What is mass killing in the context of farmed animals?

Emergency animal diseases where mass killing may be considered include diseases that are exotic (currently not present in Australia), a variation of an endemic disease (already present in a population), a zoonotic disease (transferable 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]. Some infectious diseases, such as highly pathogenic strains of avian influenza, have substantial impacts on welfare, causing severe disease and high mortality rates. Therefore, mass killing in these cases may be needed to reduce the suffering of infected animals and prevent the spread to non-infected animals.

In Australia, the response to an emergency animal disease outbreak is generally managed by the relevant state or territory authority, but in some cases may be jointly coordinated with the federal government. Where mass killing is required, the chosen method must consider the animal welfare implications while also 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. 

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 when transport restrictions are in place or staff shortages at abattoirs causing on-farm overcrowding or feed shortages, and
• natural disasters. 

Disruptions to the usual supply chain process such as transport restrictions during emergency animal disease 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 commercially 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 (stunning ensures the animal is unconscious prior to killing so it does not experience pain, suffering or distress)
3. confirmation of death. 

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

In the event of transport restrictions or limitations, animals already in-transit who are 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 depending on the size and/or age of the animals. Whatever the method, it must ensure that animals are killed immediately and effectively while avoiding pain and distress. After applying any killing method, it is critical that all animals are checked to confirm they are dead before they are moved for appropriate disposal. When death cannot be confirmed, or there is any doubt, the same method, or a secondary humane method, must be immediately applied. 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
• greyed 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. 

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, age and size
• 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. In many cases, 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].

If on-farm mass killing of poultry is required, careful consideration must be given to the type of production system (e.g., breeder farm, hatchery, meat chicken farm or layer hen farm), housing type, and the size and age of the birds involved. 

Further research is needed to develop more effective and humane methods of mass killing. Until then, current methods should be limited to the following options [5, 6, 7, 8]: 

• Gas killing (‘modified atmosphere killing’): Different types and combinations of gases (CO₂, N₂, Argon) can be used to induce hypoxemia and death of poultry.
  • Carbon dioxide gas can be used in whole sheds, partial sheds (using partitioning) or in containers for various types of poultry (e.g., meat chickens and layer hens). Although this method is effective, exposure to high concentrations of carbon dioxide is aversive and painful. Special consideration is also required for birds that may exhibit resistance to carbon dioxide gas, such as young birds or waterfowl like ducks and geese.
  • Nitrogen and argon in gas form can be used in containers either alone or in combination with CO₂. Emerging methods may also allow the effective use of nitrogen for whole-shed gassing. These gases are physiologically inert and work by displacing oxygen thus creating an anoxic (no oxygen) environment. Research indicates welfare is much better with inert gases than for CO₂, and it is also more effective for waterfowl. 
• ‘High expansion’ foam (‘dry’ foam): High expansion foam has been effectively used internationally in whole poultry sheds as well as smaller enclosures and containers. This approach creates large, gas-filled bubbles creating pockets of gas around the animal. When combined with nitrogen gas this method has been found to be effective and humane for chickens. It should not be confused with ‘water-based’ foam (‘low-to-medium’ expansion foam), which creates bubbles small enough to be inhaled resulting in suffocation. Water-based foam is not a humane method of killing.
• Low atmospheric pressure stunning/killing (purpose-built systems): This method has been validated for use in meat chickens and shown equal to improved welfare outcomes compared to carbon dioxide gas. Further research is needed for this method in layer hens and other poultry species. 
• Non-penetrating captive bolt devices (a device that induces unconsciousness via a concussive blow to the animal’s skull): These devices are only suitable for smaller number of birds but are effective for larger birds, such as turkeys.

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 <10kg.
• penetrating captive bolt devices in pigs  
• firearms for pigs ≥5kg in outdoor systems. 

Following application of a killing method using 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. 

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 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]. 

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]. 

One method of mass killing that RSPCA considers inhumane is any form of ventilation shutdown. This method causes prolonged suffering and distress to animals before death. Ventilation shutdown requires sealing a shed by closing air inlets and/or by turning off ventilation fans leading to a rise in internal shed temperature from the animals’ body heat. Additionally, carbon dioxide levels increase from animal’s respiration and toxic gases accumulate from manure build-up in the shed. Over several hours or days animals eventually die from hyperthermia (extreme heat) and hypoxia (lack of oxygen). Variations of VSD, known as VSD+, involve the addition of heat or CO₂ to speed up the process but this does not improve animal welfare outcomes.

Ventilation shutdown methods have been used overseas for the mass killing of poultry and pigs during emergency animal disease outbreaks.