All You Need to Know About the Hidden C6-C7 Malformation That’s Bringing Horses Down

What if you were to learn that your horse is living with a hidden malformation? A skeletal abnormality that could be affecting it every day, changing the way it moves, creating a string of other physical problems, and possibly underlying the hard-to-pinpoint problems you’ve been noticing for months or even years ?

And that might even be causing a level of inherent instability that could be putting the rider in danger?

Sadly, this isn’t a hypothetical question. Instead it’s a reality that is only now being slowly uncovered.

And like the proverbial stone rolling down a mountain, the issue is gathering momentum as the equine industry, owners, breeders and researchers learn about it.

  • It’s a skeletal malformation and it can’t be corrected.
  • It’s congenital, ie inherited, so is present from birth.
  • It has been in some lines of TBs for hundreds of years.
  • It creates biomechanical issues due to asymmetry and lack of anchor points for key muscles.
  • At its worst, it can contribute to neurological issues such as Wobbler syndrome.
  • Some horses are so unstable, they are more prone to falling (not good news for jockeys).
  • It can cause constant pain and associated behavioural changes.
  • It’s primarily found in Thoroughbreds, Thoroughbred crosses and Warmbloods, but has also been identified in European breeds, Quarter Horses, Arabs and Australian Stock Horses.

 

The problem behind this is a congenital malformation of the C6 and C7 cervical vertebrae (ie, base of neck) – and it’s pretty nasty.

I’ve written about the work of Sharon May-Davis on this blog before and here I’m going to do so again. Through her many dissections per year, gross anatomist Sharon has become the first person to comprehensively document and quantify this problem. 

In doing so, and publishing her findings in peer-reviewed journals, she has triggered a minor research avalanche as others take up the subject.

 

Those of us fortunate to attend Sharon’s many equine dissections in Australia, New Zealand, Japan, and Europe have been learning about this for some time. For bodyworkers and hoof trimmers, it has dramatically changed our work. I believe I’ve worked on several horses with this problem, including an eventing horse, a dressage prospect, and a TB intended for a child.

It is, not to put too fine a point on it, an extremely serious problem that is in some cases grave for the horse concerned and can potentially cause injury or loss of life for the rider.

 

The following is an amended version of an article that I wrote for the Winter 2017 edition of Equine News, a NSW, Australia print magazine that sponsored one of Sharon’s series of public lectures on this issue.

Questions, thoughts or comments? Join us at The Horse’s Back Facebook Group. 

 

A hidden problem: this OTTB had the C6-C7 malformation but presented few outward signs.

 

Twenty years of research

Sharon May-Davis’s path with this research began some 20 years ago. In February 1996, a Thoroughbred called Presley came down unimpeded in a race in Grafton, NSW, fracturing his pelvis, a hock bone, and right front fetlock.

Three years later, Sharon examined his bones, and saw something strange in his last two cervical vertebrae and his first ribs.

Fast forward to 2014, when Sharon published the first of her four peer-reviewed papers in the Journal of Equine Veterinary Science, concerning a congenital malformation in the sixth and seventh cervical (neck) vertebrae.

Although the problem had been mentioned briefly in papers, this was the first time that a researcher had accurately described and quantified the problem in its various forms.

Sharon’s unique perspective, gained as an anatomist who dissects between 15 and 20 horses per year, had certainly placed her in a position to do so.

 

The horse’s seven cervical vertebrae – made simple

Horses have seven vertebrae in their necks, labelled C1 to C7. Of these, four have unique shapes. Most horse people are familiar with C1, the first vertebrae known as the atlas, as it can be both seen and felt by hand with its distinctive ‘wing’ at the top of the neck.

Cervical vertebrae from C1 (top) to C7 (bottom), view from above (L) and below (R).

Almost as well-known is C2, the second vertebrae, known as the axis.

Both atlas and axis have unique shapes for a special reason: they support the heavy skull and anchor the muscles that control the head’s movement.

Heading down the neck, C3, C4 and C5 are broadly similar in shape, with each being a bit shorter and blockier than the one above.

However, C6 and C7 are both slightly different on the ventral (lower) side, for here they provide insertion points for muscles arising from the chest.

 

  • C6 has transverse processes (the protrusions extending outwards) that are different to those of neighbouring bones, with two distinctive ridges running the vertebrae’s length. C6 also has two large transverse foramen, the openings that the arteries pass through.
  • C7 is the shortest and squattest cervical vertebrae of all. Its transverse processes are shorter, while there are also two facets that articulate with the first ribs. C7 has no transverse foramen.

 

At least, that’s how the vertebrae should be in a normal horse.

 

So, what is wrong with the malformed C6 and C7 vertebrae?

In certain horses, these last two vertebrae are rather different, being malformed.

Sharon has identified the manifestations of this problem as a congenital (inherited) malformation affecting some Thoroughbred horses, and horses with Thoroughbred blood in their ancestry.

In C6, there is a problem with the two ridges of the transverse processes, as one or both can be partially absent.

When both are partially missing, it is common for one or two ridges (ie, parts of the transverse processes) to appear on C7 instead.

Also, the articular processes (the oval surfaces on the upper side, where each vertebrae links to its neighbours) can be radically different sizes. There can also be an additional arterial foramen or two.

The level of asymmetry can be radical.

 

 

The secondary problems this malformation causes

Being at the base of the neck, the asymmetry of C6 and C7 can cause alignment problems all the way up the vertebral column, leading to osteoarthritis of the articular facets.

It can also contribute to Wobbler Syndrome (Cervical Vertebral Stenotic Myelopathy), due to narrowing and/or malalignment of the vertebral foramen/canal, the opening through which the spinal cord passes. Not all Wobbler cases have this particular malformation, though.

A further problem is that the lower part of the longus colli muscle, which is involved in flexing the neck, would normally insert on the transverse processes of C6 and C7. When these processes are malformed, the normal insertions are not possible.

This means there is a serious symmetry problem in the junction of the thorax and neck, which can have a deeper effect on the horse’s neurology and proprioception, as well as respiration.

 

Asymmetry and narrowed vertebral foramen (canal) contribute to DJD and Wobbler Syndrome.

 

In a few cases, horses with both the C6 and C7 problem also have malformations of the first sternal rib, on one or both sides. This can cause problems beneath the scapular and further issues with muscular attachments.

Associated stability problems can have far-reaching consequences for the horse, not to mention some serious safety issues for the rider. The safety issue can’t be stated often enough.

(Add to this the fact that the horse’s nuchal ligament lamellar does not attach to C6 and C7, and often only feebly to C5, then you can see that this is a high level of instability in a critical area. Read more about Sharon’s findings on the nuchal ligament here – How the Anatomy Books Unintentionally Fail us Over the Nuchal Ligament.) 

Read on for information on for signs that this problem may be present in the living horse…

 

© All text copyright of the author, Jane Clothier, www.thehorsesback.com. No reproduction of partial or entire text without permission. Sharing the link back to this page is fine. Please contact me for more information. Thank you!

 

Why isn’t the C6-C7 problem more widely known ?

Why hasn’t this problem been noticed in regular veterinary interventions?

The answer is quite simple. While neurological issues may have been diagnosed, the exact cause has often remained hidden. 

Both Thoroughbred horses and Warmbloods are known to have higher incidences of Wobbler Syndrome than other breeds, and while this is certainly not always due to C6-C7 malformation, the malformation has been found in some when dissected.

For example, the following dissection image appears in a veterinary account of large animal spinal cord diseases. It clearly shows a malformed C7 vertebrae, very similar to the one in the above image, but without giving any further categorisation.

The difficulty lies in the deep location of the lower cervical vertebrae. While normal radiographs can show all or some of C6, they are unable to penetrate the deeper tissues beneath the shoulder to image C7.

 

Photo from another online article: the familiar asymmetry of a malformed C7 is clearly visible in a dissected set of vertebrae.  (c) veteriankey.com (click image to access full article.)

 

Nevertheless, the malformation can be identified in radiographs of C6, once you know where to look.

Since Sharon’s first paper appeared, the School of Veterinary Medicine, University of California, Davis, has reviewed its history of radiographs from horses with Wobbler Syndrome.

Researchers found that 24 cases out of 100 (close to 25%) showed malformation of one or both C6 transverse processes. This study also clarified how to identify the problem on standard radiographs of C6.

In another study, the Faculty of Veterinary Medicine, Utrecht University, completed CT scans on horses’ necks and found the various forms of the malformation in 26 horses out of 78 (33%). Unlike radiographs, the CT scans enabled identification of the C7 and first rib issues, although of course this imaging was conducted post mortem.

 

Is this rare, or are many horses affected?

While the problem has been identified primarily in TBs, it affects most breeds with TB blood in the ancestry to some degree.

Sharon May-Davis reports that to date, published, peer-reviewed journal papers have tallied 136 out of 471 horses as exhibiting congenital malformation of C6.

These have been in a range of breeds including Thoroughbreds (39%), Thoroughbred crosses (27%), Warmbloods and European breeds (30%), Quarter Horses (11%), and Arabs (11%). Standardbreds have also shown the problem, although the numbers included in studies are very small.

A common question is whether it’s known which TB lines predominantly carry this problem. The answer is: Yes. However, it is now so disseminated amongst the modern equine population beyond TBs, that it is of little help to identify them.

 

“Eight Belles… might have been genetically predisposed to breaking down.” 

Exploring the views of a TB lineage expert, this American article from 2009 asks why certain TB lines were prone to breaking down on the track – Eight Belles Breakdown: A Predictable Tragedy

 

It must be remembered that these horses are those already brought to veterinary attention and/or euthanized for a related or unrelated reason, so the percentages may be higher than those for the general horse population. At the same time, the malformation might have played a major part in the horses’ decline, due to the many locomotory and postural problems it can lead to.

 Questions, thoughts or comments? Join us at The Horse’s Back Facebook Group. 

 

‘Gift Horse’, the Trafalgar Square sculpture by Hans Haacke, displayed the malformation, presumably having been modelled on a modern-day skeleton. The George Stubbs anatomical drawing on which it was styled did not. Image (c) bowlofchalk.net

 

How do we identify these horses in life?

It’s all very well looking at these bones post mortem, you might say. Yet how can I tell if my horse has this problem? Or a horse that I might want to buy?

Some answers are forthcoming. As Sharon has frequently assessed horses before dissecting them – usually from video – she has been able to observe that many of these horses lack stability. (Indeed, in many cases, it is this very instability has directly led to the horse being euthanized, and ending up on the dissection table.)

As her research has progressed, she has also been able to identify many biomechanical and locomotion traits that make these horses ‘suspicious’ or at least ‘of interest’. Unsurprisingly, these problems have been particularly noticeable in horses with both a malformed C6 and C7.

For owners and equine professionals, here are some signs that can raise initial suspicions. All can also be caused by other problems, so a group of signs is more common than an individual indication.

  • Some of these horses have a problem with standing square in front, and will always keep one foot further forward. This can persist despite all attempts to improve the horse’s body and to train the horse to halt squarely.
  • Horses with the more serious malformations will often stand base-wide. Such horses can become very unbalanced on uneven ground, and sometimes in work. They easily become unbalanced when a hoof practitioner works on a forefoot. 

 

A bilateral C6 – unilateral C7 horse showing a toe-out stance and hoof distortion. The ventral part of the transverse process was transposed onto the left side of C7.

 

  • With such asymmetry in the skeletal structure, these horses have serious lateral flexion issues that can’t be overcome. When required to elevate the forehand, many will experience difficulties, due to the absence of correctly inserted musculature and incorrect articulation through the joints of the lower neck. 

 

The horse may have one very prominent, widely positioned scapula.

 

  • A high level of asymmetry may be seen in the shoulders, with one scapula sometimes positioned very wide, with no improvement after chiro, osteopathy or bodywork. This is particularly so with the C6-C7 problem and associated first sternal rib abnormalities.
  • The ventral aspect of the neck may show some scoliosis.

    There may be scoliosis along the entire spine.

  • There may be an obvious scoliosis to the underside of the neck.
  • The problem may lead to heavily asymmetric loading of the forefeet, so may be accompanied by a severe high foot/low foot issue (this is not in itself a sign of the C6-C7 problem).

 

 

 

 

 

If you suspect your horse has the C6-C7 issue

First, note that many horses do just fine with a C6 problem. It is those with the bilateral C6 and unilateral/bilateral C7 issue that tend to show the more worrying problems.

If your horse is showing ongoing signs of instability, it’s important to seek veterinary advice, so that neurological issues can be ruled out. (As this a recently recognised problem, it may be worth printing out the abstracts from the journal articles listed at the end of this page and handing them over.)

If the more severe malformations are identified by radiograph, it is important to remember that in some cases this can cause discomfort and pain to the horse, and it is not going to improve over time. 

Since this article was published, Sharon has allowed me to publish her paper with a preliminary protocol for radioagraphing for this issue. Read it here: By Popular Demand: Here’s How You X-Ray for the C6-C7 Malformation.

On the contrary, the cervical vertebrae of some older horses with the C6 and C7 malformations often display advanced osteoarthritis of the articular processes, as shown in the header image of a 19-year-old Thoroughbred’s malformed C7.

Questions, thoughts or comments? Join us at The Horse’s Back Facebook Group. 

 

Where does this knowledge take us?

At the moment, that question is wide open. The findings published by Sharon May-Davis have triggered ongoing research on an international level. There are certainly ramifications for breeders in more than one equine sporting industry.

Connections have been made with a number of falls on the racetrack that have caused injury, and worse, to both horse and jockey, as well as other runners. Similar things can be said for the sport of eventing, where unforced errors can have equally catastrophic effects.

It is entirely possible that at higher levels, pre-purchase examination radiographs will come to include a check on C6. While it’s not possible to radiograph the deeply positioned C7, we do at least know that this will only be present if the C6 anomaly exists.

Vets in some countries are proving faster at picking this up than others. While papers are being published, it clearly takes some time for information to filter down.

And until more is known, this problem is being unknowingly propagated every breeding season.

Of course, many horses harbouring the milder manifestations of this problem at C6 level are functioning very well. All horse owners can do is be aware that this issue exists, make use of this information if a problem arises, and await further research findings.

 

Since this article was published, Sharon has allowed me to publish her paper with a preliminary protocol for radioagraphing for this issue. Read it here: By Popular Demand: Here’s How You X-Ray for the C6-C7 Malformation.

© All text copyright of the author, Jane Clothier, www.thehorsesback.com. No reproduction of partial or entire text without permission. Sharing the link back to this page is fine. Please contact me for more information. Thank you!

 

Sharon May-Davis’s research includes the rarely documented arthritis affecting the elbow joint of ridden and driven horses – Revealed: the Common Equine Arthritis You Won’t Read About in Textbooks.

Plus, read about the effects of hard race training on Thoroughbreds – Buying an Ex-Racehorse: Can You Spot the Major Physical Issues? – and advice from a seasoned trainer on rehabbing your horse once it’s arrived at your stable – 8 Golden Rules for Helping Your Thoroughbred Get Right Off the Track.

 

Literature on the malformation

 

Peer reviewed journal articles on the C6-C7 and related first sternal rib issues.

 

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Comments

  1. Jan Easterday says:

    Fascinating article

  2. Deborah Miller says:

    Oh Wow!
    This has answered so many unanswered questions relating to my 23yr old mare! V long story short – she is a 16.3hh, TB x Hannoverian and has this condition.
    She has been in full retirement for 3 years now and has been fine (apart from sleep deprivation/narcoleptic symtoms due to lack of laying down) and I have known her days are very much numbered 🙁 She is a dominant mare and until this spring has been able to maintain this position with no physical problems but when her long term field mate left her in Spring the added physical stress of ‘keeping order’ led to fore limb unsoundness (something that was an occassional and unanswered problem throughout her ridden ‘career’ but is now clearly explained above) it has taken several weeks of danilon to get her ‘sound’ again but then a flock of sheep escaped into their field, causeing havoc and aggrevated the unsoundness again so is back on Danilon. I have vowed that I would care and provide for her as long as she was sound and did not need daily meds so I fear we are now bordering on that and this may well be her last summer :(( Your article has clarified so much though I do now feel that I will be able to make the right decision at the right time alongside my vet – thank you.

  3. CM Newell says:

    Minor quibble–congenital and inherited are not synonymous.
    Congenital conditions *may* be inherited, but not all of them are.
    For example, cerebellar hypoplasia in kittens may be the result of a feline distemper virus infection of the dam during pregnancy.
    Inherited conditions are congenital in that they are present at birth, but may not manifest until later (for example, Huntington’s Disease in humans.)

    That said, I expect that there is at least some degree of inheritance at work in cervical vertebral problems of the horse.

    • Thank you. The error is mine.

      This issue has been identified as passing down certain lines, although it does not manifest in every horse in those lines. Thankfully, the skeletons of many outstanding racehorses have been preserved, enabling identification of the issue in some prolific lines.

  4. This is so helpful. My horse is a 18 year old mare tb warmblood x who is 5 months into retirement because of undiagnosed “instability issues”. She won’t square up in front and exhibits much of the behaviours you’ve described. She would fall after being ridden usually at the walk or just standing around (mounted) after a hack. Then it got more serious. Falling at the canter – like no legs were underneath her at all. That is when we pulled all riders and began the attempts of figuring out what was wrong. We never found anything except she won’t stand up straight now. She doesn’t lock out her knees super well anymore either. Her issues were WITH riders so she does ok hanging out with no riders anymore obviously. I so appreciated this information.

    • Hi, thanks for telling us this. It does sound likely, doesn’t it? I hope it helps to have some inkling of what could have been happening. Retirement is a good decision and I’m sure you’ll notice if she exhibits pain in the paddock.

    • Hi Heather,

      I replied to this earlier, I thought, but it hasn’t appeared.

      Your mare certainly sounds like a candidate for the full C6-C7, doesn’t she? We’ll be posting details of the radiographic protocol next week, so if you were to seek additional confirmation, that might help.

      However, it does sound as if you’ve made the right decision based on the signs and in the interests of safety for riders and horse. It’s possible that she may worsen further with advancing years, but you’re clearly watching her closely.

      Thank you for posting this.

  5. Vicki McKague says:

    As a Canadian Sport Horse Association (CSHA) performance horse breeder, I would like to know which Thoroughbred lines predominantly pass on this problem. I think it is important even if so many performance horse breeds have Thoroughbred in them. As a sport horse breeder I have never seen an issue with any of my equines, but I would like to be able to knowingly avoid certain lines if I know of a “possible problem” (which I still feel hasn’t become a serious issue yet).

    • Sharon May-Davis says:

      Hi Vicki,
      I got a clear warning not to name the Thoroughbred sire line !

      Even within the last month we (European colleagues and myself) got another warning to be careful with our research and to go slow. With a potential 30% in Warmbloods, there are a lot of breeders that stand to lose if this congenital malformation became a reason for loss of purchase. Which by the way has just happened at a major European Warmblood auction!

      Remember, mine and other necks are on the line and when I got my first informal death threat in 2005, I had my children in the car.

      So I would recommend the congenital malformation to be detected by radiographs with the right technician who can read them.

      At this point in time, the radiographs and CT scans have all revealed an abnormal vertebral canal. In other words, not one had normal vertebral canal aperture for the spinal cord to pass through nor Thoracic Inlet aperture. Furthermore, the preliminary radiographic protocols written and accepted for a Japanese journal will be posted next week in English. This paper describes the radiographic view and also, as per a previous paper (re “Longus colli”) the potential influence on the trachea caused by this congenital malformation.

      This later paper describes asymmetrical: biomechanical forces; entheses patterns, neurological dysfunction among other issues and the table that Jane posted identifies these also. This paper along with the 1st sternal rib paper notes clinical findings pre and post mortem along with a maternal link. Including embryonic mortality and confirmed C6 and C7 spontaneous abortions of 3rd trimester foals.

      The 3 spin off papers from UC Davis (De Rouen et.al), Utrecht Holland (Veraa et al.) and Italy (Santinelli et al.) confirm these congenital malformations and took the findings into other breeds; Warmbloods, Standardbreds, Arabians and Quarter Horses to name a few. So in total 7 papers on the subject.

      In fact, I have personally seen or know of it in New Zealand, Japan, UK, USA, Europe and of course Australia.

      Research is a slow and time consuming journey and it took 15 years from discovery to publication and I think this was a miracle considering the number of road blocks I encountered.

      Cheers
      Sharon

      • Hello Sharon, this is a fascinating subject. I am an equine thermographer and thermal imaging can show root nerve compression/damage through changes in heat patterns in the corresponding dermatome, I would love to discuss this further with you. Regards Elaine Hall

      • Fay Seltzer says:

        I was told that a study at New Bolton Center from 1976-80 might also be related to this issue. They were trying to recreate wobblers.

    • Deborah Miller says:

      It is not known to be a serious problem because people are unaware of it – it doesn’t mean it isn’t there! The symptoms can be extremely subtle to start with and can progress very slowly. However, the symptoms are also the same/similar as many other issues that can present in horses or even develop as a consequence of the condition e.g. muscle soreness in the neck – this could be ‘blamed’ on any number of things and vets ‘treat’ the symptoms but not find the true underlying cause. It is a very complex condition and must present itself on many different levels thus making it even more difficult for the vets. More research and understanding of this condition can only lead to better veterinary care and could lead to identifying the ‘wonky’ gene for future breeding programs. Please do not think that because you have never seen any issues with your equines that it isn’t present – it really can be that subtle especially in the younger horses whose bodies can initially ‘adapt’.

  6. Fay Seltzer says:

    A friend sent me this link. I was just forced to retire my homebred 8 yr old, once in a lifetime horse, due to as the vet put it “severe proliferative osteoarthritis of the cervical facet joints at c6-c7”. It explained his erratic behavior through out his life. The attending vet at New Bolton Center in PA, USA said his will progress and eventually severely limit the spinal cord. The one thing it really explained is his lack of topline muscling. Some vets would notice the dragging of the right hind and blame it on stifle issues. The answers came when a professional rider finally believed what I was saying, and knew exactly where to go for a diagnosis.

    • I am so sorry to read this, Fay. I know it won’t help with the loss of your horse (or at least, your riding partnership), but I do hope Sharon’s research helps you to make more sense of what’s happened. If you still have the radiographs, we’ll be uploading more details on interpreting them next week (see Sharon’s response to an earlier question).

      • Fay Seltzer says:

        I feel like I’ve joined a support group, I thought my horse was just a freak thing, and now we are learning it is much more common than thought. When the vet said it was from the breeding, I was shocked, and got a bit defensive. I had a wonderful mare that was not diagnosed with this, and as far as I was aware, neither was the stallion. Now I’m learning that many have mild symptoms, and it’s possible that my mare learned to deal with it, and the untimely paralyzing in the stallion, my have been the indicators that we were previously not aware of. When I buy a new horse, I feel like it would be a better bet to xray the neck rather than the hocks.
        AND, yes, I do have the x rays.

  7. Colleen Callahan says:

    Wow, so fascinating and heartbreaking at the same time. Thank you for all of your hard work. Between this and the research being conducted by Equiseq on the genes involved in muscle myopathy, I am losing hope that there are any sound Thoroughbreds (or any horse for that matter). : (

  8. Hihi,
    Is there any reason to be worried for the cold blood, smaller breeds, like the icelandic horse?
    Or has there been no research on these breeds?

  9. betsy baker says:

    So typical of the litigious society we have become. I get that it represents income to the folks whose line it may predominate, but SERIOUSLY?!?! What is more important, the fatness of your wallet or the well being of the animal and the people who may be around them?

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