Equine odontoclastic tooth resorption and hypercementosis (EOTRH) is a progressive and painful dental condition that occurs in some horses.

Primarily affecting senior horses, it typically involves the gradual degeneration of the incisors and canine teeth. Over time, the roots of these teeth are resorbed or dissolved.

The teeth also become excessively calcified with the buildup of cementum, sometimes resulting in a bulb-like appearance. The weakened teeth may fracture or may need to be extracted to prevent discomfort for your horse.

Horses affected by EOTRH may demonstrate a resistance to biting or chewing hard feeds. [9] They may become generally irritable and experience other behavioural changes due to the pain caused by the condition.

EOTRH can also result in poor appetite or weight loss linked to difficulty with chewing and eating. [9] Horses diagnosed with this condition should work with an equine nutritionist to design a feeding plan that takes into account their dental health and chewing ability.

If your horse has EOTRH, submit their diet for analysis online and our nutritionists can help you design a diet for free.

What is EOTRH?

Equine odontoclastic tooth resorption and hypercementosis is a condition that typically affects the equine canine and incisor teeth positioned at the front of the mouth on the upper and lower jaws.

It has also been reported to occur in the cheek teeth (molars and premolars) positioned at the sides of the mouth. [1][2]

EOTRH typically affects multiple teeth and negatively impacts the entire external and internal structure of the tooth. It also compromises the periodontal ligament that connects the roots of each tooth to the gums (gingiva). [1]

Tooth roots affected by EOTRH are dissolved or broken down via resorption by cells called odontoclasts.

Odontoclasts are cells that are responsible for the breakdown of the roots of teeth. In humans, they are responsible for resportion of the roots of baby teeth so they can fall out and make way for permanent adult teeth.

EOTRH can also cause an overproduction of the hard material (cementum) that covers the roots, a process referred to as hypercementosis. [3][4][5]

Eventually, EOTRH leads to additional changes in the gum tissues and bone surrounding the teeth. [6][7] Bacteria will subsequently infiltrate the compromised tooth structure and surrounding bone, contributing to the disease process and the destruction of the teeth. [8][9]


Although EOTRH was initially observed in clinical settings, it was first identified in dental tissue specimens in 2008 and is associated with periodontic disease. [3][4]

The condition is believed to be underdiagnosed since it can progress without significant symptoms. It may go undetected for months or years until at which time it may have reached an advanced state. [5]

Older horses appear to be at a greater risk for developing EOTRH. A study of 142 horses with an average age of 21 found that 94% had minor and 64% had moderate to severe indications of EOTRH, upon radiological examination of their teeth during a routine dental check. [6]

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The Progression of EOTRH

Teeth affected by EOTRH display one of three patterns of disease progression: tooth root resorption as the dominant process, hypercementosis as the dominant process, or a combination of these two processes. [9]

When resorption is the dominant process, odontoclasts are continuously being activated by a type of immune signalling molecule known as a cytokine. This activation is in response to mechanical stress exerted on the periodontal ligament.

Cytokine activity is also stimulated by an invasion of microorganisms that cause infection in the periodontal ligament. [9]

When hypercementosis is the dominant process, an abnormal amount of cementum (calcified tissue) is deposited on teeth and their roots. This process is considered a responsive action in an attempt to stabilize the damaged tooth. [9]

In all three types of EOTRH reported, resorption and hypercementosis are both evident to some degree in affected teeth when analyzed microscopically. [9] It is believed that resorption happens inside the tooth pulp cavity in advance of hypercementosis which occurs on the outer surface of the teeth.

EOTRH most commonly progresses from the incisors positioned at the corner of the mouth, to the middle and central incisors at the front of the mouth. [5]

Signs and Symptoms of EOTRH

The clinical symptoms of EOTRH can vary significantly between horses. It is common for some horses to show little or no visible indication of the disease even when they have advanced areas of breakdown in the tooth structure, known as dental lesions.

EOTRH-affected horses may have difficulty biting, chewing, and swallowing due to pain. [9] They may also experience a reduction in appetite and weight loss due to difficulty eating normally. [9]

Other signs of EOTRH include: [9]

  • Refusing to bite hard treats
  • Drooling
  • Bad breath
  • Behavior changes
  • Irritability or discomfort
  • Increased head shaking
  • Hay dunking
  • Resistance to dental exams

During dental work, horses with EOTRH may exhibit signs of discomfort with the dental speculum and require additional sedation.

Severe Symptoms

When EOTRH becomes severe, several physical manifestations of the disease are typically evident and include:

  • Inflammation in the gum tissues (gingivitis) at the base of the teeth and gum recession. [5]
  • Sores (fistulas) that appear as red dots may develop in the gums and small drainage tracts may be visible if an infection is present. [5]
  • The teeth may display an excessive buildup of cementum on their surface. Cementum accumulation can also extend under the gums causing the area of the gums above the tooth roots to have a bulbous appearance. [5]
  • Tooth displacement, fracture, and loss are all commonly associated with EOTRH. [5]

Risk Factors and Causes

EOTRH typically affects senior horses over the age of 15, although x-ray images have indicated signs of the condition in middle-aged horses of 11 to 13 years of age. [5]

Geldings are more likely to be affected by EOTRH than mares and stallions. [5]

Multiple types of horses including warmbloods, Thoroughbreds, Icelandic horses, Haflingers, and Arabians are affected by EOTRH. [5]

The exact cause of EOTRH is not yet known, although multiple factors are believed to be involved with the development and progression of the condition. [9]

Biomechanical stress on the periodontal ligament is thought to contribute to the development of the disease. [9] Mechanical stress can result from chewing forces over time.

Areas of stress are more commonly observed in older rather than younger incisor teeth and might contribute to the development of lesions. These damaged areas could permit bacterial micro-organisms to propagate or grow inside the teeth. [9]

Excessive dental work may promote EOTRH if the periodontal roots of teeth have been damaged. [10]

Oral Bacteria

At this time, there is a limited understanding of the differences in the oral microbiome of horses with a healthy dental status and those affected by EOTRH. However, research has found that bacterial microorganisms including Treponema and Tannerella contribute to infection in EOTRH affected teeth. [8]

In one study, the DNA of Treponema species was present in 78.2% of horses with EOTRH and only in 38% without the condition. [8] The DNA of Tannerella species was present in 38.4 percent of horses with EOTRH and only in 19% without the condition. [8]

Pituitary Pars Intermedia Dysfunction

EOTRH is more prevalent in horses with the endocrine condition Pituitary Pars Intermedia Dysfunction (PPID; Equine Cushing’s disease). It is also more prevalent in horses with other metabolic disorders, presumably due to hormonal abnormalities. [5]

Research indicates that small animals affected by Cushing’s disease are more likely to have loose periodontal ligaments due to the effects of the hormone cortisol. In excess, this hormone weakens the periodontal ligaments thus contributing to the development of periodontitis. [5]

Some researchers propose that EOTRH-affected horses with abnormal cortisol, insulin, and glucose levels are at a greater risk of periodontal disease becaus