsciousness under
very stressful conditions for the animal.
Raj and Gregory (1995) demonstrated that pigs
showed no aversion to 30% CO 2 in air but a marked aver-
sion to 90% CO 2 in air. When pigs are introduced to 80%
or higher concentrations of carbon dioxide, there is a
period of about 10 seconds when there are little or no
signs of behavioural reaction. There then follows a
period of breathlessness and hyperventilation for a fur-
ther 10 seconds, and at this point, there may be vocalisa-
tion and escape behaviour. These signs are followed by
loss of posture and onset of convulsions. Based on
observed behaviours and measures of brain activity,
there is a considerable period of time, up to 15 seconds,
before the pig loses consciousness.
The use of other gas mixtures has been investigated.
Raj and Gregory (1995) and Raj (1999) reported that
no aversion was shown by pigs to an environment of
90% argon . The European Food Safety Authority report
(2004) recommended the following gaseous atmospheres
for the stunning/killing of pigs:
• A mixture of 30% CO 2 and 60% argon or nitrogen in
air, or
• with 90% argon or nitrogen (or other inert gas) in air.
In both cases, the maximum residual concentration
of oxygen should be 2% by volume. Pigs should be
immersed into these recommended gas concentrations
within 10 seconds from leaving the atmospheric air, and
they should be exposed to gas mixtures for a minimum
of 3 minutes. These regimes will, however, only give an
effective stun for less than 50 seconds so bleeding must
commence within 25 seconds. An exposure time of
7 minutes to these recommended atmospheres will result
in a stun/kill, but this is too long a period to be commer-
cially viable in practice.
Currently, however, carbon dioxide is the only gas
widely used for stunning animals commercially. It is usu-
ally stored in cylinders or bulk tanks as a liquid under
pressure. It is also available in solid form for which a
converter is necessary. The gas is non-flammable and
has a higher specific gravity than air, sinking to the bot-
tom of any container, a fact which has to be borne in
mind when it is being used for anaesthesia or euthanasia
purposes. It presents no hazard to the operator.
Inhalation of carbon dioxide induces respiratory and
metabolic acidosis and, hence, reduces the pH of the cer-
ebrospinal fluid (CSF) and neurons thereby exerting its
neuronal inhibitory and anaesthetic effects. These are meas-
ured as quiescent EEG recordings and abolition of soma-
tosensory evoked potentials in the brain. A concentration

Free bullet pistol
The free bullet pistol is frequently used to humanely
destroy horses, and sometimes cattle, humanely. Bullets
may be of the hollow pointed type, frangible iron plastic
composite missiles or powdered iron bullets fired from a
small-bore rifle (0.22 calibre), 9 mm or 0.38 calibre
handgun, or a small instrument held against the fore-
head. The benefit of hollow point projectiles is a greater
enhancement of the mushrooming effect or expansion
on impact; more energy is imparted to the tissues with
increased tissue destruction, and there is less likelihood
of the missile exiting the head.
The points of application are in general the same as
those for the captive bolt pistol, in the case of horses this
being high up in the forehead immediately below the
roots of the forelock where two lines from the medial
canthus of each eye to the base of the opposite ear cross.
The direction of aim in horses is slightly below the right-
angle plane to the forehead.
Great care must be taken to avoid accidents when
using free projectiles, and each instrument should be
fitted with a safety device. In future, the use of the free
bullet is likely to be increasingly discouraged for reasons
of safety.

Water jet stunning
This experimental form of percussive stunning, described
by Lambooij (1996), utilises a fine jet of water to pene-
trate the skull and mechanically destroy the brain by
the induction of laceration, crushing and/or shock-
waves to such an extent that immediate unconscious-
ness is induced. The 0.5 mm jet, applied at pressures of
3500-4000 bar at a similar site as for the captive bolt,
drills through the skin and skull in 0.2-0.4 seconds.
Destruction of the brain results in convulsions, which
can be controlled using an immobilising current of
400 mA applied using 40 V. Initial work has indicated
that it may be possible to produce meat of superior qual-
ity, compared to either electrical or CO 2 stunning, using
this method.

Carbon dioxide and other gas mixtures
Carbon dioxide was first used to induce pre-slaughter
anaesthesia in animals in 1904 but was not used success-
fully on a commercial scale until 1950. Since then the
method has been modified in several different ways and
is now widely used throughout the world, although not
as extensively as it could be, probably because of the high