Mass euthanasia, or the humane killing of large numbers of farm animals, is commonly required in an emergency animal disease outbreak to control and prevent further spread of the disease. An emergency animal disease is a disease that is either exotic to Australia, a variation of an endemic disease (disease that is already present in a population), a serious infectious disease of unknown cause, or a severe outbreak of an endemic disease that will have serious social or trade implications at a national level .
In an emergency animal disease outbreak in Australia, the response to that disease is coordinated by the relevant state or territory authority. It is critical in these events that animal welfare is considered a priority when determining the best course of action.
Why is mass euthanasia required?
- infected herds/flocks of animals with a disease, including diseases which cause significant illness/death, can spread to humans or spread must be controlled by law
- herds/flocks nearby infected herds/flocks where it is too risky to await a disease outcome investigation
- pre-emptive control of other herds/flocks close to infected animals
- animal welfare reasons, such as where transport to an abattoir cannot occur due to transport restrictions causing on-farm overcrowding or feed shortages; and
- natural disasters.
What are the animal welfare concerns with mass euthanasia?
When an emergency animal disease outbreak occurs, movement of animals on and off the property is often completely stopped or limited, meaning that in most circumstances mass euthanasia has to occur on farm. This restriction or limitation on transport, especially in intensively produced animals such as pigs or poultry, means that farms are at high risk of feed shortage and overcrowding.
In situations where mass euthanasia is required, there are three critical points where animal welfare must be considered :
- Animal handling and restraint prior to killing
- The stunning/killing method used
- Confirmation of death.
It is important that animals are handled as little as possible to avoid any unnecessary stress. Any euthanasia method that requires minimal handling of animals should always be considered if possible.
In the event of transport restrictions or limitations, animals being transported in trucks or trains should be offloaded and provided feed, water and shelter as soon as possible. Depending on the situation, animals may then be returned to their property/farm of origin or directed to a suitable holding yard or facility.
The euthanasia method chosen must consider all factors relating to the situation. On one farm, several different methods may be needed to euthanase animals of different sizes and ages. Whatever the method, it must ensure that animals are killed immediately and effectively while avoiding any potential pain and stress.
It is critical following any euthanasia method that all animals are checked to confirm they are dead before they are moved for appropriate disposal. At least three signs of death should be checked in each animal, these signs may include:
- No rhythmic respiratory movements
- No corneal reflex or ‘blink’
- Dilated pupils and/or glazing of the eyes
- No heartbeat or pulse
- Pale mucous membranes that do not refill after touching
- No withdrawal reflex/pain response (not a reliable sign)
- No jaw or tongue tone; and
- Onset of rigor mortis (occurs several hours after death) .
What are the current methods of mass euthanasia used?
When mass euthanasia is required, there are numerous factors that must be taken into consideration to determine what method is most appropriate for each specific situation. Some of these factors include:
- The animal species and age
- Number of animals;
- Whether domesticated or wild animals;
- The type of farming or housing system;
- Available facilities and equipment;
- Practicality and efficiency of the method;
- Training required;
- Human health and safety;
- Handling and stress level on the animals;
- Type of disease and sampling considerations;
- Biosecurity and legal considerations; and
- Disposal and decontamination requirements [1, 4].
Considerations must also be made for the order in which animals are euthanased, which in most cases means that affected/infected animals will be euthanased first, then animals who have been in direct contact with suspect and infected animals, and then other susceptible or at risk animals. In some cases, due to animal welfare considerations, certain animals must be given priority during euthanasia. Animals that require special welfare considerations may include animals that cannot access feed or water; sick and distressed animals; unweaned or young animals; animals in parturition (labour) or late pregnancy; as well as aggressive or potentially dangerous animals .
If on-farm mass euthanasia of poultry is required, consideration must be given to the type of production (breeder farm, hatchery, meat chicken farm or layer hen farm), the type of housing, and the size and age of the birds.
- Carbon dioxide gas has been used for most ages of poultry in previous emergency avian disease outbreaks. If using carbon dioxide gas, special consideration must be given for birds that may be more resistant to carbon dioxide gas such as young birds or water-based birds (ducks or geese).
- Foam (water-based air-filled or high expansion gas-filled) has been used for floor-reared meat chickens as well as in some previous emergency avian disease outbreaks and has demonstrated results comparable to carbon dioxide gas.
- Firearms, i.e. captive bolt guns, may be more appropriate than gas for larger birds such as turkeys.
Other euthanasia methods that have shown potential in research include foam with a gas (carbon dioxide or nitrogen) or carbon dioxide gas mixed with an inert gas (argon or nitrogen). However, these methods need further research to develop commercially viable systems for the mass euthanasia of poultry on farm.
If on-farm mass euthanasia of pigs is required, consideration must first be given to whether or not the affected pigs are housed in indoor or outdoor production systems, and then to the different sizes and ages of pigs produced on the farm. Special consideration must be given to sows who are farrowing (in labour) or nursing, and sows with unweaned piglets should be euthanased together simultaneously.
- Injectable barbiturates – for unweaned piglets or piglets <5kgs
- Carbon dioxide gas – for unweaned piglets or piglets <5kgs
- Non-penetrating firearms – non-penetrating captive bolt guns for piglets <5kgs
- Penetrating firearms – captive bolt gun in all other pigs >5kgs, or a gun in outdoor systems where pigs are unable to be caught or handled
- A secondary method of euthanasia – pithing (destroying the brain stem) or exsanguination (bleeding out) should always be done following the first method, especially in adult pigs, and only when the animal is unconscious, to ensure the animal has been effectively killed.
What is the RSPCA’s view on mass euthanasia methods?
RSPCA Australia defines humane killing as when an animal is either killed instantly or rendered insensible until death ensues, without pain, suffering or distress. RSPCA Australia is opposed to all inhumane methods of killing.
 Animal Health Australia (2007) AUSVETPLAN operational manual – livestock welfare and management. Version 3.0.
 Berg C (2012) The need for monitoring farm animal welfare during mass killing for disease eradication purposes. Animal Welfare 21:357-361.
 AVMA (2019) AVMA guidelines for the depopulation of animals: 2019 edition. Version 2019.0.1.
 OIE (2019) Killing of animals for disease control purposes. Terrestrial Animal Health Code 1:7.6.
 Animal Health Australia (2015) AUSVETPLAN operational manual – destruction of animals. Version 3.2.
 Alphin R, Rankin M, Johnson K et al (2010) Comparison of water-based foam and inert-gas mass emergency depopulation methods. Avian Diseases 54:757-762.
 Benson E, Malone G, Alphin R et al (2007) Foam-based mass emergency depopulation of floor-reared meat-type poultry operations. Poultry Science 86:219-224.
 Meyer R, Morrow M, Stikeleather L et al (2013) Evaluation of CO2 application requirements for on-farm mass depopulation of swine in a disease emergency. Agriculture 3:599-612.