Twitch Efficacy and Endorphin Levels

Q.

Over cocktails this weekend, with some local veterinarians and vet tech colleagues, your name came up in regard to a question we had about using a lip twitch. Since it works by releasing endorphins, we were wondering: If the twitch is taken off when the effect seems to be wearing off, how long before it will likely be effective again? One of the local veterinarians remembers that many years ago you and your students studied the endorphin release during twitch application, so you might have the answer to our question. Also, how long after first applying the twitch does it take for the endorphins to reach effective levels, and then how long are the levels effective?

Jenn Wrigley, CVT, Kennett Square, Pennsylvania


A. What a great question! I can’t imagine many folks converse about lip twitches over cocktails. At first I was bracing for the usual adult horse reproductive behavior gems! 

Yes, back in the 1990s we did look at endorphin levels and behavior during a number of activities, including twitch application. We found that there is quite a bit of variation in the intervals, as could be expected. For your first question, for most horses that are not upset, it typically takes just a few minutes of rest after removing the twitch before reapplication can achieve a worthwhile effect. For the few horses we evaluated in a second go-round, after 10 to 15 minutes of rest off the twitch, reapplication resulted in a reasonable rise again in endorphins, along with the corresponding behavior—the droopy lip, glassy eye, and relaxed facial expression. 

Based on that work, we find it helpful to pay attention to the horse’s behavior; at the first sign that effectiveness may be waning, we try to soothe the horse, and at a moment of calm we gently remove the twitch, massage the lip, offer a little sweet treat, and allow the horse to take a break. On the second application, after a 10- to 15-minute break, you can expect it to take a minute or so longer than on the first round to become effective, and the duration of effectiveness is usually shorter. 

That brings me to your second question: When first applying a twitch, how long does it usually take for the endorphin levels to rise to be effective? A good rule of thumb is 3 to 5 minutes. The interval from first application to effectiveness is very similar to the response to a standard intravenous (IV) dose of the sedative xylazine. So, just as with the usual sedation approach, it’s best to wait until it takes effect rather than to barrel right into the procedure. The typical duration of effectiveness of the first round of twitch application is also similar to that of IV xylazine—about 12 to 15 minutes, with pretty wide variation among individual horses. And any given horse can be expected to have some variation from one scenario to another. 

We all know that there are situational factors that modify the twitch’s effectiveness: for example, how skillfully and respectfully the handler applies the twitch, how well the horse handles the application, how painful the horse may be, how disturbing or uncomfortable the particular procedure and the situation is for that individual horse, and how calmly and reassuringly the people involved behave. So while it is good to have some rule-of-thumb expectations, for the best results have the handler continually observe the individual horse’s behavioral signs of effectiveness (e.g., the eye, lip, overall facial expression, and muscle relaxation) and have the person performing the procedure respect the handler’s advice.

About the Author

Sue McDonnell, PhD, Certified AAB

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania's School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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