Weekend Wellness: Understanding Equine Kissing Spines

What is kissing spines, why do horses get it, and can you prevent it from affecting your horse?

One of the most common causes of back pain in horses is due to overriding (or impinging) dorsal spinous processes, or “Kissing Spines.” Although kissing spines is common, often painful, and many are aware of this condition, the mystery remains: Why do horses get kissing spines and can you prevent it from affecting your horse?

While many questions about kissing spines currently remain unanswered, Etalon Equine Genetics is excited to announce a successful research study that suggests a genetic component may be partly responsible (Patterson Rosa, 2022). The discovery of a genetic variant associated with kissing spines has led to a commercially available test that evaluates your horse’s risk for developing high-grade kissing spines. While this test cannot conclusively determine if your horse will develop kissing spines or not, it does let you know if your horse faces a genetic predisposition to the condition.

Photo courtesy of Etalon Diagnostics

What is kissing spines in horses?

Your horse’s entire spine is made up of an average of 54 individual vertebrae that are connected by strong ligaments and are moved by the surrounding muscles. The back itself is composed of 18 thoracic and 6 lumbar vertebrae.

In a normal horse, these vertebrae and their upper bony projections, known as the dorsal spinous processes, are evenly spaced to allow for flexion and extension of the back. A horse diagnosed with kissing spines will have two or more of the dorsal spinous processes that are too close together, touch, or even overlap which results in bone-on-bone grinding – painful! It is the lack of space between the vertebrae that can cause the horse to have decreased mobility through the spine and experience varying degrees of pain depending on severity.

How do I know if my horse has kissing spines?

As with any medical condition, early detection of kissing spines is key. For this reason, it’s important for horse owners to know and be able to recognize common signs of kissing spines. Early risk assessment using Etalon’s kissing spines genetic test is also invaluable for owners to have as much information about their horse as possible.

Many owners associate common back pain symptoms with kissing spines, and for good reason. Horses with the condition are three times more likely to exhibit back pain compared to an average horse.

Due to the varying degrees of kissing spines, mild cases may show little to no outward symptoms, while more severe cases may cause a horse to exhibit significant pain responses. However, diagnosing kissing spines based on symptoms can be deceptive as the opposite is true in some horses. Evidence of kissing spines may appear on X-rays, yet the horse displays no sign of back pain. It is because of these inconsistencies in symptoms that it is even more important to understand your horse’s risk of developing the disease before it becomes a long-term complication.

Common symptoms of kissing spines in horses include:

• Back pain upon palpation
• Reluctance to be saddled and/or girthed
• Sensitivity to being brushed
• Bucking or rearing under saddle
• Cross cantering and/or difficulty cantering
• Hollowing the back
• Hind end lameness
• General poor performance

This list is by no means exhaustive and it is recommended that you consult your veterinarian if you suspect your horse’s behavior indicates they may have kissing spines.

Photo courtesy of Etalon Diagnostics

How is kissing spines diagnosed?

Your veterinarian can often diagnose kissing spines based on a combination of your horse’s clinical symptoms and radiographs (X-rays) of the back. Properly angled X-rays allow your vet to assess the distance between the spinous processes and determine if there is evidence of the disease.

X-rays are the most commonly used diagnostic tool when evaluating a horse for kissing spines, but your veterinarian may also recommend combining X-rays with other modalities. This may include ultrasonography, nuclear scintigraphy (bone scan), thermography, and local anesthetic injections to come to the most accurate diagnosis.

Your veterinarian will use their findings to grade the severity of a horse’s kissing spines on a scale of 1 to 4. A grade 1 diagnosis indicates only a narrowing of the interspinous space(s) whereas a grade 4 case would involve severe remodeling of the spinous processes.

How is kissing spines treated?

The first step in treating kissing spines, regardless of severity, starts with breaking the pain cycle for horses experiencing discomfort. This may be achieved through medical treatments such as oral or injectable (mesotherapy) anti-inflammatories, muscle relaxants, local corticosteroid injections, chiropractic adjustments, acupuncture, or shockwave therapy.

Mild cases of kissing spines can sometimes be resolved using strategic physical therapy. Typically, this looks like daily exercise that includes encouraging the horse to move in a relaxed, rounded frame. These exercises can be helpful in rehabilitating horses with kissing spines of all severities.

For horses with severe cases of kissing spines, surgical intervention may be warranted. Surgical interventions for kissing spines aim to create more space between the affected vertebrae. A surgeon will elect a procedure based on the severity of the kissing spines and which would likely produce the best long-term results. Currently, there are two common types of surgery to address the disease.

One procedure called an interspinous ligament desmotomy (ISLD), involves cutting the ligament that runs in between the spinous process at each affected site. This procedure is less invasive, can be performed while the horse is standing using local anesthesia and sedation, and has a shorter recovery time.

The other two procedures, osteoplasty and partial ostectomy, involve reshaping or removing part of the affected spinous processes respectively. Due to the more invasive nature of this surgery, horses must be placed under general anesthesia and require a longer recovery time.

What causes kissing spines and can it be prevented?

The exact cause of kissing spines has been the subject of debate since it was first identified. Theories of its origin have ranged from a predisposition for the condition due to the horse’s age, sex, height, or breed, to poor saddle fit, rider fitness, and improper training. It was previously speculated that some horses were genetically prone to kissing spines, and recent research has now indicated that may indeed be the case.

The research team at Etalon Diagnostics, together with veterinary experts, analyzed radiographs and phenotypic data (observable traits that are determined by genetic makeup and environmental factors such as gender, height, and color) of affected and non-affected horses. Each case consisted of independently reviewed kissing spine grades from 155 Warmbloods and Stock-type horses who were diagnosed with kissing spines based on clinical symptoms and X-rays. The horses in the study underwent genetic analysis that compared their genotypes to 50 control horses who were kissing spines negative (validated by veterinarian exam and x-ray).

70,000 locations on the genome were examined, and a region on Chromosome 25 linked to the development of kissing spines was found. A single nucleotide variant (SNV) within this region increased the average grade of kissing spines by one for each copy of the chromosome, called an allele, with the mutation. For each of the horse’s two copies of the allele (one from each parent), the data indicates an average increase in one severity grade of kissing spines thus confirming the link between genetics and the severity of the disease.

While it’s thought that age and sex may also play a role in a horse’s susceptibility to developing kissing spines, research did not indicate those as being universal risk factors. It does seem, however, that height may potentially have an impact. Biomechanically speaking, as a horse gets taller and body mass increases, there is a disproportionate increase in the forces applied to the spine relative to a smaller horse. Also, the soft tissue structures that support the back, such as the supraspinous and interspinous ligaments, do not increase in strength in a relative manner to the increased size. This may partially explain why there are greater rates of kissing spines in taller horses but this potential link warrants further research.

Illustration courtesy of Etalon Diagnostics

Kissing spines often occur within the last few thoracic vertebrae (T10-18), which are where a rider and saddle are positioned on the horse’s back. Because of this, there is reason to believe that poor saddle fit, improper training, rider skill, fitness, and weight could all be playing a role in the development of the disease. These are all examples of non-heritable risk factors that horse owners can be mindful of in the big picture of preventing the development of kissing spines.

Yet, fossil observations of the Equus occidentalis, an extinct horse ancestor that lived prior to evidence of human domestication, demonstrate signs compatible with kissing spines diagnostics (Klide, 1989) indicating that neither riding nor saddle fitting alone, can be solely responsible for kissing spines. The development of kissing spines is likely multifactorial and requires further studies to clarify common components in risk and severity.

It is currently thought that kissing spines is due to some combination of both heritable and non-heritable risk factors. The results of Etalon’s study indicate a genetic component for kissing spines. The benefit of this finding is that horses that carry a genetic risk variant can be identified and subsequently, not used for breeding. Selective breeding can decrease the prevalence of heritable kissing spines in the future.

How can I test my horse for kissing spines susceptibility?

Based on the research done by Etalon Equine Genetics, we now offer a Kissing Spines Susceptibility Test that can evaluate your horse’s genetic risk for developing kissing spines. Knowing your horse’s risk factor allows you to make informed decisions regarding their riding and breeding careers.

It’s important to note that this test, which identifies a risk variant for kissing spines, is not causative. This means that horses who inherit this genetic variant are not automatically destined to develop kissing spines. This is only one of many factors that are linked to the development of the disease.

What it does mean, however, is that recent research suggests that there is a genetic correlation between a risk variant for a higher grade of kissing spines and the development of the disease. This means that a horse who carries the risk variant faces a higher possibility of developing a more severe grade of kissing spines should the condition occur.

What will my horse’s results look like?

There are 3 possible test outcomes:

n/n – Decreased Kissing Spines Susceptibility (KSS) variants detected. Horse has lower risk for developing kissing spines, more likely to fall into the category of grade of 0 (evenly spaced) to 1 (narrowing of the interspinous space).

KSS/n – Moderate Kissing Spines Grade Susceptibility (KSS) variant detected. Horse has moderate risk for developing higher grade kissing spines, more likely to fall into the category of grade of 2 (densification of the margins), and has a 50% chance of passing variant to any offspring.

KSS/KSS – Increased Kissing Spines Grade Susceptibility (KSS) variants detected. Horse has increased risk for developing higher grade kissing spines, more likely to fall into the category of grade of 3 (bone lysis adjacent to the margins) to 4 (severe remodeling), and has a 100% chance of passing variant to any offspring.

If you are interested in testing your horse for kissing spines, you can order a test here.