By Manuel G. Himenes Jr., DVM
Horses have a unique dental structure. Their teeth are divided into two major sections: the incisors, which are the teeth seen in the front of the horse's mouth, and the cheek teeth, made up of the premolars and molars. The molars and premolars are lined up tightly against each other, creating the appearance of one chewing surface. This alignment of teeth is called the dental arcade. The incisors are separated from the cheek teeth by a large space. Canine teeth can be found in the space between the incisors and premolars. The incisor teeth are responsible for the grasping and tearing of food, while the cheek teeth are used for grinding of feed.
The horse chews in a circular motion with the lower jaw sliding along the upper teeth. This circular motion maintains the occlusive surfaces of the teeth. This motion is reduced as the horse eats smaller feed particles. Horses eating grains and pelleted feeds may chew in almost an up-and-down fashion.
The horse has evolved with the hypsodont tooth seen in the modern horse. Hypsodont teeth are tall and continue to erupt from the gum after forming. These teeth form in the jaw of the horse until the horse is approximately seven years old. Cementum, a hard material that forms part of the tooth, and bone are deposited, which pushes the tooth out of the skull and through the gumline. The part of the tooth that can be seen in the horse's mouth is called the clinical crown, while the portion in the jaw is called the body or reserve crown. Equine teeth have an interweaving fold of hard enamel and dentin. The enamel is important for grinding feedstuff. The chewing surfaces of the teeth must have enamel-to-enamel contact. If the teeth are offset by abnormal wear, tooth loss or abnormal growth, the enamel-to-enamel contact will be disrupted and the enamel will come in contact with the much softer dentin. This results in the dentin wearing away rapidly and deformation of the softer tooth.
This continuous eruption and wearing of teeth in the horse results in an increase in dental problems and diseases of the oral cavity as the horse ages. While horses rarely have problems with tooth decay or gum disease, their continuously erupting teeth present unique problems. If a tooth is lost or worn abnormally, the opposing tooth will continue to erupt from the gumline. This will result in the tooth impacting the empty socket left by a missing tooth or the dominant tooth continuing to wear into the opposing arcade, magnifying the abnormality. Either of these conditions will result in mouth pain and abnormal chewing patterns. This abnormal chewing can result in weight loss, tooth infection or impaction colic.
Dropping of feed while chewing, nasal discharge, foul-smelling breath, weight loss and facial swellings might all be indications of dental disease. While the symptoms of dental disease are often obvious, the actual cause of these symptoms requires a careful and detailed examination. For this reason, horses should have an annual oral exam performed by a veterinarian. It is important to do a complete oral exam and not just look at the teeth, as other disease processes may be present in the horse's mouth.
Examination of the horse's mouth will often require that the horse be sedated and a full mouth speculum used to open the horse's mouth. All of the tooth surfaces should be examined, along with the gums, tongue, hard palate and soft palate Abnormalities found on an oral exam may need to be examined further by radiography or ultrasound. Problems are evaluated to assess the importance to the horse's overall health. In the older horse, a general physical exam may be needed to determine the cause or effect of dental disease.
Abnormal occlusion, missing teeth and abnormal chewing patterns can result in the creation of a dominant tooth. As the dominant tooth erupts, it wears down the opposing tooth. If not treated, the dominant tooth can wear the opposing tooth down to the gum line, resulting in feed packing into the gum line. This can result in infection of the tooth socket and its sinus.
When more than one tooth is dominant, the dental arcade will have high and low sections. This is referred to as a wave mouth. This condition requires that the dominant tooth be cut to the level of the rest of the teeth in that arcade. The opposing surface that has been worn away is left to grow back to the level of the teeth in that arcade. It is important to identify and cut back the dominant tooth; leveling the opposite arcade that has been worn away will worsen the problem. Dominant teeth will require monitoring for the life of the horse. Other dental anomalies, such as step mouth and shear mouth, also require the overriding tooth to be cut back and monitored.
Unopposed teeth will need to be cut back routinely to prevent the tooth from impacting the opposite gum. This may occur after a tooth is lost or in abnormal occlusion occurring when the upper and lower jaws are not aligned properly, resulting in the first and last tooth being partially unopposed. This tooth will have a section that is worn away, while the rest of the tooth continues to grow and develop a pointed surface at the unopposed end--referred to as dental hooks. These hooks occur at the front and rear of the dental arcade. If left untreated, they will eventually impact the opposite gum and cause severe discomfort to the horse. Cutting or rasping down these hooks to the level of the rest of the arcade will prevent these hooks from creating a problem.
The incisors of the horse also continue to erupt as the horse ages. These teeth can become loose or wear abnormally. Loose incisors are often painful and might need to be removed. Uneven incisors will inhibit the normal side-to-side chewing motion of the horse, while incisors that are too long will prevent the normal occlusion of the cheek teeth. It is important that the incisors be evaluated and treated after the cheek teeth have been examined. Small changes in the incisors will have a large effect on the cheek teeth, especially the rear molars.
Often, the older horse will need a tooth removed. This might be due to an infection, or because the geriatric horse does not have enough reserve crown to hold the tooth in place. In the latter case, the tooth will be loose and cause pain. X rays may be required to identify the infected or loose tooth, which can be removed using a molar extractor in the standing, sedated horse. Some infected teeth must be removed by repulsion. This means that the sinus above the tooth must be entered surgically and the tooth driven out using a mallet and dental punch. This procedure requires general anesthesia and leaves a large, open tooth socket in the horse's mouth that must be packed after surgery until the defect is healed, which may take several weeks. Packing is usually not required in the "worn out" tooth as the socket left behind is minimal.
Chewing and grinding of feed are essential to your horse's health and survival. Proper dental care will help your horse eat better, be healthier, and have fewer mouth problems. An annual oral exam should be an integral part of every horse's health program.
Manuel G. Himenes Jr., DVM, owner of Oahu Equine Veterinary Clinic in Hawaii, specializes in equine practice and is a member of AAEP and AVMA.