Page 12 • HORSES and PEOPLE • Phone: 07 5467 9796 • admin@horsesandpeople.com.au www.horsesandpeople.com.au • HORSES and PEOPLE • Page 13www.horsesandpeople.com.au • HORSES and PEOPLE • Page 13

and

Page 12 • HORSES and PEOPLE • Phone: 07 5467 9796 • admin@horsesandpeople.com.au

equinedentalvets.com.aucommitted to advancing horse health

Brought to you by...

Equine Dental Vets an organisation committed to advancing horse healthOur group is made up of Equine Dental veterinarians from across the globe and we have over 300 members right here in Australia.

Most of our members provide afterhours and emergency care for all equine health issues and using our dedicated search on our website you can find the Equine dental vets nearest you at the touch of a button. This is particularly useful if you are away from

home, for example when travelling to shows and competitions.

We are proud to provide you this healthcare information in partnership with Horses and People magazine.

www.equinedentalvets.com.au

Watch the video at:www.equinedentalvets.com

Dental Anatomywith Dr Shannon Lee

by Dr Shannon Lee BVSc, MANZCVSc, MICEVO

Dental Anatomy

what you need to know Why is it important to learn about the internal workings of the horse’s head? How does equine anatomy differ from human?

Dr Shannon Lee from Advanced Equine Dentistry explains that a lack of understanding of equine anatomy amongst horse owners can have serious welfare consequences.

Anatomy, the branch of science concerned with the function or internal workings of bodily structures, is a subject that some people find fascinating, while others consider it not relevant to them and their horses. However, the more one understands anatomy, the more educated and informed decisions one can make when it comes to their horses’ health and welfare.

Discoveries made as a result of advances in anatomical knowledge have led to incredible breakthroughs in all medical fields, for example better imaging and understanding of the blood supply, or the mineral composition of teeth.

Last month, to highlight the importance of regular dental care for all horses, I explained what teeth are made of, the type of tooth equines have and how they differ from human teeth.

In this second part, we will summarise some of the remaining structures that complete the horse’s head and oral cavity, and which allow the horse to pick up, chew and swallow feed.

Diet-driven adaptation

The horse evolved over 55 million years from a forest dwelling animal the size of a dog to the creature we know today.

Recent research shows that the evolutionary path of the modern horse was affected by the food available to its prehistoric ancestors, and that the evolutionary changes in tooth anatomy lag behind dietary changes by a million years or more. To show this they studied the sharpness of the cusps of the molars of fossil horses to see how they’ve been worn down by chewing.

The marked change of horse teeth evolution is the jump to brachydonth teeth that, once erupted, do not change much to hypsodonth teeth that continue to erupt through the horse’s lifetime to counteract the wear of chewing touch grasses containing silica, which is abrasive and causes increased tooth wear (see Part One of the Dental Anatomy series in the April issue).

Remember, that horse teeth do not ‘grow’ longer, but they ‘erupt’ - they are pushed out just like you push an eraser out of its cardboard sleeve as it wears through use - gradually getting shorter and shorter. Understanding this helps to explain why a horse’s jaws are so large, the head so big and in a state of constant change.

Anosignathia

Anosignathia is the term that describes the unequal width of the horse’s top and bottom jaws. This is another adaptation that took place as the horse evolved, and its skull grew to accommodate hypsodonth teeth and provide the most effective mastication. In contrast with humans, where both jaws are of similar widths, in the horse the mandible (bottom jaw) is roughly 30% narrower than the maxilla (upper jaw). This is an adaptation seen in nearly all grazing species, such as sheep and cattle, and the reason why sharp points can develop that need regular rasping by your equine dental vet.

“Understanding dental anatomy will allow you to make more informed choices about your horse’s general health and wellbeing.

Molars

Pre-Molars

Incisors

Canines

“The horse’s lower jaw is 30% narrower than its upper jaw, an adaptation seen in nearly all grazing species.

www.horsesandpeople.com.au • HORSES and PEOPLE • Page 15Page 14 • HORSES and PEOPLE • Phone: 07 5467 9796 • admin@horsesandpeople.com.au www.horsesandpeople.com.au • HORSES and PEOPLE • Page 15

and

Page 14 • HORSES and PEOPLE • Phone: 07 5467 9796 • admin@horsesandpeople.com.au

and

Qualified & Experienced

Licensed

Regulated

Insured

Continually updated skills and knowledge

www.equinedentalvets.com.au

Get the facts:www.equinedentalvets.com.au

Choose wisely

Beware of “Tooth Fairies”

Cranial nerves

From the giant blue whale to the tiny shrew, mammals share the same 12 nerves to provide innervate on to the head. These 12 cranial nerves support motor function and sensory stimulation to the skin, muscles, teeth, eyes, gums, tongue etc. The difference lies in the length of each nerve, which in the horse, for example, is much longer than in say a human.

The 12 cranial nerves are:

1 Olfactory nerve (responsible for smell)

2 Optic nerve (vision)

3 Oculomotor (muscles that move the eye)

4 Trochlear (muscles of the eye)

5 Trigeminal (teeth and gums)

6 Abducent (lateral eye movement)

7 Facial (muscles of facial expression)

8 Auditory (hearing and balance)

9 Glossopharyngeal (swallowing)

10 Vagus (supplies the major organs)

11 Accessory (supplies muscles of the neck)

12 Hypoglossal (muscles of the tongue and neck)

The sinuses

As we mentioned earlier, horses evolved to have very large heads and the large sinus cavities allow their skulls to remain relatively lightweight.

A horse’s sinuses are made up of six separate compartments on each side of the head with a volume of approximately one litre. These compartments interconnect to allow the drainage of fluid, however, they ultimately drain into the nose through a single small opening called the nasomaxillary notch.

Sinus disease can present as a swollen face and is commonly caused by a tooth root infection. It can be divided into two types, primary and secondary. In primary sinusitis, the problem originates within the sinus itself and usually begins due to inflammation affecting drainage. In secondary sinusitis, the problem originates either outside the sinus (such as an infected tooth root) or within the sinus but due to an underlying problem (such as a mass, for example a tumour).

If you think your horse may be suffering from a sinus issue, make sure you contact your equine dental vet and seek help.

The tongue

The horse’s tongue is a mass of muscle that is anchored by the hyoid bone (voice box), and the bodies of the left and right mandible. The shape is not uniform, it thickens substantially at about the region of the first cheek teeth and this lump is known as the lingus.

The tongue is innervated by the hypoglosal nerve (cranial nerve 12) and works in conjunction with the lips and jaws to help with prehension (picking up food) and to puch food directionally within the mouth as chewing occurs (mastication). Tongue injuries in horses are not uncommon, in fact, just like people do, many horses sometimes bite the tip of their tongues when eating.

Bones of the skull

The horse’s skull is made up of 14 major bones - individual plates that join at suture sites (junctions where bone plates interlock). These bones serve two major functions: they protect the brain and other key structures involved in mastication, and they provide anchor points for the muscles involved.

Like all bones they can be affected by trauma, inflammation and infection. When these conditions occur along suture lines it is not uncommon for a horse to develop what is called a suture periostitis, a painful condition that usually appears as a swelling between the horse’s eyes.

Lumps and bumps on either the mandible or the maxilla can be the result of the normal remodelling of bone associated with tooth eruption in young horses, or they can be a sign of a painful tooth root infection.

Any bone can fracture as a result of injury to the regions of the sinuses, the mandible and the bone surrounding the incisors - these are all fairly common in horses. Infection as a result of bacteria entering these fractures is not unusual and the reason why it is always wise to seek veterinary advice early.

Injuries due to eating things like wire, rose bushes, blackberry and other plants, as well as equipment or tack injuries, for example those caused by tongue ties and severe bits, such as twisted wire bits, are also common.

The lips

A horse’s lips are very mobile and they are prehensile, that means they can be used to explore and to pick up feed. Around the lips, the vibrissae (whiskers) help gather information during grazing and rubbing, warning the horse to prevent eye, ear and facial injuries or lacerations. They are an important sensory organ that should not be trimmed or clipped.

Muscles for mastication

In a natural setting, horses spend 15 to 19 hours grazing and chewing every day, and have evolved to survive on a diet of tough fibrous grasses, so it is no surprise the mastication muscles are very powerful and large. These include the masseter muscle, which is the obvious round muscle below the horse’s eye forming the horse’s cheek, the pterygoids, which sit inside the jaw, and the smaller temporal muscles found on the horse’s forehead. In horses with a chronic chewing issue, one can sometimes see an asymmetry in these temporal muscles. These three muscle groups work to close and move the jaw, while the digastricus muscle, which is innervated by both cranial nerves five and seven, works to open the horse’s jaw.

The temporomandibular joint

The temporomandibular joint (TMJ) is the joint between the lower and upper jaw of the horse. It resides just below and in front of the base of the horse’s ear on either side of the head. Movement within the joint is easy to see when the horse is chewing. Like any other joint it can be affected by disease or degeneration.

“Tongue injuries due to eating things like wire, rose bushes, blackberry and other plants, as well as equipment or tack injuries, for example those caused by tongue ties and severe bits, such as twisted wire bits, are also common.

parotid gland

Facial nerve

Jugular veinMaseter muscle

Splenius muscle

Brachiocephalic muscle

Facial artery and vein

Muzzle

Vibrissae (whiskers)

Parotid duct

Nasolabial levator muscle

Canine muscle

Cigomatic muscle

Auditory nerve

Image source: Atlas de Anatomia Topografica de los Animales Domesticos, by Peter Popesko

www.horsesandpeople.com.au • HORSES and PEOPLE • Page 17Page 16 • HORSES and PEOPLE • Phone: 07 5467 9796 • admin@horsesandpeople.com.au www.horsesandpeople.com.au • HORSES and PEOPLE • Page 17

and

Page 16 • HORSES and PEOPLE • Phone: 07 5467 9796 • admin@horsesandpeople.com.au

and

This month’s contributor to the health feature from Equine Dental Vets

Shannon Lee, BVSc, MANZCVSc, MICEVODr Shannon Lee is a veterinarian whose focus is equine dentistry. His career has taken him to most corners of the World and he is currently the only dedicated equine dental vet in Victoria.

Shannon travels internationally to present, teach and consult, and was heavily involved in control of the Australian Equine Influenza epidemic outbreak in 2007.

When not working, Shannon enjoys spending time with his dog and riding his motorcycle.

Further information can be found at: www.advancedequinedentistry.com.au.

www.equinedentalvets.com.au

Equine Dental Vets

Qualified & Experienced

Licensed

Regulated

Insured

Continually updatedskills and knowledge

Get the facts:www.equinedentalvets.com.au

Real vets who really care about horses

Blood vessels

The horse’s head is woven with a complex network of blood vessels that supply freshly oxygenated blood to the tissues and returns oxygen depleted blood to the heart. Some examples of the important blood vessels that relate to the horse’s dental anatomy are the facial artery and vein, which need to be identified and avoided during surgery, and the palatine artery, which runs along the roof of the mouth next to the cheek teeth and can be damaged during oral extraction of teeth, including wolf teeth. Palatine artery rupture causes the horse to lose a lot of blood and can be very distressing for horse owners.

The hyoid

The hyoid apparatus is the horse’s voice box and, together with the larynx, it is suspended from the skull via the temporohyoid joint (THJ), which is located in the temporal region of the horse’s head between the larynx and the base of the ear.

The palate

The horse’s palate is made up of two parts, the hard palate and the soft palate. The hard palate is…. and the soft palate is made of

These form the roof of the horse’s mouth. The roof of the mouth is ribbed with a split running down the middle dividing left and right halves. These ribs are shaped as sand dunes, they are called rugae and they help push food in an upwards direction.

Defects of the palate, such as cleft palate, are sometimes seen in foals and this is commonly identified when milk runs out a newborn foal’s nostrils when it suckles. Cleft palate results when the two halves of the palate have not fused completely (this condition can involve either the soft palate or the hard palate), and food or

liquids can enter the nasal cavity through the split. Surgery is really the only way to attempt to fix a cleft palate and many foals with cleft palate are euthanised.

The pharynx

The soft palate acts as a barrier between the mouth and the pharynx, except when the horse is swallowing food. This helps to prevent food from entering the horse’s airway.

The gingiva and mucosa

These are tissues, soft layers that cover the area around each tooth and the bone. They are capable of rapid turnover of cells to cope with the abrasive nature of the horse’s oral cavity. Gingivitis is inflammation of the gingiva and is one of the first steps in periodontal disease (the most common disease of the horse). When a horse is affected by periodontal disease, endotoxins produced by bacteria travel the pathway of blood vessels and are deposited randomly into organs. An understanding of the anatomical pathways at both a macro and microscopic level is the key to correct and successful management and treatment.

So, that’s a very brief look at some of the other anatomical structures involved in the horse’s dental system.

A greater understanding of anatomy allows your equine dental vet to understand how dental work will affect your horse’s general health and wellbeing. As you can see, it’s a complex subject and one of the reasons why equine dental vets undertake at least five years of full-time university study before being able to begin seeing consults and treating patients.

For more information or to seek help visit www.equinedentalvets.com.

The salivary system

The horse has three pairs of salivary glands, the parotid gland, which is the largest and is in very close contact with a number of other very important structures, including the internal and external carotid arteries, the submandibular gland and the sublingual, as well as some diffuse tissue. These glands are responsible for the production of saliva to lubricate and moisten the mouth, and help with the digestion of feed. The horse’s saliva provides a natural stomach acid buffer that helps prevent gastric ulcers, so the longer the horse spends chewing every day, the more their stomachs are protected from acid exposure.

Saliva is carried from the glands via ducts to the mouth. Some of you will have had a horse who develops large swellings under the mandible at certain times of year and, while there are many potential causes for this and you should always consult your vet, one seasonal cause that is well-documented but poorly understood is idiopathic grass glanders, which are caused by some unknown irritant in the grass that leads to reaction and swelling.

Horses can also develop calcified lumps within a salivary duct. These are called sialoliths and they often contain a piece of food in the middle around which calcium and other minerals deposit as a result of changes in the pH of the horse’s saliva. Most of these are removed via surgery and many will recur. Sometimes people ask what the two small lumps are under a horse’s tongue, most likely they are looking at the sublingual papillae which secrete saliva under the tongue.

“

“Did you know that a horse can produce more than 25 litres of saliva each day?

Horses can also develop calcified lumps within a salivary duct. These are called sialoliths and they often contain a piece of food in the middle around which calcium and other minerals deposit as a result of changes in the pH of the horse’s saliva.

CerebelumBrain

Ethmoid labyrinth

Sinus cavities

Esophagus

Larynx

Epiglotic cartilage

Thyroid cartilage

Sumandibular (salivary) gland

Hard palate

Soft palate

Incisors

Canines

Cartilage

Spinal cord

Atlas

Image source: Atlas de Anatomia Topografica de los Animales Domesticos, by Peter Popesko