Flank biting is not common, but it occurs frequently enough that you are bound to come across a horse that does it sooner or later. These horses bite or “attack” an area of their chest, sides, flanks or stifles, sometimes violently, for no obvious reason.
A research survey of 58 horses with the syndrome reported that it is more common in geldings and stallions than mares and usually occurs before sexual maturity. Flank-biting stallions commonly also show personality problems, such as screaming, striking, agitation, and over-aggression.
While a chemical imbalance in the brain is possible, it is often induced by stress, which includes poor management, too little exercise, sexual frustration and lack of social interaction with other horses. Muzzles, neck cradles, bibs and the like have all been tried — and while they may help at least until the aggression manifests itself in another way — these devices are not cures. The underlying frustration usually remains and eventually manifests itself in another form. Likewise, tranquilizers/drugs only treat the symptom, not the cause.
So, rather than resorting to drugs or quick-fix devices as first-line treatments, we recommend management changes:
A low-energy-density diet (primarily hay and grass), with nutritional supplementation with high-dose thiamine, folic acid and vitamin B6 (stress-reducing vitamins).
Increase exercise, formal and turn-out.
Add social interaction. Be sure the horse can at least see other horses and activity. You can provide a turnout friend if it is safe for the companion animal. Pony geldings are often a good choice, but be certain the stallion does not exhibit any aggression toward his new friend. Some stallions will accept this, others will not.
If the horse exhibits no other behavior or personality problems beyond flank biting, look for another cause. The common-sense explanation is that there is a trigger, whether it is pain, burning, itching, or something else. Research in children and laboratory animals shows self-biting often results when the nerves carrying messages of sensation are severed.
Irritating the injured nerve often makes things worse, while injecting lidocaine (a local anesthetic) can prevent the behavior. It is believed the cut nerve ending sends a barrage of messages to the brain and spinal cord that triggers the biting in an attempt to relieve the pain.
It might be similar to how an intense itch causes a nearly irresistible urge to scratch. This is not to say that flank-biting horses are running around with severed nerves. However, there is a good chance the areas they target for biting have some type of abnormal sensation caused by nerve irritation.
With two flank-biting horses who had evidence of irritation in the area of the spine/spinal nerves on the same side as the biting, a combination of local injection of Sarapin (a drug that relieves neuromuscular pain) into spasmed muscles along the spine and into acupuncture points eliminated the problem. Conversely, we know of a mare that would flank-bite for a few hours following an acupuncture treatment.
There are no drugs that will predictably stop self-biting behavior in people or laboratory animals. In lab animals, the behavior has been stopped or eliminated by feeding tryptophan at five times the dietary requirement and by injections of a magnesium salt. However, since we don’t even know the daily requirement for tryptophan in the horse’s diet — and overfeeding of any amino acid for prolonged periods is not advisable — we really don’t recommend that you use this method.
We don’t suggest regular magnesium injections either, but we do recommend you have the mineral balance of the horse’s diet analyzed. If the calcium:magnesium ratio is not at our recommended 2:1, supplementing magnesium is reasonable.