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YOUR GUIDE TO THE 2017 AAEP CONVENTION SPONSORED BY A9 TheHorse.com/AAEP2017 March 2018 THE HORSE AAEP Wrap-Up MICHELLE ANDERSON T he modern horse's dentition and digestive system result from millions of years of evolution in response to changing food sources and climates. Likewise, how today's equine veterinarians treat those teeth must evolve and improve constantly, said Pa- draic "Paddy" Martin Dixon, MVB, PhD, FRCVS, Dipl. EVCD (Equine). Dixon presented the Frank J. Milne State-of-the-Art Lecture on the evolution of horses and equine dentistry. Dixon is a professor of equine surgery at the Univer- sity of Edinburgh's Royal (Dick) School of Veterinary Studies, in Scotland, where he studies dental and upper airway disorders. The Evolution of Horses & Their Teeth Hyracotherium (aka eohippus, or the "Dawn Horse") evolved in North America circa 55 million years ago. The rabbit-sized animal lived in forest land, browsed on soft, leafy vegetation, and had brachydont (short crowns, well-developed roots, and narrow canals in the roots) teeth similar to those of humans or dogs. These were unlike the hypsodont teeth (high-crowned with cementum extending past the gum line toward the occlusal, or chewing, surface, providing extra material for wear and tear) of today's horses. The climatic event that extinguished dinosaurs preceded by some millions of years an increase in atmospheric carbon dioxide, which favored growth of fibrous plants, such as tundra grasses. These replaced the lusher, more nutritious veg- etation Hyracotherium consumed. These plants' cell walls contained more structur- al carbohydrates, including cellulose. In response to these changes, Hyracotherium and its descendants adapted their diet. "Cellulose is the most common food substance on earth, but not a single living mammal can break it down," Dixon said. Rather, they rely on the complex com- munity of microbes that live in the gut for help. As mastication (chewing) and digestion adapted to changing vegetation, the gut microbiome evolved to help equid ancestors digest cellulose, Dixon said. At the same time, early equine ances- tors transitioned from consuming large, infrequent, nutrient-rich meals to smaller, frequent, less nutritious meals, more like the grazing we see in horses today. "Horse have a low-efficiency gut and eat a low-energy diet," Dixon said. "Because of this, they spend a lot of time grazing—16 to 18 hours a day—and this causes a lot of wear on the teeth." Many grasses and plants contain silica and other phytoliths (minute mineral particles), which also cause tooth wear. So, the short-toothed prehistoric equine ancestors evolved to have more durable, constantly erupting, longer teeth. A Look at Dental Tissues Horses have incisors (used for cutting and tearing), cheek teeth (premolars and molars used for grinding), wolf teeth (that were originally premolars), and canine teeth (used for defense in other species). Teeth erupt slowly above the gum line with age until they expire. Dixon de- scribed dental and related tissues: Enamel The cells that produce enamel, the hardest body tissue, die as the tooth erupts, meaning enamel is dead and can't heal if damaged. An erupted tooth's complete enamel content is laid down and wears throughout a horse's lifetime. Therefore, the crowns of some horses over 25 might have little enamel left and a reserve crown (everything lying below it) composed entirely of cementum and den- tin. "This results in the 'smooth-mouthed' horse," said Dixon, with little to no cheek teeth ridges left to chew properly. Cementum This is the softest of the calcified dental tissues, covering the root and anchoring it to the periodontal ligaments. In horses, cementum plays a structural and protective role by sur- rounding the tooth. It forms under the gingiva (gums) as the tooth erupts and, so, later becomes part of the clinical (vis- ible) crown. Cementum is also deposited on the reserve crown roots once it erupts. Dentin As the main tooth structure, this living calcified tissue without blood sup- ply supports enamel. Cells called odon- toblasts build predentin, which becomes dentin. In healthy human teeth, dentin is very sensitive and never exposed. But in the equine tooth, enamel and cementum wear away, leaving dentin unprotected on the occlusal surface. For unknown rea- sons, exposed equine dentin doesn't cause pain like humans experience, Dixon said. Pulp Tooth pulp is soft, sensitive, and vascular and contains the blood ves- sels necessary to deliver nutrients to the dentin-building odontoblasts that give horses' teeth sensation. Periodontal tissue These tissues (including the gingiva and periodontal ligament), alveolar bone (socket), and ce- mentum attach the teeth to the jawbones, allowing for flexibility and tooth eruption. All these tissues and the complete tooth structure have evolved "to grind cellulose down to 2- millimeter sizes so the gut mi- crobes can break it down," Dixon said. Domestication and Dentistry As humans started using horses for transportation, warfare, and work, and entertainment, they developed early veterinary theories. Written evidence documents ancient treatments, but few of The Evolution of Equine Dentistry BIANCA MCCARTY