Field euthanasia of livestock

Summary

​Transmissible spongiform encephalopathies (TSE) are rare, but fatal diseases associated with degeneration of the brain. These diseases are commonly referred to as ‘mad cow disease’ or ‘scrapie’ in cattle and sheep, respectively. Australia is free of TSE and the risk of introduction is deemed negligible, but active, on-farm surveillance is required to demonstrate this ‘disease-free’ status and ensure continued access to international markets.

TSE surveillance requires post-mortem, microscopic examination of brain material collected from animals exhibiting signs of neurological disease. Euthanasia by captive bolt or firearm can render the brain unsuitable for collection of samples and so injectable barbiturates such as pentobarbitone are commonly used in this situation. Barbiturate residues remain in the carcass for many years and can cause the death of carnivorous and scavenging species that consume the carcass, all of which makes carcass disposal problematic. Euthanasia of heavily sedated animals using saturated KCl (SS-KCl) or saturated MgSO4 (SS- MgSO4) has been proposed as a low risk alternative, but scientific studies to evaluate whether these methods are humane have been lacking.

This study evaluated the physiological, behavioural and nociceptive responses of heavily sedated sheep, that were euthanased with either pentobarbitone (n=10), SS-KCl (n=11) or SS- MgSO4 (n=10). The potential for conscious perception of pain was evaluated by measuring cortical brain activity using electroencephalography (EEG). Physiological variables including the time until end of rhythmic breathing and cardiac arrest, as well as behavioural responses were also recorded (kicking and paddling).
In this study, there was no evidence of nociception associated with the euthanasia with pentobarbitone, SS-KCl or SS-MgSO4. Based on this, all methods were deemed humane, however, the use of SS-MgSO4 rather than SS-KCl is recommended.

Although no conscious perception of pain was evident for any group, animals euthanased with SS-KCl consistently displayed severe, behavioural response to this agent. These behaviours were observed after the animal was unconscious and included sustained kicking, paddling and tetanic convulsions which are visually unappealing and pose a threat to the safety of personnel involved in the process.

A smaller volume of SS-MgSO4 (1-2ml/kg) was required to achieve brain death (permanent isoelectric EEG waveform) compared with SS-KCl (1-7ml/kg). The volumes required are expected to be larger in a field setting because EEG analysis is not available and infusion should continue until clinical death has been confirmed (i.e. loss of reflex activity).
The use of SS-MgSO4 instead of pentobarbitone decreases the risk of secondary poisoning of carnivorous animals. Saturated salts are extremely unlikely to cause any adverse reactions or death of animals consuming contaminated carcasses. Similarly, the risk of secondary poisoning associated with xylazine is also very low, because experimental models show very high tolerance for ingestion of xylazine.

The recommendations for field veterinarians are as follows. SS-MgSO4 is a safe, rapid, humane and practical method of euthanasing livestock that enables preservation of brain material. This method could also be used where a fire arm or captive bolt is not available. The SS-MgSO4 must only be administered intravenously to an animal that is heavily sedated with xylazine. The infusion should continue until death has been confirmed.