Killing mammals using electricity - two stage application

Electricity can be used to stun and kill animals. This method involves stunning the animals with electricity and death is caused either by bleeding (cutting the major blood vessels between the heart and brain), or by electrocution (by applying an electric current to stop the heart). Bleeding is not recommended during a disease control operation as it is a biosecurity risk, and so electrocution should be performed.

The purpose of electrocution is to kill the animal by stopping the heart from pumping blood around the body (known as ventricular fibrillation). If this happens, the brain will be starved of oxygen and rapidly die. When an appropriate electric current is passed through the heart it goes into a state known as ventricular fibrillation, meaning the heart muscles fibres contract in a rapid, uncoordinated manner instead of in a regular, coordinated way; blood circulation stops and, if this state persists, death will soon occur. When an animal is electrocuted it becomes rigid with slight body tremors and then gradually relaxes. There should be no further movement. It cannot be guaranteed that every animal will go into cardiac arrest: if an animal exhibits the signs of a head-only stun, such as clonic paddling movements of the legs, then it should be stunned immediately using a captive-bolt device followed by pithing to prevent recovery. Equally, it may be possible for the animal to go into a cardiac arrest without being effectively stunned. The symptoms of this are very difficult to observe as the animal may be paralysed and will die very quickly, but the presence of eye movement or corneal reflex (reaction to touching the surface of the eye) are reasonable indicators. Should this situation occur, the animal must be re-stunned immediately, the equipment checked before further use, and the placement of the electrodes carefully monitored.

Please also see the HSA publication Electrical Stunning of Red Meat Animals on-line guide http://www.hsa.org.uk/electrical-stunning-of-red-meat-animals-introduction/introduction-1

The two-stage application of electric current may be performed to kill young and adult sheep and goats, and calves and pigs over one week of age. It comprises the application of current to the head by scissor type tongs which stuns the animal, followed immediately by the application of the tongs across the chest in a position that spans the heart which causes death by cardiac arrest. The heart is particularly susceptible to low frequency current e.g. 50 Hz, but not to higher frequencies. Therefore, it is important to check that the frequency of the stunning equipment is set to 50 Hz. The application of electricity to the animal’s head during the first stage results in unconsciousness and seizures. During the first (tonic) phase, when current flows through the brain, the animal collapses and stops breathing, with the front legs extended rigidly and the hind legs flexed into the body. The second (clonic) phase sees the animal relax and start involuntary kicking of both the fore and hind legs. The electrical current is applied for a minimum of three seconds. Stage two is performed as soon as possible after the head-only stun when the animal is unconscious and the tongs are applied across the chest so they span the heart for a further 8-10 seconds causing the  death of the animal via cardiac arrest. It is vital that the operator ensures the animal is unconscious before the second application is performed, as electrocution is known to be an extremely painful experience. The electrodes should be applied firmly for the entire duration of time and the pressure should not be released until the stun is complete. Two team members are required to perform the two-stage killing method with electricity. The first is responsible for the application of the electrodes to the animal, and the second can manipulate the position of the animal to allow the second application to be made. Stunning equipment is developed and designed to be efficient in a specific context. Manufacturers are therefore required (EC 1099/2009) to provide detailed instructions to users, concerning the conditions under which equipment should be used and maintained, to ensure optimal animal welfare.

Stunning and killing parameters

The frequency of the current should be no greater than 80Hz because, as frequency increases, ventricular fibrillation is less likely to result. The appropriate voltage and current required depends upon the species and age of animals as in Table 1.

Table 1: Minimum electrical parameters for stunning and killing livestock.

NB: These parameters have been developed for clean animals with relatively low resistance. High resistance caused by thick fleece or dirt may affect the efficacy of a stun and parameters should therefore be adjusted in accordance with Ohm’s Law to ensure the animals are adequately stunned/killed.

It is important to keep the contact resistance as low as possible to maximise the flow of current. The overall resistance to current flow is due to two factors; the tissues of the body and also the contact between the electrodes and the skin. The conductivity of the exterior of the animal may be improved by wetting the skin or fleece. From the perspective of the operator, it is possible to minimise the contact resistance by applying electrodes in the correct position and maintaining constant pressure for the duration of the application. Table 2 shows the variation in resistance (Ohms) caused by fleece cover on sheep.

Table 2: Approximate electrical resistance of sheep with varying fleece cover.

Very often there is a build-up of grease and dirt on the electrodes. This is especially likely when the equipment is used on-farm. This build-up increases the electrical resistance and must be regularly removed. Failure to clean electrodes will lead to corrosion, further increasing resistance. The electrodes must be thoroughly cleaned to the manufacturer’s specifications regularly, to maintain optimum electrical contact with the animal. Although resistance falls once the current begins to flow, it is the initial resistance that must be overcome to deliver the recommended current in order to produce an immediate stun. Table 3 below shows the variation in stunning parameters required for an effective stun if resistance is high.  For a more detailed explanation of electrical stunning and the parameters required please see the HSA publication Electrical Stunning of Red Meat Animals    on-line guide   http://www.hsa.org.uk/electrical-stunning-of-red-meat-animals-introduction/introduction-1.

Table 3: Voltage and current required to effectively stun pigs and sheep if resistance is either low or high.

Modern electrical stunning equipment is constant current by design and will automatically adjust the applied voltage to maintain the required current, this has obvious welfare benefits. However, it remains critical to reduce contact resistance by keeping the electrodes clean. Electrical stunning equipment should be regularly tested by a qualified electrician to ensure that the output to the tongs is isolated from earth.

The handler and slaughterman must wear appropriate protective clothing (rubber boots) to minimise the risk to their own health and safety. Animals should be located in a pen which has a reliable supply of electricity. The animals’ death should be confirmed following the two-stage application of electricity using the criteria described in Figure 4. The person performing the kill should ensure the absence of brainstem reflexes for every animal. If there is any doubt surrounding the success of a stun or kill, an alternative method such as death by free-bullet firearm or captive-bolt followed by pithing, should be performed immediately.

Considerations when using electrical methods during depopulation due to disease control

There are advantages in using electricity to stun and kill animals during a disease control operation. A major advantage over firearms and captive-bolt devices is the great reduction in risk to biosecurity. The method is non-invasive, ie. the skin remains unbroken. Therefore, there is less concern regarding the presence of contagious pathogens entering and contaminating the environment. Unlike the methods described in the previous section, the brain also stays intact, meaning it could potentially be used for diagnostic analysis post-mortem.

The two-stage application of electricity is particularly well suited for use with adult pigs. As mentioned above, due to the conformation of these animals’ skulls, problems can arise when using mechanical killing methods. However, when using electricity this is not an issue.  This method is also less hazardous for the handlers and killing personnel.  Compared to mechanical methods, post-stun/kill convulsions of animals are reduced which means there is less risk to human safety.

There are disadvantages associated with the method. The two-stage method requires a reliable supply of electricity and the electrodes need to be applied and maintained in the correct positions to produce an effective stun and kill. The procedure may be physically demanding for the slaughterman and animal handler. If a large number of animals are to be killed, operator fatigue and poor electrode placement are a real concern. It is recommended that, where possible, the killing operative and handlers swap between tasks, or take regular breaks to prevent fatigue.

An important consideration regarding the welfare of animals is the risk of high contact resistance.  This is a particular concern when applying the method to adult unshorn sheep as the fleece may prevent the required voltage from being administered, especially as the tongs are applied to the chest. In addition, sheep breeds with high levels of wool cover on the head can increase the difficulty in maintaining low resistance contact. If necessary a saline solution may be applied to aid conduction. The horn position of sheep, particularly males, may also impede the electrode placement during stage one. It is also very important to clean the electrodes regularly to maintain minimal resistance. Using a wire brush, powered wire wheel or cleaning solution after use on 20–25 animals will help to maintain the equipment. Between killing operations, the tongs should be stored in a dry environment with the electrodes protected from potential damage.

Conclusion – species and situations

This method is suitable for use with small ruminants (although those with full fleece and horns may be problematic), pigs and calves (over one week of age).

Next: Methods suited for young animals (neonatal lambs, calves and piglets) - Blow to the head