W.E. Morgan Morrow1, R.D. Munilla2, and R. Bottcher2
1Department of Animal Science and 2Biological and Agricultural Engineering
North Carolina State University
Raleigh, NC 27695


The task of euthanizing pigs is always difficult. Generally people don't like doing it, yet, to alleviate the suffering of some pigs and to protect their healthier pen-mates, some pigs must be euthanized. To do the job properly the technician must choose the right euthanasia agent for the weight of the pig and perform the job cleanly and consistently. A good euthanasia agent should:

Result in death without pain
Produce a rapid loss of consciousness
Produce a rapid death
Be reliable
Be safe for personnel
Minimize animal stress
Be non-reversible
Be non-traumatic for the operator
Be economic

To achieve all these objectives is difficult if only because any agent that is effective at killing a pig also has the potential for harming the human operators. The American Veterinary Medical Association has a panel on euthanasia which has reviewed the situation and recommended a variety of agents suitable for swine. In our opinion, the captive bolt is appropriate for euthanizing swine weighing more than 30 lb. However, we believe that we can design a better system for euthanizing piglets.

Problem Statement:

The current methods for euthanizing culled piglets are emotionally distressful to some personnel in swine nursery facilities. Most commonly, piglets are put down by electrocution or by inducing cerebral trauma with a blow to the head; when properly administered both methods are considered humane and cost effective. However, they require direct handling and observation of the piglets during the euthanizing process. A method or device to euthanize the piglets that is less stressful to nursery personnel is required.

Design Goals:

Design Constraints:

Design Conceptualization:

Basically only methods fully or conditionally recommended by AVMA can be employed. These are: injection of barbiturates or chloral hydrate, cerebral trauma induced by penetrating captive bolt or gun shot wound, electrocution, decapitation, and inhalation of CO, CO2, N2, Ar, or gaseous anesthetics,

To preclude observation of the death event, all design solutions must be based on some kind of container. The requirements for minimal handling and observation of the animals essentially preclude the use of injectables (only intravenous injections work reliably).

Achieving cerebral trauma (penetrating captive bolt, gunshot), electrocution, decapitation, or cervical dislocation semi-automatically would require restraining yolks and/or harnesses of varying degrees of complexity. Electrocution would require placing two electrodes, one on the head and the other on the hindquarters, in a manner which insures high conductivity and allows a strong current to pass through the heart. Cervical dislocation would require a double yolk, which could firmly and separately grip the head and the body and then be activated to separate them abruptly. Decapitation and using a captive bolt would only need one point of restraint, the neck or the head respectively, and should be easier to implement. However both methods violate the constraint on blood letting and mangling, and probably would be considered too gruesome. All of these restraining devices would have to be made adjustable to fit the large range of sizes of the piglets. The only other alternative would be to make and market several different sizes, which seems cost-prohibitive. Even if an adjustable device could be designed cost effectively it would still require singular insertion and extraction (i.e., non-minimal handling) of each piglet. It must also be noted that the AVMA findings indicate that animals experience varying degrees of stress when they are physically restrained.

The use of inhalants would eliminate the need for restraining devices. In fact, since the respiration volumes of the piglets are proportional to their body sizes and weights, it seems likely that a single device could be used to put down 12 one pounders or a single 12 pounder in one cycle. This aspect would provide a "one size fits all" feature and would be well suited to the demand for culling, which is inversely proportional to piglet age. Because a gassing chamber would require minimal internal fixtures, it seems likely that a disposal liner could be easily fitted. This feature, along with the absence of tissue damage, should make for easy clean up and minimize personnel's contact with the cadavers.

The risk of low level exposure to carbon monoxide or to the AVMA recommended inhalant anesthetics could be minimized through the use of detection devices, and alarm and shutoff circuits; these devices are readily available for CO. Catastrophic failures with large volumes of gas however, would create a liability concern and increase the cost of manufacture of the chamber. Large volumes of compressed toxic gas would also provide an opportunity for suicide attempts. To illustrate, a 2 ft3 chamber using halothane would need to have storage capacity for a least 40 ft3 of gas (~20 charges) to be practicable; CO would only require 2 ft3, but that would be enough to achieve a 0.5% atmosphere in a small bathroom for one hour and kill someone. In spite of this risk, a euthanizing device based on the safe generation and management of CO has much to recommend it. If small amounts of gas could be generated during operation and stored for safe disposal or venting immediately after the euthanizing procedure, the device's potential for use in suicide attempts would be greatly reduced. The risk to operating personnel could be easily handled by redundant CO sensor and cut off circuitry that would disable the gas generation process in the event of any CO leaks.

1) Use of commercially compressed CO gas cylinders

2) Chemical interaction of sodium formate and sulfuric acid

3) Exhaust fumes from the incomplete combustion of any carbon based fuel.

The first method must be excluded from consideration because of the inherent risk for accidental or deliberately induced catastrophic failure of the storage system. The second method can be disregarded because the chemical reaction of sodium formate and sulfuric acid produces an irritating mixture of CO and acid fumes and would have to be scrubbed before it could be pumped into the gassing chamber. In addition, the safe storage and handling of sulfuric acid presents a problem as does the accurate control of the gas generation process.

On the other hand, CO generation by combustion of carbon based fuels is a relatively simple task. With the right feedstock and controls an exhaust with a 10-12% concentration of CO can be obtained. This gas mixture could then be easily filtered and cooled, and pumped into the chamber. After each euthanizing cycle the chamber would be flushed with fresh air drawn in by a compressor. This mixture of CO and air would be stored in a small cylinder, from where it could be recycled into the chamber (along with some fresh, CO mixture) for use in the next cycle. After each day's operations the gas mixture could be safely vented outdoors.

AVMA Guidelines on Carbon Monoxide

Carbon monoxide (CO) combines with hemoglobin to form carboxyhemoglobin. This blocks the uptake of oxygen by red blood cells.

Clinical signs of CO toxicosis, as originally described, are due to its action upon the blood system. In people, initial symptoms are headache sometimes combined with nausea, followed by depression progressing to unconsciousness. Because CO stimulates motor centers in the brain, unconsciousness may be accompanied by convulsions and muscular spasms.

Carbon monoxide is a cumulative poison. Distinct signs of CO toxicosis are not induced until the concentration is 0.05% in air, and acute signs do not occur until the concentration is approximately 0.2%. In human beings, exposure to 0.32% and 0.45% of CO for one hour will induce unconsciousness and death.

When CO is produced by combustion, oxides of nitrogen and hydrocarbons, oxygenates of hydrocarbons, and heat must be controlled to prevent discomfort to the animal. This may be done by passing exhaust gases through a water chamber and a metal gauze filter with a cloth screen. The water chamber cools the gas, removes some carbon particles, and entraps the oxides of nitrogen, hydrocarbons, and oxygenates of hydrocarbons. The cloth filter removes carbon particles, allowing relatively clean, nonirritating CO gas to enter the chamber.

An idling engine running on a rich fuel mixture will produce the highest percentage of CO in exhaust gas. Carbon monoxide produce by an internal combustion engine is just as effective as cylinder CO and considerably less expensive. Chamber concentration of CO from piped exhaust gas can quickly reach 8%, resulting in 70% saturation of hemoglobin. Carbon monoxide must be considered extremely hazardous for personnel because it is highly toxic and difficult to detect. An efficient exhaust or ventilatory system is essential to prevent accidental exposure of human beings.

Advantages-(1) Carbon monoxide induces rapid and painless death; (2) hypoxemia induced by CO is insidious so that the animal is completely unaware of it; and (3) unconsciousness occurs without pain or discernible discomfort, when properly administered.

Disadvantages-(1) Safeguards must be taken to prevent exposure of personnel; (2) during chemical generation by sodium formate and sulfuric acid, irritating vapors of sulfuric acid must be removed by passing the CO through a solution of 10% sodium hydroxide; and (3) exhaust gases must be filtered and cooled to prevent discomfort to animals.

Recommendations-Carbon monoxide used for individual or mass euthanasia is acceptable for small animals, including dogs and cats, provided that the following precautions are taken: Personnel using CO must be instructed thoroughly in its use and must understand its hazards and limitations; the CO generator and chamber must be located in a well-ventilated environment, preferably out-of-doors; the chamber must be equipped with internal lighting and viewports that allow personnel direct observation of animals; the gas generation process should be adequate to achieve a CO concentration throughout the chamber of at least 6% within no more than 20 minutes after animals are placed in the chamber; sodium formate- and sulfuric acid-generated CO must have the irritating acid vapors filtered out by passing it through a 10% solution of sodium hydroxide;

If CO generation is by combustion of gasoline in an engine, (1) the engine must be operating at idling speed with a rich fuel-air mixture; (2) prior to entry into the chamber, the exhaust gas must be cooled to less than 125oF (51.7oC); (3) the chamber must be equipped with accurate temperature gauges monitored by attendants to assure that internal temperature of the chamber does not exceed 110oF (41.3oC), and (4) the exhaust gas must be passed through water and cloth filtration processes to remove irritants and carbon particles before entering the chamber. Exhaust gas piped into a chamber from a cruising vehicle is not acceptable.

Take-Home Message.

Euthanasia is a complex issue. It is an unpleasant task yet one that must be done if we are to properly care for our animals. Researchers at NSCU are investigating the issue and are developing a safe and humane system based on the generation of carbon monoxide.

The CO gassing chamber described above will meet all the design criteria. This design could operate in batch mode and would provide minimal handling of the piglets, no tissue damage, and easy cleanup and disposal. Its only objectionable feature, relating to the use of CO, would all but be eliminated by the gas production and management system and the multiple failsafe devices. It represents by far the best overall solution to the problem.