While many horse owners are becoming aware of the importance of equine dental care through media attention and increasing veterinary education, the majority might not have a working knowledge of the horse's dentition or masticatory apparatus (all the components needed to chew stemmed roughage). But if an educated horse owner wishes to make informed decisions about health care for his or her equine companion, he or she needs to understand the basics of equine dentition and how the teeth affect overall health--after all, proper mastication is the first step in the long process of digestion. Armed with this fundamental knowledge of dentition, the owner will be better able to ask the right questions and have appropriate expectations for their equine health care providers.
Back to Basics
Let's start with the basics: anatomy. A normal mature horse (at least 6 years of age) should have between 36 and 42 teeth, depending on gender and presence or absence of wolf teeth. The incisors, the teeth you see immediately inside the horse's lips, are useful for nipping grass and for defense and grooming, but they have an even more important function in the biomechanics of mastication. Their health, wear patterns, and physical length can determine how efficiently the grinding teeth, farther back in the mouth, can do their job.
Most males have four canine teeth immediately behind the incisors and in front of where a bit sits in the open area of gums. These areas are called the "bars" in horseman's language or the interdental space in anatomical terminology. These teeth are used by stallions in the wild as weapons. They can be quite long and sharp. In the domestic horse it is prudent to smooth any sharp edges to prevent oral injury, particularly from tack pinching sensitive tissue against these sharp teeth.
Occasionally mares have canines, but they are typically much smaller and only on the bottom jaw.
The "wolf teeth" are small teeth that are often, but not always, present just in front of the first large grinding teeth in both sexes. They usually come in at an early age, usually by the time a horse is a year old. A typical horse has two, but one or none is not uncommon and horses with four wolf teeth are prevalent.
These small, sharp teeth serve no useful purpose for the horse and can cause discomfort when tack is introduced. Most veterinarians recommend removing them before the horse is introduced to a bit or any headgear. The canines and wolf teeth account for the normal variability in teeth number between individuals.
Continuing our tour toward the back of the mouth, the next teeth we encounter are the premolars. These teeth, along with the molars just behind them, comprise the roughage grinding apparatus of the horse. These teeth are the workforce in the horse's mouth. Together, the premolars and molars (cheek teeth) total 24 teeth arranged in four arcades of six teeth; two rows on the top and two on the bottom. These teeth are large and uniquely designed to perform their task of grinding stemmed roughage into a uniform bolus (a chewed mass of food) suitable for swallowing. They have special corrugated surfaces that make their grinding function more effective. Teeth are designed to last the life of the horse, and they will if they wear optimally. However, disease or abnormal wear patterns can develop, causing pain or shortening the functional life of these teeth.
In addition to the hard tissue structures of the horse's mouth, soft tissue components play an equally important role in proper mastication. The horse's prehensile lips allow him to pick up very small or fine items with great precision and dexterity and move the items into the oral cavity for processing. The lips are incredibly sensitive and can discriminate with mind- boggling accuracy. Any owner who has tried to medicate their horse through feed knows this. How does that animal manage to sift through the medication and take only the feed?! Blame the prehensile lips.
Next the tongue, cheeks, and palate must all function in synchronous harmony. The fantastically muscular tongue and cheeks function opposite each other to position the feed bolus between the upper and lower arcades for optimum grinding efficiency. Additionally, the hard palate is strongly ridged in an alternating pattern to act as an auger, keeping feed moving from front to back. If you've ever seen a horse try and spit something out, you know how difficult it is to override this strong front-to-back mechanism.
Once the properly chewed feed bolus reaches the back of the mouth (the oral pharynx), it is conducted down the esophagus, continuing to the next stage of digestion. If you watch a horse eat, you will see him grinding with his jaws rhythmically moving side to side. Periodically the motion stops. This is usually when he is swallowing. There is a furrow that runs the length of the horse's neck near the front. This furrow contains the large jugular vein, the carotid artery, and the esophagus. You can actually see the feed bolus move all the way down the esophagus if you very closely watch the left side of the horse's neck in the furrow during this momentary break in chewing.
Summarizing, the lips select the feed and move it into the mouth. Other soft tissues conduct the feed through the mouth, allowing the grinding teeth to pulverize it, mix it with saliva, and move it to the back of the mouth for swallowing.
Sounds simple, right? On one level it is simple. But when you start to examine the many mechanistic elements involved in the process of chewing and preparing stemmed roughage for swallowing, a much more intricate, although no less elegant, apparatus emerges.
The next conceptual step is that of biomechanics. The horse and other herbivores, due to their very different diets, have evolved with specifically adapted teeth and a very effective chewing pattern to break down the coarse ration. Their teeth are termed hypsodont. That is they have large crowns, and the bulk of each crown remains deeply embedded in the bones of the skull and jaw, erupting a few millimeters each year to replace the same amount as it is worn away at the chewing surface. But horses do not produce new tooth material. There is a finite amount of tooth to last the horse's lifetime.
They chew in a roughly elliptical pattern that utilizes crushing forces in a side-to-side motion. There is also an important, although smaller, forward and backward component to the pattern, but it is far less obvious to the casual observer. If you were to spend a little time watching your horse eat his hay or pasture grass you will see that his lower jaw opens then moves toward one side or the other, then simultaneously closes and moves back toward center. Due to this strongly "sided" movement, horses are only able to chew on one side of their mouth at a time. This last motion, as the jaw closes and moves the lower cheek teeth strongly across the uppers, is extremely effective at breaking, tearing, and grinding stemmed roughage.
The health of the system--in addition to the type of feed the horse eats--determines the number of grinding strokes needed to produce a feed bolus that is safe and appropriate to swallow. A horse with compromised dentition might take much longer to masticate his feed. Luckily for the horse, the masticatory apparatus and the larger digestive system have a lot of potential for compensation. One part of the system can make up for deficiencies in another.
For instance, recent studies have shown that horses don't necessarily need to grind roughage perfectly in order to get the needed nutrition. The lower digestive tract is apparently capable of extracting nutrients from a wide range of chewed particle sizes. There is, of course, a critical limit to this compensatory capability. The horse must be able to chew roughage well enough to swallow the bolus. Extremely coarse stems cannot be safely swallowed, and the horse might "choke" if he tries to swallow poorly chewed feed.
Choke is a potentially fatal esophageal impaction and a true veterinary emergency. "Choke" is horseman's lingo for esophageal impaction. It occurs when a horse swallows a feed bolus that is too coarse or large or sticky to pass smoothly down the esophagus from the pharynx to the stomach. The bolus gets stuck at some point along the length of the esophagus and subsequent swallowed feed gets stopped at the level of the blockage and slowly backs up toward the mouth. Depending on the sensitivity of the horse, he might stop eating immediately, making effective treatment or even spontaneous resolution very likely. Horses that continue to eat after a blockage--or if the owner does not notice the signs, allowing the problem to continue for hours to days--face a very serious condition that could be fatal in prolonged situations.
Signs of choke include sudden refusal to eat, stretching the head and neck forward or side to side or toward the ground, lying down, rolling, general distress, coughing, eye rolling, pawing, etc. The hallmark sign, which distinguishes choke from colic, is feed (or feed-tinged saliva) running from the nostrils. But this is not always observable.
As noted, some cases of choke resolve spontaneously. Some are easily treated with heavy sedation, passage of a stomach tube, and esophageal lavage. Others require surgery, and some are not treatable. Complications include aspiration pneumonia, chronic strictures (abnormal narrowing) of the esophagus, esophageal diverticulae (weak parts of the esophagus that pouch outward), perforation, and death.
Know your horses' eating habits. If you notice these signs, especially shortly after feeding, evaluate the situation and be ready to call your veterinarian. Choke is a veterinary emergency.
Hypsodont teeth are adapted to tolerate quite a bit of structural compromise before they become nonfunctional. Malocclusions (abnormal relationships between opposing teeth) have to progress to severe proportions before an owner will observe any difference in the horse's behavior or see loss of body condition. In addition to this built-in adaptability within the dental system, horses are, in general, quite stoic when it comes to their feed intake. Most horses will continue to eat, even in the face of great discomfort, until it becomes mechanically impossible to do so. Remember, they evolved as prey animals having to graze on the move. A wild horse that loses condition and becomes weak is easy prey for predators. This system served the wild horse well for thousands of years and still does. However, our domesticated horses are not subject to the same diets or lifestyles as wild horses are.
There are three major reasons to provide your horses with skilled comprehensive dental care: 1) comfort, 2) health, and 3) longevity.
I'd wager that not a single owner reading this article would knowingly allow their horse to be in pain. Many malocclusions, such as hooks, crooked or displaced teeth, stepped teeth, diseased teeth, fractured teeth, periodontal disease, and other dental abnormalities can cause minor to excruciating pain.
It is not uncommon for horses to be in chronic pain for years without the owner being aware of the situation. The sooner these problems are diagnosed and treated--or even better, prevented altogether--the less pain the horse has to tolerate. Additionally, it is important that the tack not cause discomfort, as it impacts the horse's head structures and the teeth in particular. Sharp enamel edges on the teeth can lacerate sensitive soft tissues if the bit, noseband, bosal, or hackamore forces that tissue onto sharp edges on the teeth.
A variety of dental abnormalities can cause disease conditions, periodontal disease in particular. Those abnormalities can cause severe pain if left untreated and can lead to premature tooth loss. Fractured teeth are often painful and many must be extracted or treated endodontically. Horses are susceptible to tooth root disease, just like people. Tooth root abcesses are almost always very painful.
Finally, the third reason for comprehensive dental care is to prolong the useful life of the teeth to last the horse through his lifetime. When horses only lived to be 20 or so, perhaps it didn't matter so much if their teeth lost effectiveness in their late teens. But now, thanks to advances in equine veterinary care and nutrition, our equine partners are living well into their 30s. But that means their teeth need to last longer, too. Here is where the beauty of truly comprehensive, lifelong dental care really shines.
Due to the nature of the hypsodont dental system, dental malocclusions in the horse are progressive in nature and usually start in the young animal. Left unaddressed, a small abnormality will progress to a large one over the years until portions of the arcade are worn out or become diseased. At that point, functionality is compromised and failure is inevitable. It is just a question of how quickly the end point is reached. This varies from individual to individual.
The take-home message here is that the fewer malocclusions to which the horse has to adapt, the more normal his teeth wear and the longer they maintain their functionality. In other words, a well- maintained equine mouth is more likely to serve the horse well into his golden years. The majority of equine dental care tactics are based on a preventative health care model. Small malocclusions, minor periodontal disease, and crooked teeth in the young horse can usually be intercepted and treated quickly and easily by a knowledgeable practitioner. Early intervention prevents major progression of malocclusions and disease states.
Every horse should receive a comprehensive oral exam by no later than 2 years of age (much earlier if an overbite or underbite is suspected). Most horses should have annual examinations thereafter, although some situations require more frequent treatments. Young horses, for instance, often need more frequent exams to keep up with their rapidly changing dentition. Horses with demanding competition schedules might also need more frequent exams to keep them comfortable and performing at their best. Find out if your veterinarian has advanced training in equine dental care or ask for a referral to a qualified practitioner in your area.