What is mass killing?

Mass killing, or the humane killing of large numbers of farm animals, is at times required in the event of an emergency animal disease outbreak to control and prevent further spread of the disease. Emergency animal diseases where mass killing may be considered include diseases that are exotic (disease that is not present) to Australia, a variation of an endemic disease (disease that is already present in a population), a zoonotic disease (diseases that can transfer from animals to humans), 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 [1].

In Australia, the response to an emergency animal disease outbreak is coordinated by the relevant state or territory authority. Where mass killing is required, the chosen killing methods must consider the animal welfare implications while meeting biosecurity requirements and minimising the risk of disease transmission [3]. It is critical in these events that animal welfare is a priority when determining the best course of action.

Why is mass killing required?

The need to undertake mass killing may be required in several circumstances such as [2, 3]:

• infected herds/flocks of animals with a disease, including diseases which cause significant illness/death, where the disease can spread to humans, or the spread must be controlled by law
• pre-emptive control of other herds/flocks close to infected animals
• animal welfare reasons, such as where transport restrictions are in place or staff shortages at abattoirs causing on-farm overcrowding or feed shortages, and
• natural disasters.

The animal welfare concerns associated with mass killing

Disruptions to the usual supply chain process such as transport restrictions during emergency animal diseases outbreaks, staffing shortages, and abattoir closures or shift reductions may result in fewer animals being able to be slaughtered than planned. This is a particular concern for intensively farmed species, such as pigs, poultry, and animals in feedlots, who are at risk of experiencing overcrowding on farm due to limited indoor housing space, potential feed shortages from reduced or no supply, and health problems. In these circumstances, where animal movement on and off the property is often completely stopped or limited, mass killing may have to occur on farm.

To ensure animal welfare is safeguarded during short, medium, and long-term disruptions, stakeholders in the supply chain must have coordinated contingency plans in place. Animal welfare must be one of the primary considerations in these contingency plans, including contingencies if mass killing is determined to be the only measure to prevent negative animal welfare outcomes. In situations where mass killing is required, there are three critical points where animal welfare must be considered [2]:

1. Animal handling and restraint prior to killing
2. The stunning/killing method used
3. Confirmation of death

It is important that animals are handled as little as possible to avoid any unnecessary stress. Any killing method that requires minimal handling of animals should always be the first consideration.

In the event of transport restrictions or limitations, animals being transported in trucks or trains, and not deemed a biosecurity risk, should be offloaded and provided with 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 killing method chosen must consider all factors relating to the situation. On a farm, several different methods may be needed to kill animals of different sizes and ages. Whatever the method, it must ensure that animals are killed immediately and effectively while avoiding pain and stress. It is critical following any killing 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 [1]:

• 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
• Onset of rigor mortis (occurs several hours after death)

Personnel must be trained and competent at recognising signs of unconsciousness and death and should be highly skilled in accurately performing the most appropriate, species-specific procedure.

Current methods of mass killing

When mass killing is required, there are numerous factors that must be taken into consideration to determine what method is most appropriate for a specific situation. Some of these factors include [1, 4]:

• 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
• Disposal and decontamination requirements

Consideration must also be given to the order in which animals are killed, which in most cases means that affected/infected animals will be killed 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 killing. These 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 [5].

Poultry

If on-farm mass killing 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.

More research is needed to develop suitable methods of killing. Until then, current methods in the event of mass killing should be limited to [3, 5, 6, 7, 8]:

• Carbon dioxide gas (whole shed, partial shed or containerised) has been used in several types of poultry (e.g., meat chickens and layer hens), however, special consideration must be given to birds that may be more resistant to carbon dioxide gas such as young birds or water birds (ducks or geese).
• Foam (high expansion air or gas-filled) has been used for floor-based poultry housing systems and has demonstrated results comparable to carbon dioxide gas.
• Low atmospheric pressure stunning/killing (portal purpose built systems) has been validated for use in meat chickens and as shown equivalent to improved welfare outcomes compared to carbon dioxide gas.
• Non-penetrating captive bolt devices are more appropriate for a smaller number of birds and for larger birds such as turkeys.

Pigs

If on-farm mass killing of pigs is required, consideration must first be given to whether the affected pigs are housed in indoor or outdoor production systems, and then to the different sizes and ages of pigs raised on the farm. Special consideration must be given to sows who are farrowing (in labour) or nursing, and sows with unweaned piglets should be killed together.

More research is needed to develop alternative killing methods that address the animal welfare challenges associated with some of the current methods. Until then, current methods in the event of mass killing should be limited to [3, 5, 9, 10]:

• Carbon dioxide gas or inert gas (nitrogen) for unweaned piglets or piglets <5kg.
• Non-penetrating captive bolt devices for piglets <5kg.
• Penetrating captive bolt devices in pigs >5kg.
• Firearms for pigs >5kg in outdoor systems.

Following application of killing method, such as captive bolt devices or firearms, and once pigs are confirmed to be at least unconscious, a secondary method of killing, such as pithing (destroying the brain stem) or exsanguination (bleeding out), should always be done, especially in adult pigs, to ensure the pigs are effectively killed.

Cattle

If on-farm mass killing of cattle is required, consideration must be given to whether cattle can be mustered into yards or killed in paddocks.

Current methods in the event of mass killing of cattle should be limited to:

• Firearms at the closest possible range for cattle who are mustered into yards, or at a distance in paddocks for cattle that cannot be mustered.
• Penetrating captive bolt devices for cattle who can be restrained safely and for young cattle (<100kg).

The use of captive bolt devices should ideally always be followed up by a secondary method of killing to ensure death (a second shot, draining of blood, pithing). Selection of the appropriate calibre of firearm and bullet or shotshell are important when gunshot is used for killing. Captive bolt also requires accurate selection of powder charge and choice of penetrating or non-penetrating bolt. The most important factor for safe and effective killing is correct placement of the bolt or bullet to cause immediate loss of consciousness and death. As such, personnel carrying out killing must be highly trained and competent [5, 11].

Sheep

If on-farm mass killing of sheep is required, sheep should be moved into yards, usually with the addition of portable panels.

Current methods in the event of mass killing of sheep should be limited to:

• Firearm
• Penetrating captive bolt for sheep who can be restrained safely.
• Where small numbers of sheep are being killed, sodium pentobarbitone injection and sedation with xylazine prior to these terminal procedures may be the preferred approach to maximise sheep welfare [5].

What is the RSPCA’s view on mass killing methods?

The RSPCA is opposed to all inhumane methods of killing. Animals should either be killed instantly or rendered insensible until death ensues, without pain, suffering or distress.

One method of mass killing that RSPCA considers inhumane is any form of ventilation shutdown because it causes prolonged suffering and distress to animals before death. Ventilation shutdown involves sealing a shed by shutting air inlets and/or by turning off ventilation fans following which the internal shed temperature rises from animals’ body heat and toxic gases accumulate from a build-up of manure in the shed. After several hours or days animals eventually die from hyperthermia (extreme heat) and hypoxia (suffocation). Ventilation shutdown has been used overseas as a method of mass killing for poultry and pigs during emergency disease outbreaks. To read more about the animal welfare issues with ventilation shutdown as a method for mass killing, click here.