Drug Residues in Horse Tissue After Intravenous Anesthesia and Intrathecal Lidocaine Euthanasia

Euthanasia of horses with intrathecal lidocaine injection preceded by intravenous anesthesia leaves low drug residues that pose little risk of toxicity to scavenging animals, according to a report recently published in the Journal of Veterinary Internal Medicine.

In remote areas where burial and rendering facilities for large animals are not available, drug residues in the tissues of horses euthanized by pharmacological means can create a health hazard if the remains are not properly disposed of. Barbiturates, for example, render tissues toxic to scavengers, carnivores, and potentially to humans (who may consume the remains in areas with scarce food resources).

Intrathecal injection of lidocaine can be used to euthanize horses under anesthesia when other pharmacological euthanasia agents are unavailable. A previous study by the same research group (from the University of California at Davis) showed that intrathecal lidocaine was effective for euthanasia of horses and caused brain death prior to cardiac death. Animals euthanized with intrathecal lidocaine are typically premedicated with xylazine and anesthetized with ketamine before the lidocaine is injected.

This study included 11 horses undergoing euthanasia because of poor quality of life, chronic pain, or debilitating disease. The horses received intravenous injections of xylazine hydrochloride, midazolam hydrochloride, and ketamine hydrochloride, followed by intrathecal injection of 2% lidocaine hydrochloride. Death was confirmed by the absence of respiration, brain electrical activity, and electrocardiographic activity.

Whole blood, cerebrospinal fluid, and samples from the triceps brachii and gluteus medius muscles were collected immediately after death and frozen at −80°C. Muscle samples from another 5 horses that had undergone anesthesia and euthanasia with the same drugs in the field were also analyzed. The muscles chosen for sampling were thought likely to constitute most of the tissue mass ingested by a scavenger or carnivore.

Residues of all 4 drugs were present in serum, plasma, and skeletal muscle samples. However, all levels were much lower than those likely to cause sedation or toxicity if ingested. The concentration of lidocaine in cerebrospinal fluid was not assessed. The authors acknowledge that although animals might be exposed to lidocaine by eating central nervous system tissue, opening the skull and vertebral column “would be unlikely to occur without considerable effort.”

A study limitation was that organs other than skeletal muscle were not sampled. Drug residues in serum were higher than those in skeletal muscle, so levels in blood-rich organs might also be higher. However, exposure to levels high enough to cause a clinical effect would require that a scavenging animal eat a very large quantity of organ tissue.

“Because [scavenging] animals live in the wild, any changes in their level of consciousness could have serious consequences to their welfare, which is the reason for concerns about residue drugs causing even a mild sedative effect,” write the authors. They recommend further studies of the long-term effects of oral ingestion of sedative and anesthetic drugs.

The study was supported by anonymous donations to the Comparative Neurology Research Group at the University of California at Davis.