Gas stunning of pigs:Gas mixtures

Equipment: alternatives, adjustments and maintenance

Mohan Raj

BVSc MVSc PhD

Gas mixtures approved in the EU

• High concentrations (not less than 40% in air) of carbon dioxide (CO2)– hypercapnia

• Carbon dioxide (no more than 30%) in association with inert gases (nitrogen, argon)– Hypercapnic hypoxia

• Inert gases– anoxia

• Applicable to Slaughter, depopulation and other situations

Equipment?

• The method may be used in pits, bags, tunnels, containers or in buildings previously sealed

Properties of CO2

• Denser than air

– Can be contained easily

• CO2 is an acidic gas and irritant to inhale

• CO2 is a potent respiratory stimulant

• Unconsciousness is induced by inhibition of brain cells through lowering of pH (acidosis)

– Recovery from acidosis is also distressing and painful

Physiological effect of CO2

• Carbon dioxide induces breathlessness

• Inhalation of carbon dioxide is distressing and painful

• Pain and breathlessness activate same brain regions

How to measure pain or breathlessness in animals?

– No verbal communication

Intrapulmonary chemoreceptors

Vertebrate animals have chemical receptors in their lungs (Intrapulmonary chemoreceptors, IPCs)

Acutely sensitive to carbon dioxide but insensitive to hypoxia / anoxia

Welfare significance (1)

Stimulation of IPC depresses respiration and the rate and extent depends upon the inhaled concentration of carbon dioxide

Welfare significance (2)

Inhalation of carbon dioxide leads to stimulation of central (brain) and peripheral (e.g. arterial) chemoreceptors

The effect of carbon dioxide on IPC is independent of the effects on central and arterial chemoreceptors, and pH of blood

Welfare significance (3)

IPC stimulation

Depression of breathing

Apnoea in animals = breathlessness / feeling of suffocation in humans?

Properties of inert gases

• Naturally occurring gases

– Nitrogen lighter than air, 78.2% in air

– Argon denser than air, 1% in air

• Colourless and odourless

• Unconsciousness is induced through hypoxia / anoxia

• Humans describe induction as a “pleasant way of losing consciousness”

Aversion testing in pigs

Mrs. Ruth Harrison

Aversion to gas mixtures

Pigs do not avoid anoxia created using argon or nitrogen

Pigs avoid high concentrations of carbon dioxide

Aversion to CO2 in pigs

No aversion to anoxia

Inference

Induction of unconsciousness with hypoxia / anoxia is non-aversive

Carbon dioxide induces breathlessness

Inhalation of carbon dioxide is distressing and painful

Time to loss of consciousness – duration of suffering can be up to 45 seconds

Other welfare concerns with CO2

• Literature

– Animals show escape attempts

– Excitation phase during induction of unconsciousness

– Animals exhibit muscle jerks during induction of unconsciousness

– Convulsions start before onset of unconsciousness

EEG in >80% CO2

EEG of pig in anoxia

Other view point

• Carbon dioxide stunning is better than badly performed electrical stunning– Isolation of individuals for electrical stunning

– Stressful restraint

– Poorly designed and maintained stunning devices

– Pre-stun shocks

– Ineffective stunning

– Low stunning currents

– Recovery of consciousness

Advantages of gas stunning• Can be applied to small groups (group

stunning system) – no need to isolate or restrain individual animal

• Automated gas concentrations & exposure time less prone to human error

• Irreversible stunning prevents chances of recovery of consciousness– Concentrations of CO2 Vs exposure times

– Three minutes exposure 80% CO2

Group stunning system

Loading / stunning systems for pigs

Group loading /

stunning system

Relaxed carcasses

Easy to shackle and bleed

Palpebral and corneal reflexes are absent

Elicitation of palpebral

reflex in a pig

Operators’ safety

Relaxed carcass

after exit and

during bleeding

Exsanguination of pigs

Brachiocephalic trunk is cut by inserting a

knife through the thoracic inlet

Disadvantages

• Availability and supply of gas

• Prolonged exposure to kill with anoxia (7 minutes)

• Less competition – not many equipment manufacturers

• Fear of incidence of PSE condition due to anoxia stunning

Recent crisis in Europe

Economics

• High capital and operational costs– Argon is more expensive than CO2 or nitrogen,

depends upon market situations

• Economic return due to improvements in carcass and meat quality is relative, e.g.– Handling and loading on farm

– Transport conditions

– Unloading and lairage conditions

– Mixing unfamiliar animals

– Movement and restraining facilities

Meat quality

• Gas stunning produces better carcass and meat quality than electrical stunning

– Depends upon how bad the pre-slaughter handling, restraining and electrical stunning systems are

• Anoxia accelerates glycolysis and onset of rigor

– Carcass chilling rate need to complement rate of glycolysis

Traditional approach

• Relied on one single stunning method to meet all requirements

• For example,

– Animal welfare

– Commercial feasibility

– Economic viability

– Meat quality and safety

• All the methods have advantages and disadvantages

Equipment

• Gas mixtures can be administered in pits, tunnels or containers

• So far, only concrete pits have been used for denser than air mixtures, can be used for argon

• Tunnels and containers are used for poultry

– Knowledge and experience can be transferred ?

Potential alternative

• Induce unconsciousness with anoxia

– A mixture of 80% nitrogen and 20% argon

• Used in the UK for stunning turkeys

– Time to onset of unconsciousness in on average 21 seconds

• Then kill with an exposure to high concentrations of CO2

• Or, induce cardiac ventricular fibrillation

DEFRA project (MH0128)

Effect of duration of exposure to 85% nitrogen and 15% carbon dioxide on duration of occurrence of quiescent EEGs (indicator of unconsciousness)

– Five exposure times: 90, 120, 180, 240, 300 & 360 s

– Total EEG power to less than 10% of pre-exposure

Duration of quiescent EEG

Duration of exposure (s) to 85% nitrogen and 15% carbon dioxide

90 s 120 s 180 s 240 s 300 s 360 s

No. of pigs used 11 17 19 18 13 9

Died 0 0 3 9 13 9

Minimum (s) 13 24 48 49 N/A N/A

Maximum (s) 58 94 155 176 N/A N/A

Mean (s) 26 49 79 98 N/A N/A

Induction of cardiac ventricular

fibrillation

• 180 s exposure to the gas mixture

• CVF (trans-thoracic) treatments

– dry or wet skin

– 110, 125, 150 or 175 V RMS

– 1, 2, 3 or 4 seconds

CVF options

• 125 V RMS applied to wet skin for 4 s

• 175 V RMS applied to dry skin for 4 s

Electrical arcing or shunting occurs between CVF electrodes

over dry as well as wet skin surfaces

However, no evidence of significant effect on the induction of

CVF under these conditions

InferenceAssuming 22 s of delay between induction of CVF and onset of

brain death, as determine from the abolition of VEPs:

• CVF will have to be applied prior to pigs exiting gas mixture after 90 and 120 s exposure

• CVF will have to be applied within 26 s of pigs exiting the gas mixture after 180 s exposure

• Exposure of pigs to the gas mixture for 240 s resulted in death in 50% and CVF will have to be applied to surviving pigs within 27 s of exiting the gas mixture

• Prolonging the exposure time to 300 and 360 s resulted in death in all the pigs

Need more research and development: two phase stunning

system involving exposure of pigs to argon-induced anoxia and kill them

with CO2?

Guide to good practice

• Ensure smooth entry and passage of animals through the system

• No bunching or grouping of animals

• Gas concentrations monitored and maintained during exposure

• Visible and audible alarm systems

• Calibrate gas monitors

CO2 stunner (old)

Poor welfare in older systems

Single race and stop-start loading system

Causes of poor welfare

• Loading system – single file

• Stop – start

• Fanning of CO2 by the flap door

• Overloading in cradles

• Compression of the chest

• Uneven exposure times

• Overflow and build up of CO2 in the vicinity

Critical control points

• Monitor and maintain gas concentrations

• Adequate exposure times according to type of animals

• Interval between end of exposure to gas and bleeding last animal in a group should be appropriate and adequate to prevent recovery of consciousness

Monitoring points

• Key stage 1: between end of exposure to gas (stunning) and shackling

• Key stage 2: during sticking

• Key stage 3: during bleeding

Carbon dioxide stunning of pigs: key stage 1

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Carbon dioxide stunning of pigs: key stage 2

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Carbon dioxide stunning of pigs: key stage 3

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Thank you