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Does Twitching Sedate Horses?

April 2017 by Horses and People Magazine

During difficult veterinary procedures, like scoping and other invasive treatments, it is often necessary to physically restrain the horse for its own and the vet staff’s safety. But, do lip and ear twitches actually sedate the horse or do they restrain through pain? And does their use carry any negative consequences? In a recent study, Benjamin Flaköll and his team of researchers set themselves the task to find out by examining 12 geldings that hadn’t experienced twitching previously, and after examining the data, they warn that ear twitching should be actively discouraged.

The study titled: Twitching in veterinary procedures: How does this technique subdue horses? by Benjamin Flakoll, Ahmed Ali and Carl Saab was presented at the 2016 Equitation Science Conference in Saumur, France and was published last October in the Journal of Veterinary Behaviour. 

When horses need to be subdued during a veterinary procedure to avoid injury, vets will resort to chemical sedatives and/or twitching. While lip twitching is most common, some veterinarians also use an 'ear twitch' which consists on manually twisting the animal's ear to restrain it.
Prior research had suggested that as well as providing some minor physical restraint and distraction, the lip twitch subdues horses with a temporary calming, analgesic effect. After being in place for some time, there is a release of endorphins that act as natural painkillers, however, it was not known if it is the twitch itself that triggers the endorphins release (in a similar way to the effect of acupuncture or acupressure), or if the endorphin release is the result of pain in the area twitched. 
The mechanism behind how the ear twitch restrains a horse had not been investigated but, anecdotally it is thought that while lip twitching does not seem to affect a horse's behaviour, after ear twitching many horses horses become 'head shy' and resent having their ears handled.
The researchers wanted to find out more, so they set out to investigate the effect of both types of twitching by examining certain stress measures and by conducting a behavioural analysis to determine any long-term effect. They did this by noting the difference in ease of handling the relevant body part (lip and ear) immediately before and after the procedure, and again four weeks later. 
The researchers chose twelve riding school geldings and divided them into two groups. The first group would receive the lip twitch and the other the ear twitch. To determine the effect of the twitch on stress, they first measured heart rate for 15 minutes and salivary cortisol at baseline (before any procedures). At this initial stage, the subjects were also tested behaviourally to determine their acceptance of having the relevant body part handled (either the lip or the ears).
Behavior was evaluated on a 3-point scale: (1) allows touch willingly (does not move away), (2) allows touch unwillingly (moves away but eventually allows itself to be touched), and (3) does not allow (moves away, does not allow itself to be touched during a 10-second period and/or becomes visibly aggressive, e.g. attempts to kick or bite, and/or exhibits signs of stress, such as heavy breathing and/or rolling eyes).
The lip or ear twitch was then applied and heart rate (HR) and heart rate variability (HRV) were measured for the duration of the procedure which lasted 15 minutes. Salivary cortisol (SC) and the horse's behaviour was again measured at the end of the procedure. Four weeks later, a third behavioural analysis would test if the horses exhibited long-term consequences. 

In regards to the stress related measures, HR, HRV and SC, they found that, in the ear twitch group, they increased significantly for the duration of the procedure, indicating a stress response and suggesting that the ear twitch immobilises the horse causing it to 'freeze' through fear and pain (although the researchers also acknowledge there is no direct way to ascertain and measure pain in horses). 

In the lip twitch group, HR and HRV significantly decreased during the first five minutes, indicating a lower stress level during the initial phase of twitching, and supporting the notion of an analgesic effect but, importantly, they increased after five minutes to the same level as the ear twitch. This supports anecdotal reports that the twitch must only be used for short periods and procedures of short duration. 

The most significant finding, however, was that behaviourally, all the horses that had the lip twitch applied did not demonstrate a change in acceptance to having their lips handled, whereas 4 out of 6 horses in the ear twitch group showed signs of sensitisation and became more difficult to having their ears touched, both directly after the procedure and four weeks later. 

Based on the results, the researchers conclude that the lip twitch and ear twitch clearly affect horses in very different ways.

Initially, the lip twitch increases parasympathetic nervous system activity (showed by a decrease in HR and HRV) and reduced stress levels, plus it has no effect on a horse’s behaviour short or long-term. This suggests that it does subdue through a temporary calming, probably analgesic effect. However, after the first 5 minutes of application, the lip twitch appears to significantly raise the sympathetic tone (indicating a stressful response), which raises questions about its suitability for periods longer than just a few minutes.

The ear twitch, on the other hand, significantly raises sympathetic nervous system activity and stress levels and makes horses harder to handle both directly after application of the twitch and over time. These results strongly suggest that the ear twitch restrains horses through a stressful and aversive mechanism, probably fear and/or pain.

Because of these stark differences, the researchers warn that the only twitch that should be used when attempting to humanely subdue a horse is the lip twitch, and use of the ear twitch should be actively discouraged among veterinarians and others involved in equine management.

Nevertheless, if it is necessary to subdue horses for more than a few minutes, veterinarians should not use the lip twitch but should consider anaesthetizing the animal.

The abstract of the study titled: Twitching in veterinary procedures: How does this technique subdue horses? by Benjamin Flakoll, Ahmed Ali and Carl Saab is published in the Journal of Veterinary Behaviour and is available here

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